REFLECTION OF LIGHT GRADE 11-LESSON 2 PHENOMENA RELATED TO OPTICS 1.i. What is reflection of light?.. ii. What are the laws of reflection? a...... b.... iii. Consider the diagram at the right. Which one of the angles (A, B, C, or D) is the angle of incidence? Which one of the angles is the angle of reflection? iv... A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection?... Page 1
2.i. A 3 cm long candle is placed 4 cm in front of a plane mirror. Draw a ray diagram to obtain the image of it. ii. What are the features of an image formed by a plane mirror? 1 2 3.. 4. 5 iii. Give 5 uses of plane mirrors. CURVED MIRRORS Page 2
Images formed by a converging spherical mirror Characteristics of the Image a) Distant object Real Inverted Smaller than object At F b) Object beyond C Real Inverted Smaller Between C and F c) Object at C Real Inverted Same size as object At C d) Object between F and V Virtual Erect Larger than object Behind mirror e) Object at F Convex mirrors e) Object at F No image Reflected rays are parallel Characteristics of the image regardless of object position Virtual Erect Smaller than object Behind mirror between F Page 3
3.Construct optical diagrams to answer these questions 1. An object 1.4 cm tall is positioned 12 cm from a concave mirror. If the radius of curvature of the mirror is 8.0cm, determine the characteristics of the image (whether it is real or virtual, upright or inverted) and find: (i) the size of the image (ii) its distance from the mirror (iii) state its characteristics 2. The focal length of a concave mirror is 5.0 cm. If an object 1.2 cm high stands 7.5 cm from the mirror determine the size and position of the mirror and state its characteristics. 3. As Q2 but the object is positioned 3.0 cm from the mirror. 4. An object 1.4 cm high is placed 9.0 cm from a convex mirror. Given that the radius of curvature of the mirror is 12.0 cm, determine the characteristics of the image (whether it is real or virtual, upright or inverted) and find: (i) the size of the image (ii) its distance from the mirror (iii) state its characteristics LENSES Page 4
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4.Construct optical diagrams to answer these questions 1. A convex lens has a focal length 2.0 cm and an object is placed (a) 2.5 cm and (b) 1.5 cm from it. Calculate the image if the object is 1 cm high. For both positions produce a ray diagram 2. A concave lens has a focal length of 2 cm. An object is placed 6 cm from the lens. Find the position and size of the image if the object 2 cm high. 3. A convex lens has a focal length of 5 cm. An object 2 cm high is placed 7.5 cm in front of the lens. (i) Draw ray diagram to find the position, height and nature of the image. (ii) Using the lens formula calculate the position, height and nature of the image. (iii) Calculate the power of the lens. (iv) If a second lens, concave of focal length 100 cm, is placed in combination with the first lens calculate the height of the new image. Refraction of light Experiment 1 ² A glass block Three pins A white sheet of paper A ruler A pencil A mathematical table showing the sine values of angles Demy papers and pastel Page 6
Experiment 2 A board containing the following data A tall glass jar with water A movable stand with a fixed pin An iron nail A meter ruler Demy papers and pastel Experiment 3 Direct a white light beam to the prism and observe how the ray gets refracted through the prism A plane mirror A prism A convex lens A white cardboard to be used as a screen A lens stand Demy papers and pastel Experiment 4 Process Vary the angle of incidence of laser beam to the water surface, and observe way the refraction takes place A round bottomed flask with water A laser torch A stand A screen Joss sticks and box of matches Demy papers and pastels 5. a. What is refraction of light? B.What are the laws of refraction? a.. Page 7
b.... C.Draw ray diagrams for refraction of light from, i. air to glass ii. air to water iii.glass to air 6. a An angle of incidence is 30 and the refractive index of glass is 1.5.Find the angle of re fraction.. b.. What is total internal refraction of light? What are the conditions required for total refraction to take place?.. c.. What do you understand by spectrum? How is the spectrum obtained? What is the cause of dispersion?...... d..arrange the different colors in the spectrum in increasing order of wavelength? e. Define critical angle? How does it relate to refractive index? h. State Snell s law of refraction? i. A coin lies at the bottom of a fish tank under 30 cm of water. Calculate the apparent position of the coin. Refractive index of water j. Write 2 phenomena related to refraction of light.. Page 8
7..i. Calculate the refractive index of the following materials. Substance Speed of Light in Substance Refractive Index Polycarbonate Sapphire 169,500 km/s 189, 900 km/s Sapphire 169,500 km/s ii. When a light beam passes from a substance with a high refractive index to a substance with a low refractive index it turns towards/away from the normal.draw a diagram to represent this. iii. When a light beam passes from a substance with a low refractive index to a substance with a high refractive index it turns towards/away from the normal. (circle the correct answer) Draw a diagram to represent this. iv.. Fill in the table. Angle of Incidence (in air) 40 80 TOTAL INTERNAL REFLECTION Angle of refraction in water (n= 1.33 water) in Perspex (n= 1.5 Perspex) Applications of total internal reflection Mirage This phenomenon is most common in desert areas which experience hot climate. We might sometimes see water at a distance during a hot day. But, this is just an illusion. Sometimes you may see the reflection of some trees, etc.. in the "water". Why does this happen? The layers of atmosphere closer to the earth's surface are hotter whereas those away from the surface are denser. Heat makes the layer rarer. So, in other words, the layers of atmosphere closer to the surface of earth are rarer and becomes denser as you move away, to higher altitude. As shown in the above diagram, the ray of light travelling from the tree moves from the denser layer of air to the rarer subsequently. As a result, at Page 9
each interference, the ray keeps moving away from the normal. This goes on until the incident angle becomes greater than the critical angle, when the phenomenon of total internal reflection takes over, resulting in the ray of light to re-enter the same medium again. This ray is extended backward, at which point we actually see the tree, inverted - a virtual image! Optic Fibre: The optic fibre consists of an outer cladding and an inner core, the core being denser than the cladding. The ray of light incident on the core-cladding interface is made to fall at an angle greater than the critical angle. This leads to a "chain reaction" of total internal reflections. Rainbow White light from the Sun has to hit the raindrops at a certain angle before a rainbow is possible. It is best if the sun is fairly low in the sky such as dawn and late afternoon. When light from the sun hits a water droplet, some of the light is reflected. This light will obey the laws of reflection.the light that is not refracted crosses the air-water interface. When this happens it slows down because the water is more dense than the air. The reduction of speed cause the path of the light to bend - this is called refraction. In this case the path of the light rays bends toward the normal line. White light is made up of a spectrum of colors, each with its own wavelength. Different wavelengths travel at different speeds and when they encounter a change to medium that is denser or less dense, the speeds are effected by different amounts. Hence, the colors separate. This phenomenon is known as Dispersion. At the rear of the raindrop, the light hits the water-to-air interface. If the angle of incidence is greater than the critical angle, Total Internal Reflection will occur. A rainbow will only be seen if this happens, otherwise the light will Page 10
Dispersion 8.. A spectrum is formed by refraction of a light ray through a glass prism. a. What is the reason for this phenomenon?. b. What is the color that gets refracted most?.. c. What is the color that gets refracted least?.. D.If you wish to see white light again, what do you do?... 9. position of the object Position of the image Nature of the image Size of the image Application Between O and F 1 on the same side of the lens Erect and virtual Magnified Magnifying lens (simple microscope), eye piece of many instruments At 2F 1 At 2F 2 Inverted and real Same size Photocopying camera Between F and 2F 1 Beyond 2F 2 Inverted and real Magnified Projectors, objectives of microscope At F 1 At infinity Inverted and real Magnified Theatre spot lights Beyond 2F 1 Between F 2 and 2F 2 Inverted and real Diminished Photocopying (reduction camera) At infinity At F 2 Inverted and real Diminished Objective of a telescope Page 11
OPTICAL INSTRUMENTS Due to overall process taking place in an optical instrument the size of the original object changes. Convex lenses capable of forming small images as well as large images are used in many optical instruments. Concave lenses which form small virtual images of objects in front are used as safety devices fitted to doors of houses to view outside from inside the houses. 1. SIMPLE MICROSCOPE/ MAGNIFYING GLASS The simple microscope uses the vertical magnified image formed by the object placed between the optical centre of a convex lens and its focus. Perhaps the simplest optical instrument is the lens magnifier. Without optical aid, we cannot see things close up. The eye will simply not focus closer than about 0.25 m (unless you are nearsighted!). But an object places just inside the focal point of a converging lens will produce a large virtual image that can be viewed more easily. The object is closer than the observer s near point so it appears larger than when it is at the near point without the lens present. Features of the image.... Page 12
2. COMPOUND MICROSCOPE Compound microscope is used to obtain the magnified images of small objects near the observer. The standard optical microscope consists of two lenses (each can be a compound lens). By placing the object to be observed very close to the focal point of the first or objective lens, a larger real (but inverted) image will be produced. This real image is then observed with a second lens, the eyepiece, which acts as a magnifier to make the image even larger. The microscope uses 2 convex lenses, the objective and the eye piece. The magnification can be changed by using different eye pieces. Features of the image 1. Real 2. Magnified 3. Inverted Uses 1. To observe a blood smear 2. To observe a plant cell 3. To observe an animal cell 4. To observe cross section of a leaf Page 13
3.ASTRONOMICAL TELESCOPE /REFLECTING TELESCOPE Telescopes are used to obtain magnified images of distant objects appearing small. The inverted final image obtained in a telescope is a distinct disadvantage when observing the objects on the Earth An instrument that has largely contributed to the progress of science is the telescope. In 1608,a Dutch spectacle manufacturer, Hans Liperche has invented the world s first telescope.as this telescope mostly used to observe heavenly bodies, it is called the astronomical telescope. Features of the image... 4. BINOCULARS Prism binoculars, consisting of two refracting telescopes, one for each eye, has a right angled prism in each telescope to convert the inverted image to an upright one by total internal reflection. Prism binoculars, from which an upright image can be seen is an ideal instrument to observe objects on the Earth. They include a pair of special reflecting prisms in the light path to: 1. shorten the physical length of the tubes holding the objectives & eyepieces 2. narrow the distance between incoming light paths to match eye separation 3. invert the inverted image so that it is upright Features of the image..... Page 14
Uses-1. To observe animals in a wild life park Advantages of prism binocular over telescope. 1. The instrument is short and could be easily handled. 2. Provides erect image. 3. No lateral inversion. 4. A three dimensional view of the object is seen as both eyes are used. 5. THE SLIDE PROJECTOR Over head projectors and slide projectors can be used as optical instruments which form still pictures on a screen. Lenses of short focal lengths in over head projectors produce large real images of information in transparencies on screens. In slide projectors, large real images of figures on slides are formed on a screen by placing slides a little beyond focus of the convex lens in the projector The projector uses a convex lens to cast an image of a slide transparency onto a screen. The slide is illuminated by a beam of light, created using a condenser lens & a compact light bulb. The distance from the lens to the screen is adjusted to produce a clear enlarged image of the slide on the screen. Because the image is inverted, the slide must be placed in position upside down so the image is the correct way up. Features of the image...... 6. THE CAMERA Images are focused on a photographic film by the convex lens in a camera. When taking photographs the rays travel through the convex lens, gets incident on the photographic film and a chemical change occurs to print the image as a stationary picture. Page 15
The camera uses a convex lens to form a real image on a photographic film. Focusing the camera adjusts the distance from the lens to the film, so that the objects at different Distances can be focused on to the film Features of the image.... 7. OVERHEAD PROJECTOR This is a display system that is used to display images to an audience. Transparencies are placed on top of the lens for display. The light from the lamp travels through the transparency and into the mirror where it is shone onto a screen for display. The mirror allows both the presenter and the audience to see the image at the same time, while looking down at the transparency while writing, and the audience looking forward at the screen. This is a display system that is used to display images to an audience. Transparencies are placed on top of the lens for display. The light from the lamp travels through the transparency and into the mirror where it is shone onto a screen for display. The mirror allows both the presenter and the audience to see the image at the same time, while looking down at the transparency while writing, and the audience looking forward at the screen. Page 16
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