Light enables organisms

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Marvin Pitts
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1 Chapter 15. Light 1. What does light do? Sunlight causes the day. Moonlight is a reflection of Sunlight. It shines to dispel the darkness of the night. Light enables organisms to see during day and night. Sunlight is free during day. At night electric light is used by humans. Light provides Brightness and glow Heat Energy Moonlight is cool in contrast to hot sunlight and electric light. 2. Light is one of the six forms of Energy. It is studied in Physics in higher classes. 6. Atomic Energy 1. Heat or Solar 5. Chemical Energy 2. Radiation or Light 4. Mechanical Energy 3. Electrical 10 x 10 Learning TM Page 1

2 3. Light, Reflection, and Shadow Chapter 15 Light Class 7 Science 2017 Light Reflection Shadow Light is a form of that travels in a straight line. It is colourless and give heat. It splits into 7 colours when passing through a prism. Two more colours are White and Black. The 10th colour is grey which is a mixture of white and black. A mirror can change the direction of light. This change is called Reflection. Sight is possible due to light. When all the light is reflected back, result is the white colour. When all light is absorbed, result is black colour. When an object that does not reflect light or allow the light to pass, comes in the path of light, a dark spot called a shadow, is formed. Shadows change in shape and size, during morning, afternoon and evening. They are longer at sunrise and sunset, and shorter at noon. 1. Real Image, is an image in the mirror or lens that can be formed on a screen. 2. Virtual Image, is an image in the mirror or lens that cannot be formed on a screen. 10 x 10 Learning TM Page 2

3 Mirrors and Lens Concave mirror Concave Lens Convex mirror Convex Lens Converge the light inwards Always forms an erect, virtual, smaller image Diverge the light outwards 1. Concave Lenses are thinner in the middle and thicker at the edges. Also called converging lenses. 1.1 They converge the rays of light or bring them to one point, to form an erect, virtual, and smaller image. 1.2 Used by dentists, in torches and front lights of vehicles. 2. Convex Lenses are thicker in middle and thinner at edges. 2.1Also called diverging lenses, because they spread light outwards. 2.2 Convex Lenses always form an inverted and a real image. Used in rear view mirrors in vehicles. 10 x 10 Learning TM Page 3

4 Plane mirror Virtual, same size, erect image. Image is at the same distance inside the mirror, but left side is seen as the right side in the image. Concave mirror curves inwards Convex mirror curves outwards Distant Image is real and inverted. Closer image is virtual and erect. Thus, depending on the distance the image can be smaller or larger, virtual or real Distant Image is erect virtual and smaller in size. Can reflect images from over a large area. Exercises 1. Fill in the blanks: 1.1 An image that cannot be obtained on a screen is called virtual image. 1.2 Image formed by a convex mirror is always virtual and smaller in size. 1.3 An image formed by a concave mirror is always of the same size as that of the object. 1.4 An image which can be obtained on a screen is called a real image. 1.5 An image formed by a concave mirror cannot be obtained on a screen. 2. Mark T if the statement is true and F if it is false: a) We can obtain an enlarged and erect image by a convex mirror. False. 10 x 10 Learning TM Page 4

5 b) A concave lens always forms a virtual image. False c) We can obtain a real, enlarged and inverted image by a concave mirror. True. d) A real image cannot be obtained on a screen. False. e) A concave mirror always forms a real image. True. 3. Match the items given in Column I with one or more items of Column II. Column I Column II a) A plane mirror The image is erect and of same size as the object. b) A convex mirror Can form image of objects spread over a large area. c) A convex lens Used as a magnifying glass d) A concave mirror Used by dentists to see enlarged image of teeth e) A concave lens The image is erect and smaller in size than the object. 4. State the characteristics of the image formed by a plane mirror. Answer. The characteristics of the image formed by a plane mirror is that the image is erect, of the same size as the object, and at the same distance as the object. 5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Answer. Such letters are : A, I, M, H, T, O, U, V W, X, Y. These letters appear in the same shape and size in the mirror because their left and right sides are exactly the same. Therefore they do not appear to be inverted in their image in the mirror. 6. What is a virtual image? Give one situation where a virtual image is formed. Answer. A virtual image can be seen but cannot be taken or reflected on to a screen. For example, the images of objects formed in the rear view mirror of a vehicle are virtual images. 7. State two differences between a convex and a concave lens. 10 x 10 Learning TM Page 5

6 Answer. A concave lens is the inner spherical surface. It is thinner in the middle and thicker at the edges. Images in a concave lens are erect, virtual and smaller than the object. A convex lens is the outer spherical surface. It is thicker in the middle area and thinner at the edges. It can form real and inverted images of distant objects. It enlarges the images of nearer objects. 8. Give one use each of a concave and a convex mirror. Answer. A concave mirror is used by dentists to see enlarged images of teeth. A convex mirror is used as rear view mirror in cars and other vehicles. 9. Which type of mirror can form a real image? Answer. A concave mirror can form a real image. 10. Which type of lens forms always a virtual image? Answer. A concave lens always forms a virtual image. Choose the correct option in questions A virtual image larger than the object can be produced by a 11.1 concave lens 11.2 concave mirror 11.3 convex mirror 11.4 plane mirror 12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be m m m m 13. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be m/s m/s m/s m/s 10 x 10 Learning TM Page 6

5 _. Light. II < Object. Lesson at a Glance

5 _. Light. II < Object. Lesson at a Glance 5 _ Light Lesson at a Glance Light is a form of electromagnetic radiation (radiation energy) which produces in us the sensation of vision. Light travels along straight line. A highly polished or shiny