Wonders of Light - Part I

Transcription

1 6. Wonders of Light - Part I Light : The fastest physical quantity, which is an electromagnetic radiation travelling with the speed of m/s. SCHOOL SECTION 25

2 SCIENCE & TECHNOLOGY MT EDUCARE LTD. Q.I (A) Choose the correct alternatives and rewrite the complete sentences : 26 MIRROR. The image formed by a concave mirror is.... (a) always virtual and erect (b) always virtual and inverted (c) virtual if the object is placed between the pole and the focus (d) virtual if the object is beyond the focus 2. A concave mirror forms a virtual image of an object placed.... (a) at infinity (b) at the centre of curvature of the mirror (c) at the focus of the mirror (d) between the focus and the mirror 3. A convex mirror always forms an image which is... than the object size. (a) larger (b) smaller (c) double (d) three times 4. No matter how far you stand from a spherical mirror, your image appears erect. The mirror may be.... (a) Plane (b) concave (c) convex (d) either plane or convex 5. In case of a concave mirror, an erect image is.... (a) real and enlarged (b) real and diminished (c) virtual and diminished (d) virtual and enlarged images can be displayed on a screen. (a) Virtual (b) Real (c) Virtual and erect (d) Virtual and inverted 7. A rear view mirror of a car is.... (a) Plane mirror (b) Concave mirror (c) Convex mirror (d) Cylindrical mirror 8. An image of an object placed at infinite distance from a concave mirror is formed at.... (a) the focus of the mirror (b) behind the mirror (c) centre of curvature (d) infinity 9. The distance of focus F from the pole P is termed as the... of the mirror. (a) radius of curvature (b) centre of curvature (c) principal axis (d) focal length 0. A concave mirror is also called as a... mirror. (a) converging (b) diverging (c) plane (d) outward curved *. A ray of light parallel to principal axis after reflection from concave mirror passes through.... (a) centre of curvature (b) focus (c) pole (d) optical centre 2. The centre of the spherical mirror is called.... (a) pole (b) centre of curvature (c) principal axis (d) focus 3. The image made by a plane mirror is a... image. (a) real (b) virtual (c) inverted (d) diminished 4. According to the new sign convention, the... of the mirror is taken as origin. (a) focus (b) pole (c) optical centre (d) centre of curvature SCHOOL SECTION

3 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 5. Turning back of light from the surface on which it falls is called.... (a) refraction (b) reflection of light (c) dispersion (d) rectilinear propagation 6. The size of the image of an object placed at the focus of a concave mirror is.... (a) erect (b) highly enlarged (c) same size (d) diminished 7. A convex mirror is also called as a... mirror. (a) converging (b) plane (c) diverging (d) inward curved 8. To obtain a real, inverted and enlarged image using a concave mirror, the object should be placed.... (a) between the focus and centre of curvature of the mirror (b) at infinity (c) beyond centre of curvature (d) at centre of curvature 9. If magnification is negative, the image is... with respect to the object. (a) inverted (b) enlarged (c) erect (d) diminished LENS 20. The image formed by a concave lens.... (a) is always virtual and erect (b) is always real and inverted (c) is virtual if the object is placed between the optical centre and the focus (d) Is virtual if the object is beyond the focus *2. The focal length of... lens is positive. (a) concave (b) bifocal (c) convex (d) double concave *22. The image of an object is formed behind... in hypermetropia. (a) cornea (b) retina (c) lens (d) iris *23. An optical device used by watch repairers is.... (a) compound microscope (b) telescope (c) simple microscope (d) spectrometer *24. The power of spectacle for myopic eye is.... (a) positive (b) negative (c) zero (d) positive and negative 25. The human eye can focus objects at different distances adjusting the focal length of the eye due to.... (a) presbyopia (b) power of accomodation (c) hypermetropia (d) myopia 26. The tendency of pupil to adjust the opening for light is called.... (a) adaptation (b) power of accomodation (c) light control (d) perception 27. A lens does not produce any deviation of a ray of light passing through its.... (a) centre of curvature (b) optical centre (c) second focal point (d) axial point at a distance of 2F 28. The power of a convex lens of focal length 25 cm is.... (a) 4 dioptre (b) -4 dioptre (c) /25 dioptre (d) -/25 dioptre SCHOOL SECTION 27

4 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 29. A person suffering from myopia.... (a) cannot see nearby objects clearly (b) cannot see distant object clearly (c) can see nearby as well as distant objects clearly (d) can neither see nearby objects nor distant objects clearly 30. A well lit object produces... incident rays. (a) two (b) three (c) finite (d) infinite 3. The change in focal length of the eye to view objects at different distances is brought about by the.... (a) retina (b) ciliary muscles (c) pupil (d) iris 32. The least distance of distinct vision is about.... (a) 30 cm (b) 35 cm (c) 20 cm (d) 25 cm Answers :. virtual if the object is placed 2. between the focus and the mirror between the pole and the focus 3. smaller 4. convex 5. virtual and enlarged 6. Real 7. Convex mirror 8. the focus of the mirror 9. focal length 0. converging. focus 2. pole 3. virtual 4. pole 5. reflection of light 6. highly enlarged 7. diverging 8. between the focus and centre of curvature of the mirror 9. inverted 20. is always virtual and erect 2. convex 22. retina 23. simple microscope 24. negative 25. power of accomodation 26. adaptation 27. optical centre dioptre 29. cannot see distant object clearly 30. infinite 3. ciliary muscles cm Q.I (B) Match the following : MIRROR. Column I Column II (i) Plane mirror (a) Rear view mirror (ii) Concave mirror (b) At laughing gallery (iii) Convex mirror (c) At a hair dresser (iv) Irregular curved mirror (d) At a dentist (i - c), (ii - d), (iii - a), (iv - b). LENS 2. Column I Column II (i) Microscope (a) Prism (ii) Telescope (b) Spectacles having convex lenses (iii) Presbyopia (c) To observe distant objects (iv) Dispersion of light (d) Observation of plant and animal cells ( e) Weakness of ciliary muscles (i - d), (ii - c), (iii - e), (iv - a). 28 SCHOOL SECTION

5 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 3. Column I Column II (i) Microscope (a) Torches and headlights (ii) Telescope (b) Observation of plant and animal cells (iii) Simple microscope (c) To observe distant objects (iv) Concave mirror (d) Watch repairer ( e) correction of eye defect (i - b), (ii - c), (iii - d), (iv - a). 4. Column I Column II (i) Myopia (a) Convex lens (ii) Hypermetropia (b) Concave lens (iii) Presbyopia (c) Cone cells (iv) Colour blindness (d) Ciliary muscles ( e) Bifocal glasses (i - b), (ii - a), (iii - d), (iv - c). 5. Column I Column II Column III (i) Myopia (a) Old age problem. Convex lens (ii) Hypermetropia (b) Near-sightedness 2. Bifocal lens (iii) Presbyopia (c) Long sightedness 3. Concave lens (i - b, 3), (ii - c, ), (iii - a, 2). Q.I (C) State whether the following statements are true or false. If false write the corrected statement : MIRROR. A virtual image can be displayed on a screen. False. A real image can be displayed on a screen. 2. A concave mirror always forms a real image. False. A concave mirror forms a real, as well as a virtual image. 3. A concave mirror always forms a magnified image. False. A concave mirror can sometimes form a diminished image as well. 4. Images formed by convex mirrors are always virtual. True. 5. The distance between the focus and the pole is called the radius of curvature. False. The distance between the focus and the pole is called the focal length. 6. Reflection from a spherical mirror obeys laws of reflection. True. 7. The reflecting surface of a concave mirror is curved outwards. False. The reflecting surface of a concave mirror is curved inwards. 8. Distances measured in the direction of the incident light are taken as positive. True. 9. If the image is erect, the height of the image is negative. False. If the image is erect, the height of the image is positive. 0. A concave mirror is used in search light. True. SCHOOL SECTION 29

6 SCIENCE & TECHNOLOGY MT EDUCARE LTD.. A concave mirror always form a real and inverted image. False. A concave mirror can also form a virtual and erect image. 2. Doctors use diverging beam of light to study teeth, ears, eyes. False. Doctors use converging beam of light to study teeth ears, eyes. LENS 3. All positive images are erect. True. 4. The principal focus of a concave lens is virtual. True. 5. The laws of reflection are equally valid for plane as well as curved reflecting surfaces. True. 6. The thickness of a glass slab is not related to the displacement of the ray of light passing through it. False. The displacement of the ray of light passing through a glass slab is proportional to the thickness of the glass slab. 7. The same laws of refraction also hold true for refraction from a curved surface. True. 8. A concave lens always forms a magnified image. False. A concave lens always forms diminished images. 9. Convex lens is called a diverging lens. False. Convex lens is called a converging lens. Q.I (D) Find the odd man out :. Kilometre, Metre, Dioptre, Centemetre. Dioptre. Dioptre is the power of a lens. Rest are units of length. 2. Cornea, Iris, Pupil, Cerebrum. Cerebrum. Cerebrum is the part of brain. The others are parts of humaneye. 3. Torches, Projector lamps, Solar furnaces, Rear view mirror. Rear view mirror. In rear view mirrors convex mirror is used. Concave mirrors are used in the rest. 4. Simple microscope, Compound microscope, Astronomical telescope, Terrestrial microscope. Simple microscope. It has a single convex lens, while in the rest two convex lenses are used. 5. Myopia, Hypermetropia, Colour blindness, Presbyopia. Colour blindness. It cannot be corrected, whereas the rest can be corrected. Q.II Define the following : MIRROR *. Centre of curvature of mirror (C). The centre of sphere of which the mirror is a part, is its centre of curvature. *2. Pole (P). The centre of the spherical mirror is the pole. *3. Principal axis of mirror. The straight line passing through the pole and centre of curvature of mirror is its principal axis. *4. Focus length of concave mirror. The rays parallel to principal axis get reflected from the mirror, and meet in front of the mirror at a single point. It is the focus of the concave mirror. 30 SCHOOL SECTION

7 MT EDUCARE LTD. SCIENCE & TECHNOLOGY *5. Focal length of mirror. The distance between the pole and the focus is called focal length of mirror. The focal length is half of the radius of curvature. f R 2 LENS *6. Centre of curvature of lens. It is the centre of the imaginary sphere, which forms the given lens. Each lens has two centres of curvature C and C 2 respectively. *7. Principal axis of lens. It is an imaginary straight line passing through the two centres of curvatures of lens. *8. Optical centre of lens. The central point of lens on the principal axis is its optical centre. When a ray of light passes through the optical centre of a lens, it passes without undergoing any deviation. *9. Principal focus of convex lens (F). When several rays of light parallel to principal axis are incident on a convex lens, they converge at a point on a principal axis. It is the principal focus of the convex lens. Every lens has two principal foci. *0. Focal length of lens. The distance of principal focus and optical centre of the lens is the focal length.. Adaptation. The tendency of pupil to adjust the opening for light is called adaptation. 2. Power of accommodation. The ability of the lens of adjusting focal length is know as power of accommodation. 3. Distance of distinct vision. The minimum distance from the normal eye, at which the object can be seen clearly and distinctly without any strain on the eye is known as the distance of distinct vision. It is about 25 cm for normal human eye. Q.III (A) Give scientific reasons : MIRROR *. Concave mirror is called converging mirror.. When rays of light parallel to the principal axis are incident on concave mirror they converge. 2. After convergence they meet at one point on the principal axis, hence concave mirror is called converging mirror. 2. Concave mirrors are used in torches and in car headlights.. Concave mirrors are used in torches and car headlights because when a source of light is placed at the focus of a concave mirror, a parallel beam of light rays are formed. 2. This helps us to see things upto a considerable distance in the darkness. *3. Concave mirrors are used in solar devices.. Solar devices like solar cooker or solar water heater uses solar energy to cook food or heat water. 2. When Sun ray falls on the concave mirror, they converge. 3. Due to convergence the intensity of Sun rays increases and the food or water is heated faster. Hence concave mirrors are used in solar devices. SCHOOL SECTION 3

8 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 4. A dentist uses a concave mirror while examining teeth.. Concave mirror produces an erect, virtual and highly magnified image of an object placed between its pole and focus. 2. Dentist uses this principle to get a clear and distinct image of teeth, hence dentist use a concave mirror. LENS 5. You cannot enjoy watching a movie from a very short distance from the screen in a cinema hall.. Ciliary muscles are responsible for changing the focal length of lens to observe near and far away objects by changing curvature of lens. 2. If the object is too close, the eyelens curvature cannot be changed enough, causing strain on the eye and may sometimes cause blurred image. 3. Hence one cannot enjoy watching a movie from a very short distance from the screen in a cinema hall. *6. A simple microscope is used by watch repairers.. A magnifying glass works on the principle of simple microscope. 2. When a object is placed within the focal length of a convex lens we get a virtual, erect and magnified image on the same side of the lens. 3. This principle is used by the watch repairer to see the small parts more clearly. Hence watchmakers use a magnifying glass while repairing wrist watches. 7. Concave lens is also called a diverging lens.. When rays of light parallel to the optical axis pass through a concave lens they diverge. 2. After divergence they appear to meet at a point on the optical axis. 3. As concave lens diverges the rays, it is called as diverging lens. 8. Convex lens is also called a converging lens.. When rays of light parallel to the optical axis pass through a convex lens they converge. 2. After convergence they meet at a point on the optical axis. 3. As convex lens converges the rays, it is called as converging lens. 9. A concave lens is used to correct myopia.. In myopia the distance between eye lens and retina increases as eyeball is lengthened or lens is curved. 2. A concave lens causes light rays to diverge before they strike the lens of the eye. 3. The power of concave lens is so closen that it creates required divergence and hence after convergence by eye lens, the image is formed on the retina. 0. Convex lens is used to correct hypermetropia.. In hypermetropia, the distance on account of either shortening of eye ball or flattening of lens. 2. A convex lens causes converging of rays before they strike the retina of the eye. 3. The power of convex lens is so chosen that it creates required convergence and hence after convergence by eye lens, the image is formed on the retina.. Old people use bifocal glasses.. Few old people, suffer both myopia and hypermetropia. 2. In bifocal lens upper part is concave lens to correct myopia and lower part is convex lens to correct hypermetropia. 32 SCHOOL SECTION

9 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 3. The concave lens is useful for distant vision and convex lens for near vision. 4. Hence old people use bifocal glasses. Q.III (B) Answer the following questions in short : MIRROR *. State the different positions of source of light with respect to concave mirror.. In torches : The source of light is placed at the focus. 2. Projector lamps : The source of light is placed at the centre of curvature. 3. Flood lights : The source of light is placed just beyond the centre of curvature. *2. What are the rules for drawing ray diagrams for the formation of image by spherical mirror? The rules are as follows :. If the incident ray is parallel to the principal axis, then the reflected ray passes through focus. 2. If the incident ray is passing through the focus then the reflected ray is parallel to the principal axis. 3. If the incident ray passes through the centre of curvature, the reflected ray traces the same path. *3. If you are given a part of hollow spherical glass, how will you convert it into concave mirror? The inner side of the hollow spherical glass is concave, hence the outer side should be coated with reflecting layer to make inner side concave mirror. LENS *4. What is meant by power of accommodation of eye?. The ability of the lens of adjusting focal length is known as power of accommodation. 2. The process of focusing the eye at different distances is called accommodation. 3. This is brought about by a change in curvature of the elastic lens making it thinner or fatter. *5. State the function of iris and ciliary muscles.. Iris in human eye controls and regulates the amount of light entering the eye by contracting and dialating the pupil. 2. Ciliary muscles adjusts the focal length of eye lens by contracting and relaxing. 6. Explain presbyopia. How does it occurs?. Presbyopia is also called as old age hypermetropia. 2. The power of accommodation of eye usually decreases with ageing. The near point of aged people recedes and they find it difficult to see nearby objects comfortably and clearly. 3. As ciliary muscles lose the capacity to change focal length of eye lens, aged people experience eye lens presbyopia. 4. It is corrected by using convex lens of suitable focal length. 7. Explain magnification by a lens with formula.. The magnification produced by lens is the ratio of height of the image (h 2 ) and the height of object (h ). Height of the image Magnification Height of the object h2 M h SCHOOL SECTION 33

10 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 2. Magnification produced by a lens is also related to the object distance (u) and image distance (v). Distance of the image Magnification Distance of the object M v u 8. Explain power of lens with suitable formula. The degree of convergence of light rays achieved by convex lens is expressed in terms of power of lens. It is the reciprocal of the focal length. P f (in metre) Unit of power of lens is dioptre. 9. What are the rules for obtaining images by convex lens? Following are rules for obtaining images by convex lens.. If the incident ray is parallel to principal axis, then the refracted ray passes through focus. 2. Ray of light passing through the optical centre passes through the optical centre undeviated. 3. If the incident ray is passing through the focus, the refracted ray passes parallel to the principal axis. Q.IV (A) Distinguish between : *. Myopia and Hypermetropia. Myopia. It is defect in which a human eye can see nearby objects clearly but is unable to see distant objects clearly. 2. In myopia, the image of the distant object is formed in front of retina. 3. The distance between eye lens and retina increases as eyeball is lengthened or lens is curved. 4. It is corrected by using concave lens of suitable focal length. 2. Convex mirror and Concave mirror. Convex mirror. In a convex mirror, the reflecting surface is on the outer side. 2. It is called as diverging mirror. 3. The focus of a convex mirror is virtual. 4. It can form only a virtual image 5. It can form only a diminished image. Hypermetropia. It is the defect in which human eye can see distant objects clearly but is unable to see nearby objects clearly. 2. In hypermetropia, the image of nearby object falls behind the retina. 3. The distance between eye lens and retina decreases on account of either shortening of eyeball or flattening of lens. 4. It is corrected by using convex lens of suitable focal length. Concave mirror. In a concave mirror, the reflecting surface is on the inner side. 2. It is called as converging mirror. 3. The focus of a concave mirror is real. 4. It can form a real as well as a virtual image. 5. It can form an enlarged, diminished as well as the same size image. 34 SCHOOL SECTION

11 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 3. Convex lens and Concave lens. Convex lens. It is thicker at the centre than its edges. 2. Convex lens is called converging lens as it converges the rays of light falling on it. 3. Convex lens is curved outwards. 4. The principal focus of the convex lens is real. 4. Real image and Virtual image. Real image. A real image is formed only when the refracted or reflected rays actually meet at a point. 2. Real images can be taken on a screen. 3. All real images are inverted. 5. Mirror and Lens. Mirror. Mirror is a opaque surface. 2. Convex mirror is a diverging mirror. 3. Mirror reflects light. Concave lens. It is thinner at centre than at the edges. 2. Concave lens is called a diverging lens as it diverges the rays of light falling on it. 3. Concave lens is curved inwards. 4. The principal focus of concave lens is virtual. Virtual image. A virtual image is formed only when the refracted or reflected rays appear to meet. 2. Virtual images cannot be taken on a screen. 3. All virtual images are erect. Lens. Lens is a transparent medium. 2. Convex lens is a converging lens. 3. When light passes through lens, it can refract. Q.IV (B) Draw a neat and labelled diagram of the following : MIRROR *. A ray diagram for object at infinity for a concave mirror. A C F P B Image position : At focus. Nature : Real, inverted and highly diminished. SCHOOL SECTION 35

12 SCIENCE & TECHNOLOGY MT EDUCARE LTD. *2. A ray diagram for object between infinity and centre of curvature for a concave mirror. A B C B' F P A' Image position : Between centre of curvature and focus. Nature : Real, inverted and diminished. *3. A ray diagram for object at centre of curvature for a concave m irror. A ns. A B B' C A' F P Image position : At centre of curvature. Nature : Real, inverted and same size. *4. A ray diagram for object betw een F and C for a concave m irror. A ns. A B' C B F P A' Image position : Beyond centre of curvature. Nature : Real, inverted and magnified. *5. A ray diagram for object at focus for a concave m irror. A ns. C B A F P Image position : At infinity. Nature : Real, inverted and highly magnified. 36 SCHOOL SECTION

13 MT EDUCARE LTD. SCIENCE & TECHNOLOGY *6. A ray diagram for object between pole and focus for a concave mirror. A' B C F B P B' Image position : Behind the mirror. Nature : Virtual, erect and magnified. 7. Convergence of rays by concave mirror. F P 8. Divergence of rays by convex mirror. P F SCHOOL SECTION 37

14 SCIENCE & TECHNOLOGY MT EDUCARE LTD. LENS *9. A ray diagram for object position at infinity for a convex lens. A 2F F O F 2 2F 2 B Image position : At F 2. Nature : Real, inverted and highly diminished. *0. A ray diagram for object position beyond 2F for a convex lens. A B 2F F 2 B' F O 2F 2 A' Image position : Between F 2 and 2F 2. Nature : Real, inverted and diminished. *. A ray diagram for object position at 2F for a convex lens. A B 2F B' F O F 2 2F 2 A' Image position : At 2F 2. Nature : Real, inverted and same size. *2. A ray diagram for object position between 2F and F for a convex lens. A B 2F F O F 2 2F 2 B' Image position : Beyond 2F 2. Nature : Real, inverted and magnified. A' 38 SCHOOL SECTION

15 MT EDUCARE LTD. SCIENCE & TECHNOLOGY *3. A ray diagram for object position at F for a convex lens. A B 2F F O F 2 2F 2 Image position : At infinity. Nature : Real, inverted and highly magnified. *4. A ray diagram for object position between F and O for a convex lens. A' A B' 2F F B O F 2 2F 2 Image position : On the same side of lens as object. Nature : Virtual, erect and magnified. 5. Convergence of rays by convex lens. 2F F O F 2 2F 2 6. Divergence of rays by concave lens. 2F F O SCHOOL SECTION 39

16 SCIENCE & TECHNOLOGY 7. Power of accommodation of human eye. A Lens pulled thin Light from distant object MT EDUCARE LTD. Light focused on retina Lens gets thicker Light from nearby object B Light focused on retina Q.V Answer the following in detail : MIRROR *. Explain the sign conventions for reflection by spherical mirrors. According to the new cartesian sign convention, the pole (P) of the mirror is taken as origin. The principal axis is taken as X-axis of the co-ordinate system. The sign conventions are as follows :. The object is always placed on the left of the mirror. 2. All distances parallel to principal axis are measured from the pole of the mirror. 3. All the distances measured to the right of the origin are taken as positive, while distances measured to the left of the origin are taken as negative. 4. Distances measured perpendicular to and above the principal axis are taken as positive. 5. Distances measured perpendicular to and below the principal axis are taken as negative. 6. Focal length of convex mirror is positive while that of concave mirror is negative. Direction of incident light Height upwards (+ve) Distance towards the left ( ve) Distance towards the right (+ve) Height downwards ( ve) P Principal axis Mirror Representation of sign conventions 40 SCHOOL SECTION

17 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 2. State the applications of a concave mirror. State the position of the source of light.. In torches and headlights : The source of light is placed at the focus to obtain a parallel beam of light. 2. In flood lights : The source of light is placed just beyond the centre of curvature so as to get intense beam of light. 3. Reflecting mirrors for projector lamps : The object is placed at the centre of curvature to obtain an image of the same size. 4. To collect heat radiations in solar devices : Heat radiations from the sun coming from infinity is brought to focus by concave mirror in its focal plane. 5. Shaving mirror, dentist s mirror : It produces as erect virtual and highly magnified image of an object placed between its pole and focus. 6. Solar furnaces : Large concave mirrors are used to concentrate sunlight to produce heat in solar furnace. LENS 3. Draw a neat labelled diagram of a normal human eye. Describe its parts briefly. Cornea Pupil Retina Macula Lens Iris Optic nerve. Cornea : The cornea is a thin membrane, which forms a transparent bulge on the surface of the eye ball. Maximum refraction of light rays entering the eye takes place from cornea. 2. Iris : It is a dark muscular diaphragm. The colour of iris is different for different people. It also controls the light entering the eye. 3. Pupil : It is a small opening of variable diameter at the centre of iris. It is useful to control and regulate the amount of light entering the eye. 4. Eye lens : It is a transparent biconvex crystalline body located just behind the pupil. 5. Retina : Retina is a light sensitive screen. It consists of large number of light sensitive cells called as rods and cones. 6. Optic nerve : Optic nerve is a carrier of signals from the retina to the brain. 4. Explain myopia with its correction.. It is the defect in which a human eye can see nearby objects clearly but is unable to see distant objects clearly. 2. In myopia, the image of distant object is formed in front of retina. 3. There are two possible reasons of myopia : (a) As ciliary muscles do not relax sufficiently, converging power of eye lens becomes high. SCHOOL SECTION 4

18 SCIENCE & TECHNOLOGY MT EDUCARE LTD. (b) The distance between eye lens and retina increases as the eyeball is lengthened or lens is curved. 4. A concave lens of suitable focal length can correct this defect. 5. The power of concave lens is so chosen that it creates required divergence and hence after the converging action of eye lens, the image is formed on the retina. 5. Explain hypermetropia with its correction.. It is the defect in which human eye can see distant objects clearly but is unable to see nearby objects clearly. 2. The image of near object falls behind retina. Two possible reasons of hypermetropia are : (a) Weak action of ciliary muscles causes low converging power of eye (b) lens. The distance between eye lens and retina decreases on account of either shortening of eyeball or flattening of lens. In this case focal length of the eye lens is too long. 3. A convex lens of suitable focal length can correct this defect. 4. The rays coming from nearby object are first converged by convex lens and then converged by eye lens to retina. 6. State the four applications of convex lens.. Simple microscope : A single convex lens of small focal length can be used as a simple microscope. A magnification of about 20 times is obtained by simple microscope. Watch repairers, jewellers, etc. use it. 42 SCHOOL SECTION

19 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 2. Compound microscope : A combinations of two convex lenses having short focal lengths are used in compound microscope. It is used to observe bacteria, viruses, cells, micro-organism, etc. 3. Telescope : A combination of two convex lenses is used in telescopes. An astronomical telescope is used to get detailed view of astronomical bodies like planets, stars etc. Terrestrial telescope is used to study terrestrial objects. 4. Optical instruments : Convex lenses are used in different optical instruments like camera, projector, spectrometer etc. 5. Spectacles : Convex lens is used in spectacles to correct defect like hypermetropia. Q.VI Answer the following questions in one sentence each : MIRROR. What is light? Light is a form of electromagnetic radiation that produces the sensation of vision. 2. What kind of mirror will a doctor use to concentrate on teeth, eyes, ears etc.? The doctor will use a concave mirror to concetrate on teeth, eyes, ears etc. 3. What does the nature position and size of the image depend on? The nature, position and size of the image depends upon the distance of the object from the surface. 4. What do you mean by ray diagram? A ray diagram is a specialized pictorial representation used to trace the path of ray of light. 5. State any four uses of concave mirror. Concave, mirrors are used in torches, headlights, shaving mirror, dentist s mirror, solar furnaces etc. LENS 6. Write any four applications of convex lens. Three applications of convex lens are. Simple microscope 2. Compound microscope 3. Optical instruments 4. Telescope. 7. Identify a lens which gives a virtual erect and enlarged image. A convex lens can give a virtual, erect and enlarged image. 8. Identify a lens which gives a virtual, erect and diminished image. A concave lens gives a virtual, erect and diminished image. 9. Focal length of which lens is taken as positive. Focal length of convex lens is taken as positive. 0. What type of image is formed by eye lens? The image formed by the eye lens is real and inverted.. Name the part of the eye that controls the amount of light entering the eye. Pupil of the eye controls the amount of light entering the eye. 2. State the meaning of lens. A lens is a transparent material bound by two surfaces, out of which at least one surface in spherical. 3. Name the cells of retina which respond to the intensity of light and colour of the object. Rods are sensitive and respond to the intensity of light and cones respond to the colour of the object. SCHOOL SECTION 43

20 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 4. What is accommodation of eye-lens? The ability of the eye-lens of adjusting its focal length is called power of accommodation. 5. What is the least distance of distinct vision for a normal eye? The least distance of distinct vision for a normal eye is 25 cm. 6. A person cannot see nearby objects, name the eye defect he is suffering from. The person is suffering from hypermetropia. 7. With advancing age a person might suffer from which eye-defect. With advancing age a person might suffer from Presbyopia. 8. When is the magnification produced by a lens : (i) positive (ii) negative.. When the image is erect the magnification produced by a lens is positive. 2. When the image is inverted the magnification produced by a lens is negative. 9. Which type of an image will be formed when an object is kept infront of a lens of focal length +20 cm in following situations :. The object distance is 35 cm? 2. The object distance is 0 cm?. The given lens is a convex lens as its focal length is positive. If an object is kept at 35 cm (between F and 2F) from the lens the image will be real inverted and magnified. 2. If an object is kept at 0 cm (between F and optical centre) from the lens the image will be virtual, erect and magnified. Q.VII Solve the following numericals : MIRROR Type A Problem based on the formula : v + u f Solved examples : *. An image is formed 5 cm behind a convex mirror of focal length 0 cm. At what distance is the object placed from the mirror? Given : Image distance (v) 5 cm Focal length (f) 0 cm To find : Object distance (u)? Formula : v + u f Solution : v + u f 44 u f v u 0 5 u 2 0 u 0 u 0cm. The object is placed in front of the convex mirror at a distance of 0 cm. SCHOOL SECTION

21 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 2. At what distance from a concave mirror of focal length 0 cm should an object be placed? So that image is formed 20 cm behind the mirror. Given : Focal length (f) 0 cm Image distance (v) 20 cm To find : Object distance (u)? Formula : v + u f Solution : v + u f u f v u 0 20 u 2 20 u 3 20 u 20 3 u 6.67 cm. The object is placed in front of the concave mirror at a distance of 6.67 cm. 3. An object placed 20 cm in front of a convex mirror is found to have an image 5 cm behind the mirror. Find the focal length of the mirror. Given : Object distance (u) 20 cm Image distance (v) 5 cm To find : Focal length (f) Formula : v + u f Solution : v + u f 5 + ( 20) f f f 60 f f 60 cm. The focal length of the convex mirror is 60 cm. HOME WORK ASSIGNMENT - A. An object is placed at a distance of 36 cm from a concave mirror of focal length 2 cm. Find the image distance. ( 8 cm) 2. An arrow is placed at a distance of 25 cm from a diverging mirror of focal length 20 cm. Find the image distance. (. cm) SCHOOL SECTION 45

22 SCIENCE & TECHNOLOGY MT EDUCARE LTD. Type B Problem based on the formula : M h 2 h v u Solved examples :. An object 4 cm in height is placed at a distance of 36 cm from a concave mirror. The image is formed 8 cm in the front of the mirror. Find the height of the image. Given : Object height (h ) 4 cm Image distance (v) 8 cm Object distance (u) 36 cm To find : Height of image (h 2 )? Formula : h2 M h v u... (As image is real.) Solution : h2 h v u h 2 hv u h h 2 2 cm The height of the image is 2 cm and it is inverted. 2. An object 2 cm high is placed at a distance of 6 cm from a concave mirror which produces a real image 3 cm high. Find the image distance. Given : Object height (h ) 2 cm Object distance (u) 6 cm Image height (h 2 ) 3 cm To find : Image distance (v)? Formula : h2 M h v u Solution : h2 h v u 3 2 V 6 v v 24 cm The image is formed at a distance of 24 cm in front of lens. 3. For a convex mirror the given data : object distance U 30 cm to the left, image distance 2 cm to the right of the mirror and height of object 5 cm. Find the height of image. Given : Object distance (u) 30 cm Object height (h ) 5 cm Image distance (v) 2 cm To find : Image height (h 2 )? Formula : h2 M h v u 46 SCHOOL SECTION

23 MT EDUCARE LTD. SCIENCE & TECHNOLOGY Solution : h 2 h v u h 2 v h u h h 2 2 cm The height of the object is 2 cm and it is erect. HOME WORK ASSIGNMENT - B. An object 0 cm in height is placed at a distance of 36 cm from a concave mirror. If the image is formed at a distance of 8 cm in front of the mirror. Find the height of image. ( 5 cm) 2. A converging mirror forms a real image of height 4 cm of an object of height cm placed 20 cm away from the mirror. Find the image distance. ( 80 cm) Type C Problem based on the formula : v + u f h2 M h v u Solved examples : *. An object 3cm in size, is placed at 20 cm in front of a concave mirror of focal length 2cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image. Also find the nature and size of the image. Given : Object size (h ) 3 cm Object distance (u) 20 cm Focal length (f) 2 cm To find :. Image distance (v)? 2. Image size (h 2 )? 3. Nature of image? Formula : v + u f h2 M h v u Solution : v + u f v f u v 2 20 v v SCHOOL SECTION 47

24 SCIENCE & TECHNOLOGY MT EDUCARE LTD. 48 v 30cm. The screen should be placed at 30cm from the mirror. The image is real. M h2 h v u h 2 v h u h h 2 4.5cm Height of the image is 4.5cm. It is an inverted and enlarged image. *2. An object of size 7 cm is placed at 25 cm in front of concave mirror of focal length 5 cm. At what distance from the mirror should a screen be placed, so that we can get a sharp and clear image? Find nature and size of the image. Given : Object size (h ) 7 cm Object distance (u) 25 cm Focal length (f) 5 cm To find : Image distance (v)? Image size (h 2 )? Formula : v + u f h2 M h v u Solution : v + u f v f u v 5 25 v v 75 2 v 75 v 37.5 cm. The screen should be placed at 37.5 cm in front of the mirror. The image is real. M h2 h v u h 2 vh u h h h cm The height of the image is 0.5 cm, it is an inverted and enlarged image. SCHOOL SECTION

25 MT EDUCARE LTD. SCIENCE & TECHNOLOGY HOME WORK ASSIGNMENT - C. An object 2 cm in height is placed at a distance of 6 cm from a concave mirror, if the focal length of the mirror is 9.6 cm. Find the image distance, nature and size of the image. ( 24 cm, h 2 3 cm; real, inverted and enlarged) 2. An arrow 2.5 cm height is placed at a distance of 25 cm from a diverging mirror of focal length 20 cm. Find the nature position and size of the image formed. (. cm,. cm) LENS Type D Problem based on the formula : P f (m) dioptre Solved examples :. Calculate the focal length of a corrective lens having power +2.5 D. Given : Power of lens 2.5 D To find : Focal length (f)? Formula : P f (m) dioptre Solution : P f f P f 2.5 f 0.4 m The focal length of the lens is 0.4 m. 2. The power of a convex lens is 2 dioptres. Find its focal length. Given : Power (P) 2D To find : Focal length (f)? Formula : P f Solution : P f 2 f f 2 m 0.5 m or 50 cm The focal length of a given convex lens is 0.5 m or 50 cm. *3. Fill in the blanks for convex lens. f (m) P (D) f (m) P (D) SCHOOL SECTION 49

26 SCIENCE & TECHNOLOGY MT EDUCARE LTD. HOME WORK ASSIGNMENT - D. Calculate the power of a convex lens of focal length 20 cm. ( 5 dioptres) 2. The focal length of a concave lens is 40 cm. Calculate its power. ( 2.5 dioptres) Type E Problem based on the formula : v u f Solved examples :. A boy was standing at a distance of 20 cm from a convex lens. If the focal length of the lens is 80 cm. Find at what distance will the image of the boy be formed. Given : Object distance (u) 20 cm Focal length (f) 80 cm To find : Image distance (v)? 50 Formula : Solution : v v v v v u f v u f + f u 80 + ( 20) v 80 v cm. The image of the boy is formed at a distance of cm, on the same side of the lens. 2. An image was formed at a distance of 60 cm on the other side of lens. If the object distance is 0 cm. Find the focal length of lens. Given : Object distance (u) 0 cm Image distance (v) 60 cm To find : Focal length (f)? Formula : v u f Solution : v u f 60 ( 0) f f f SCHOOL SECTION

27 MT EDUCARE LTD. SCIENCE & TECHNOLOGY 7 60 f f 60 7 f 8.57 cm. The focal length of the lens is 8.57 cm. HOME WORK ASSIGNMENT - E. An object is placed at a distance of 5 cm from a convex lens. If the focal length of the lens is 60 cm. Find the image distance. ( 20 cm) Type F Problem based on the formula : M h h 2 v u Solved examples : *. An object 6 cm tall is placed in front of a convex lens at a distance of 8 cm. If the image is formed at a distance of 9 cm on the other side of lens. Find the height of the image. Given : Object Height (h ) 6 cm Object distance (u) 8 cm Image distance (v) 9 cm To find : Image height (h 2 )? Formula : M h2 h v u Solution : h2 h v u h 2 v h u h h h 2 3 The image height is 3 cm and it is inverted. *2. Fill in the blanks for convex lens. h h M h h M HOME WORK ASSIGNMENT - F. An object 0 cm tall is placed in front of a convex lens at a distance of 30 cm. If the image is formed at distance of 5 cm on the other side of lens. Find the height of the image. (.67 cm) SCHOOL SECTION 5

28 SCIENCE & TECHNOLOGY MT EDUCARE LTD. Type G Problem based on the formula : v u f M h h 2 v u Solved examples : *. A 3 cm tall object is placed perpendicular to principal axis of a convex lens of focal length 5 cm. The distance of the object from the axis is 8 cm. Find the nature, position and size of the image. Also find its magnification. Given : Height of the object (h ) 3 cm Focal length (f) 5 cm Object distance (u) 8 cm To find :. Image position (v) 2. Height of image (h 2 ) 3. Nature 52 Formula : v u f h2 M h v u Solution : v u f v u + f v v v 90 v 90 v 90 cm The image is formed at 90 cm on the other side of the lens. M h2 h v u h 2 v h u h h 2 5 cm The height of the image is 5 cm. The image is real and inverted. M v u M 90 8 M 5 cm The image is 5 time magnified. The negative sign shows that image is inverted. SCHOOL SECTION

29 MT EDUCARE LTD. SCIENCE & TECHNOLOGY *2. An object of height 5 cm is held 20 cm away from converging lens of focal length 0 cm. Find the position, nature and size of the image formed. Given : Height of the object (h ) 5 cm Object distance (u) 20 cm Focal length (f) 0 cm To find :. Image position (v) 2. Size of image (h 2 ) 3. Nature of image Formula : Solution : v u f h2 M h v u v u f v f + u v v v 20 v 20 v 20 cm The image is formed 20 cm on the other side of the lens. M h2 h v u h 2 V h U h h 2 5 cm The height of the image is 5 cm. It is real and inverted. *3. An object is placed at a distance of 0 cm from a convex lens of focal length 2 cm. Find the position and nature of the image. Given : Object distance (u) 0 cm Focal length (f) 2 cm To find :. Image position (v) 2. Nature of image Formula : Solution : v v v u f v u f + f u SCHOOL SECTION 53

30 SCIENCE & TECHNOLOGY MT EDUCARE LTD. v v 60 v 60 v 60 cm The image is formed at a distance of 60 cm on the same side of the lens, it is virtual and erect image. HOME WORK ASSIGNMENT - G. An object of height 0 cm is held 30 cm away from converging lens of focal length 5 cm. Find the position, nature and size of the image formed. ( 6 cm, 2 cm, real inverted and diminished) 2. A 5 cm tall object is placed perpendicular to principal axis of a convex lens of focal length 50 cm. The distance of the object from the axis is 0 cm. Find the nature, position and size of the image. Also find its magnification. ( 2.5 cm, 6.25 cm, virtual, erect and magnified) 54 SCHOOL SECTION

31 MT EDUCARE LTD. SCIENCE & TECHNOLOGY ACTIVITY BASED QUESTIONS ACTIVITY : 6. (For question refer to Text Book page No. 62) Image cannot be seen, because for a human eye to see an image light is required. ACTIVITY : 6.2 (For question refer to Text Book page No. 63) Fig (a) shows converging of rays. Fig (b) shows diverging of rays. ACTIVITY : 6.3 (For question refer to Text Book page No. 64). Image in spoon can be seen. 2. As the spoon is moved away from concave spoon, the image becomes smaller and smaller. 3. As the spoon is moved away from a convex spoon, the image becomes bigger and bigger. ACTIVITY : 6.4 (For question refer to Text Book page No. 68). The image is formed at the focus. 2. The distance between the screen and the lens is the focal length. ACTIVITY : 6.5 (For question refer to Text Book page No. 69). Similarity : (a) Human eye and camera both have the power of adjusting the focal length. (b) They both contain convex lens or converging lens. 2. Difference : Human eye is an organ, while camera is a device. ACTIVITY : 6.6 (For question refer to Text Book page No. 72). Negative power indicates that the person has myopia. 2. Positive power indicates that the person has hypermetropia. 3. Most of the students have negative powered spectacles. SCHOOL SECTION 55

32 SCIENCE & TECHNOLOGY MT EDUCARE LTD. HOTS QUESTIONS & ANSWERS. Pritesh has spectacles the power of the spectacles is -2.5D. (a) Name the eye defect. (b) What is wrong with his eyeball? (c) Which spectacles will he use? (a) The eye defect is Myopia or short - sightedness. (b) The eyeball is lengthened or lens is curved. (c) He will use concave lens of suitable focal length (-2.5D). 2. Ramesh was not able to see far off objects but after 60 years he could not even see the nearby objects. (a) State the earlier and new defect. (b) What will he do to rectify this defect? (a) The earlier defect is myopia and after 60 years Ramesh has presbyopia (old age hypermetropia). (b) Now he will use bifocal glass. 3. Swapnil who has a normal eye tries to read a book keeping it at a distance of 2 cm. Will he be able to read the book comfortably? (a) Swapnil will not be able to read comfortably as he is keeping the book very close to the eye. (2 cm) (b) The distance of distinct vision is about 25 cm. At this distance the book can be read clearly without any strain on the eye. 4. When lights are switched on suddenly in a dark room we get irritated? (a) In a dark room the pupil of the eye is dilated. (b) When light is switched on suddenly more light enters the eye. (c) As rods have to suddenly adapt to brightness they get agitated. (d) Hence we get irritated. 5. Tarun has two lenses of focal length 0 cm and 20 cm. Of these which is more powerful? (a) Power of a lens is inversely proportional to focal length (b) Hence the lens of focal length 0 cm is more powerful. 6. What kind of mirror will be used in the rear view mirror of vehicles? Why? (a) Convex mirror will be used in the rear view mirror. (b) The images will be virtual and diminished. (c) So the driver can clearly see the vehicles behind him. 7. Mohan cannot decide the lens given by the teacher as converging or diverging lens. How are you going to help him? (a) If the lens is thick at the centre and thin at the edges, it is a converging lens (convex lens). (b) If the lens is thin at the centre and thick at the edges, it is a diverging lens (concave lens). 8. A boy is standing at a distance of 40 cm from convex lens of focal length 20 cm. What is the image distance? (a) The boy is standing at 2F of a convex lens. (2F is twice focal length.) (b) The image will be at 2F 2. (c) Hence the image distance is 40 cm. 56 SCHOOL SECTION

33 S.S.C. Marks : 30 CHAPTER 6 : WONDERS OF LIGHT - I SCIENCE Duration : hr. Q.I [A] Choose the correct alternatives and rewrite the complete 4 sentences :. A lens does not produce any deviation of a ray of light passing through its.... (a) centre of curvature (b) optical centre (c) second focal point (d) axial point at a distance of 2F 2. The power of spectacle for myopic eye is... (a) positive (b) negative (c) zero (d) positive and negative 3. The power of a convex lens of focal length 25 cm is.... (a) 4 dioptre (b) 4 dioptre (c) /25 dioptre (d) /25 dioptre 4. If magnification is negative, the image is... with respect to the object. (a) inverted (b) enlarged (c) erect (d) diminished. Q.I [B] Match the columns : 4 Column A Column B. Plane mirror (a) Rear view mirror 2. Concave mirror (b) At laughing gallery 3. Convex mirror (c) At a dentist 4. Irregular curved mirror (d) At a hair dresser Q.I [C] State whether True or False : 2. If the image is erect, the height of the image is negative. 2. Convex lens is a diverging lens. Q.I [D] Find the odd man out : 2. Simple microscope, Compound microscope, Astronomical telescope, Terrestrial microscope. 2. Cornea, Iris, Pupil, Cerebrum.

34 Q.II Define the following : 2. Focus point of concave mirror. 2. Distance of distinct vision. Q.III Answer the following : (Any Three) 6. Distinguish between Convex lens and Concave lens. 2. Concave mirrors are used in solar devices. Why? 3. A dentist uses a concave mirror while examining teeth. Why? 4. You cannot enjoy watching a movie from a very short distance from the screen in a cinema hall. Why? Q.III Draw a neat and labelled diagram of the following : (Any One) 2. A ray diagram for object between pole and focus for a concave mirror. 2. Human eye. Q.IV Solve the following numericals : (Any Two) 4. An object 2 cm in height is placed at a distance of 6 cm from a concave mirror, if the focal length of the mirror is 9.6 cm. Find the image distance, nature and size of the image. 2. A 5 cm tall object is placed perpendicular to principal axis of a convex lens of focal length 50 cm. The distance of the object from the axis is 0 cm. Find the nature, position and size of the image. Also find its magnification. Q.V Answer the following questions in brief : (Any One) 4. Explain the sign conventions for reflection by spherical mirrors. 2. Explain myopia with its correction. Best Of Luck