Introductory topics in Light

1. Introductory topics in Light Please send comments/feedback to sandiego@aidindia.org

2. Introduction to Light • We learn a lot about the world by looking around. How do we see things? We see an object when light from it enters our eyes. But what is light? Light is a form of energy. It can travel from place to place following a few simple rules. By understanding these rules we can understand the seven colours of the rainbow, why the sky is blue, or eclipses of the sun and the moon. In this chapter we are going to learn some of these beautiful and strange things that light does when travelling from one place to the other. We'll also learn about some instruments that use these simple rules of light to see faraway stars and tiny creatures that our eyes can't see.

3. Sources of Light • We can't see anything in a dark room, but when we turn on a light bulb or light a match we can see the things in the room. The glowing bulb or the burning match are sources of light. • A light bulb or a match is man made. Such a man made source of light is called an artificial source of light. Burning wood, coal, oil and cooking gas are some other examples of artificial source of light. The chemical compounds in the sparkle burn and produce coloured light. When the current flows through the thin filament of an electric bulb, it emits light. Light is emitted when current passes through the vapour in tube lights, mercury and sodium vapour lamps. • There are other sources of light which are not man made. The sun and other stars emit light too. They are called natural sources of light. • Objects such as books, tables, flowers, moon and planets scatter the light falling on them. We see them by the light scattered from them. We cannot see such objects in the dark. These are called non-luminous objects.

4. Propagation of light through media • If we hold a piece of glass in front of a burning candle we can see the candle but if we hold a piece of wood in front of it we can't see it. That means light from the candle can travel through glass and reach our eyes but the light cannot travel through the piece of wood. • Light can travel through media such as glass, water and air. They are called transparent objects. Light can also travel through vacuum. • Light cannot pass through objects such as wood, stone and metal. We call these opaque objects. • Objects like wax or oilpaper transmit a part of the light falling upon them. Such objects are called translucent objects.

5. Rectilinear propagation of light • Light travels along a straight line. The velocity of light in air or vacuum is 3,00,000 Km/s • We can do a simple experiment to verify that light travels in a straight line. Take two square hard boards of equal size and make pin holes at their centres. Fix the two hard boards vertically with two stands. Place one hard board in front of a burning candle. Peep through the pin hole and make sure that you can see the flame. Place the second hard board in between the candle and the first hard board. Now try to see the flame through the first pin hole. If you can't, move the second hard board till you can see the flame. Are the two pin holes and the flame in a straight line, now? • We see shadows everywhere. When we walk, our shadow walks with us. On a hot summer day we sit under the shadow of a tree. You may have noticed, in general shadow is formed when light falls on any opaque object. This happens because light travels in a straight line.

6. Activity 1 – Pin hole camera • Make a pinhole in the middle of one side of a hard board box. Cut off a square on the opposite side and paste an oil paper over it. Point the pin hole towards a distant object. Observe the inverted image formed on the oil paper. Why is the image inverted? • How does a pinhole let you see the images carried by light and why are these images upside-down and backward? The simplest way to understand pinhole images is to take a look at what happens to light rays reflecting from an object,let's say a tree that you are looking at with your pinhole viewer. Sunlight bounces off the leaves, the trunk, the ground beneath the tree. A pinhole lets through only a portion of these reflected light rays. Most of the light that reflects from the tree is blocked by the cardboard surrounding the pinhole. Only a few of the rays of light that bounce off a particular leaf are heading in just the right direction to shine through the pinhole of your viewer. Light rays from leaves at the top of the tree must slope down at a steeper angle to pass through the pinhole. These rays hit the bottom of the screen. Rays of light from the base of the tree trunk must slope up to pass through the pinhole. These upward moving rays will hit the top of the screen. Think about this: if the rays from the top of the tree hit the bottom of the screen, and the rays from the bottom of the tree hit the top of the screen, then seems like you should see an upside-down picture of the tree on your screen. (And you do!) The same logic applies to right and left so that the image is also reversed this way. The tree has more points than the top and bottom. The same rules apply to light rays reflecting from every point on the tree's leaves and trunk: only the light rays that are heading in just the right direction will pass through the pinhole. On the other side, these rays of light line up geometrically to make an upside-down and back

7. Activity –2 Playing with Shadows • Reflect the sunlight with a plane mirror and make it fall on a small hole in hard board sheet. Let the light beam coming out by the hole fall on a white screen. Place a small ball in the path of the light between the hole and the screen. Observe the dark shadow of the ball on the screen. Draw the outline of the shadow with a pencil. Observe what happens to the size of the image if the screen is moved a little away from the ball. Explain the reason for this. • Make the hole on the hard board larger and repeat the experiment. Observe that the shadow formed consists of a central dark region called umbra and a partially dark region called penumbra around it. Mark the shadows on the screen. What do you understand from this experiment? Solar eclipse is nothing but moon's shadow on earth

8. Colors of Light • You might have seen the rainbow on a rainy day either in the morning or evening on the side opposite to the sun. The seven colours violet, indigo, blue, green, yellow, orange and red are seen from the bottom of the rainbow. White light is dispersed into seven colours by the raindrops. This forms the rainbow. • Pass sunlight slanting through one side of an equilateral glass prism and observe the colours of light on the other side of the prism. List the colours in order. Isaac Newton observed the pattern of colours of white light and called it spectrum. • Why do the objects have different colours? Of the seven colours of white light a few colours are absorbed and certain other colours are reflected by the objects. The colours so reflected are seen by us as the colours of the objects. For example, when white light falls on a green leaf, green colour alone is reflected; and the rest of the colours are absorbed. That is why the leaf appears green

9. Optical Instruments • Human eye is an amazing device with which we can see a lot of things. We can see stars millions of miles away, we can pass a hair through a needle hole just by looking carefully. But there is a limit to how much details we can see at a distance, or how small an object is visible to us. • An ant has several thousand eyes. The moon has mountains and valleys and big crators on its surface. We can't see those with bare eyes. Our eyes can not see our face! Or a bicycle coming from the other corner of a street! Optical instruments are clever devices to help us see things that we don't see with bare eyes. For example, mirror is an optical instrument that we use every day.

10. Mirrors • Mirrors can be of different types: • A plane mirror is a plane glass whose one side is coated with mercury. • A convex mirror is a spherical glass whose inner surface is coated with mercury. The convex mirror is used as rear view mirror in vehicles. • A concave mirror is a spherical glass whose outer surface is coated with mercury. The concave mirror is used in the headlight of vehicles to reflect the light and render parallel beam. fig: A boy standing in front of a large concave mirror. A magnified virtual image is formed. When he moves back the image inverts!)

11. Our eyes • The lens inside our eye is soft and can change shape. That is necessary for focussing objects at various distances on the retina. When we are looking at a distant object the lens stretches and becomes thin. As an object moves closer and closer the lens becomes thicker and thicker to focus the object on retina. • If the lens can not become thick enough to focus nearby objects or the eyeball flattens, they appear to be out of focus. Only distant objects are seen clearly. This defect is known as long sightedness. To rectify this defect spectacles having convex lenses are used. • If, on the other hand, the shape of the eyeball is elongated or the lens can not become thin enough, we can see the nearby objects clearly; the distant objects cannot be seen clearly. This is defect is known as short sightedness. To rectify this defect spectacles having concave lenses are used

12. Other optical instruments • Microscopes are used to magnify very tiny objects. A microscope is essentially made up of two convex lenses. Telescopes are used to see distant objects clearly. A telescope is made up of convex lenses and possibly concave mirrors. Cameras, projectors, binoculars and periscopes are examples of optical instruments.

13. Basic ideas learnt • Light is a form of energy. • Natural sources of light emit light of their own. • Non luminous objects are seen by the light scattered from them. • Objects can be classified into transparent, opaque and translucent media. • Light travels in a straight line. • The velocity of light in air or vacuum is 3,00,000 Km/s. • The shadow of objects are formed because of the rectilinear propagation of light. • White colour is the composition of seven colours: violet, indigo, blue, green, yellow, orange and red. • Short-sighted persons use spectacles with concave lenses. • Long-sighted persons use spectacles with convex lenses. • Microscopes are used to magnify very tiny objects. • Telescopes are used to see distant objects