ICSE Class 7 Physics Chapter 05 Light and Shadows

Light and Shadows

We interact with our surroundings through the five senses of touch, hearing, sight, taste and smell. Among these, sight is probably the most useful. We can see the world around us because something, which we call light, exists in nature. When the light from an object enters our eyes, we see the object. You will learn about the exact nature of light in higher classes. Here, we will study only how light is produced and how it behaves.

Sources of Light

Anything that emits (gives out) light is called luminous, from the Latin word lumen for light. If it emits light on its own, without receiving any light from outside, it is called a source of light. During the day, the sun is our principal source of light. The flame of a candle, the filament of an electric bulb and a fluorescent tubelight are some other sources of light. All luminous bodies, however, are not sources of light. For example, the moon is a luminous body, but it does not emit light on its own. It reflects some of the light which reaches it from the sun. Thus, the moon is not a source of light.

Sources of light may be divided into three categories - high-temperature sources, fluorescent sources and bioluminous sources.

High-Temperature Sources

When any material is at a high temperature, it emits light. The material may be a gas, liquid or solid.

Fig. 5.1 Light is emitted by (a) the hot gases of stars, (b) molten iron and (c) the glowing coil of a heater.

In the chapter on heat we have discussed how a flame is produced by burning gases. We could also say that a flame is the light emitted by gases at a high temperature. The gases are raised to that temperature by the energy released in the chemical reaction of burning. However, burning, or combustion, is not the only reaction that can raise the temperature of gases to such a degree that they emit light. The sun and the stars, for example, are composed of very hot gases that emit enormous amounts of light, but the gases are not heated by combustion. The energy that raises them to very high temperatures comes from nuclear reactions.

The lava (molten rock) flowing out of a volcano glows a fiery red. So does molten iron in iron and steel mills. These are examples of hot liquids emitting light.

The filament of an electric bulb emits light because it is heated to a very high temperature by the electric current flowing through it. An electric bulb is also called an incandescent lamp. 'Incandesce' means to glow with heat. Other solids also emit light when they are very hot. For example, iron glows red when it is heated by a blacksmith. The coil of a heater also glows red when it is hot.

Fluorescent Sources

Tubelights, CFLs (compact fluorescent lamps), large vapour lamps used for lighting in public places, and halogen lamps (halogens: a group of similar elements) used in automobile headlights are called fluorescent sources. They use different processes through which electrical energy is converted to light without producing too much heat.

Fig. 5.2 (a) Incandescent lamp, (b) halogen lamp, CFL and tubelight

Bioluminous Sources

Some living organisms are able to produce small amounts of light. The process by which they do so is called bioluminescence, combining the Greek bios, which means living, with lumen. In this process, chemical energy changes directly to light, almost without producing any heat. That is why bioluminescence is also called 'cold light'. Deep in the oceans, where sunlight does not reach, a large number of small creatures use bioluminescence for many of their activities. Fireflies and some bacteria are also capable of bioluminescence.

How Light Travels

Unlike sound, light does not need a material medium to travel. It can travel through vacuum, the way thermal radiation does. That is how it reaches us from the sun.

Transparent, Translucent and Opaque Substances

Everyday experience will tell you that light can pass or travel through some mediums (materials) and not through others. Light can pass through clear glass, water and colourless polythene sheets, for example, but it cannot pass through wood, bricks or metals. Materials that light can pass through are called transparent. We can see through such materials because light from objects on the other side can pass through them and enter our eyes. Materials that light cannot pass through are called opaque. We cannot see through opaque materials because they block the light from objects on the other side.

There are some materials through which light can pass only partially. Such materials, for example, tracing paper and ground glass, are called translucent. We can see things dimly through such materials.

Rectilinear Propagation of Light

On a misty night, you may have seen the beams of light from the headlamps of a car. Or you may have had fun watching the powerful beams of light rotated in the sky by circus companies or the organisers of a fair. You may even have seen beams of sunlight making their way through the clouds or through the leaves of trees. Perhaps you have noticed that all such beams of light are always straight. Light always travels in straight lines. This is called the rectilinear propagation of light.

Fig. 5.3 Light travels in straight lines.

Look at a candle flame or an electric lamp through a drinking straw. Then bend the straw and try to look at the same object. You will not be able to see it because light will not be able to pass through the bent straw. You can try other variations, like looking at an object through a bent rubber tube. You will always come to the same conclusion - that light travels in straight lines.

Pinhole camera

This simple camera works because light travels in straight lines. Make one yourself to see how it works.

Take a toothpaste box. Use black chart paper to make a sleeve into which the box can slide. Fold in the flaps of the box and staple them to make a tube. Cut the tube into two unequal parts. Stick black chart paper and translucent sticky tape (or tracing paper) across the two parts, as shown in Figure 5.5. The sticky tape acts as the screen. Make a hole in the chart paper with a drawing pin. Stick the shorter tube to the sleeve. Then slide the other tube into the sleeve with its open end to the outside. Point the pinhole at bright objects, such as trees outside your window during the day, or an electric lamp at night. You will see inverted images of the objects on the screen.

Fig. 5.5

Look at Figure 5.6. You will see that rays of light coming from the top and bottom of an object cross the hole and continue to move in straight lines. Thus, rays from the top of the object fall at the bottom of the screen and those from the bottom of the object fall at the top of the screen. That is how the image formed is inverted. You may try the following variations.

1. Point the camera at some object, and change the distance between the pinhole and the screen by sliding the movable part of the camera in or out. You will find that the size and brightness of the image changes with the distance between the pinhole and the screen.

2. You can check that the images of distant objects are smaller than those of objects nearby.

3. If you make the hole larger, you will find that the image becomes brighter but somewhat hazy.

Fig. 5.6 The size of the image depends on the length of the camera.

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