7th Class Science Light NCERT Summary - Light and Shadows

Light and Shadows

LIGHT

• Light is needed to see things. We may shave eyesight, but we cannot see anything without light. We need a source of light to make objects visible to us.

SOURCE OF LIGHT

• An object that gives out light is called a source of light. These sources of light can be categorised in two ways:

(i) Natural sources of light: Sun, moon, stars, glow-worm are some examples of natural sources of light.

(ii) Artifical sources of light: Candle, oil lamp, gas lamp, torch, electric bulb are some example of artificial sources of light.

• Some light sources are brighter than the others. For example, a candle is brighter than a firefly.
• The modem internationally accepted unit of brightness of a source is lumen (1 watt is equal to about 700 lumen).
• Light from a source such as a candle or an electric bulb or the Sun spreads in all directions. It gets dimmer as v/e move away from it. The brightness on given surface is measured in units of "lumen per unit area".
• Sources of light can be categorised as hot and cold sources also. For example, an electric bulb is a hot source while a tubelight is a cold source of light. The reason for this difference is the method light is produced them. For example, the electric bulb has a wire inside it called the filament. Unless the filament is heated to a high temperature, it will not glow. The filament is heated by passing an electric current through it. A hot filament which glows is said to be incandescent. A tubelight does not use a filament but a different mechanism to glow. Hence, it is a cold source.
• The only objects in the sky which are natural sources of light are the Sun and the stars. The Moon is only a cold non-luminous mirror for the Sun's light. It is a reflector of the Sun's light.

HOW LIGHT TRAVELS

• Light travels in a straight line. To find out how light travels, let us perform an activity. Take a lit candle and place it on a table. Take a rubber tube and look through it at the flame. First, stretch the tube straight and look. The flame can be seen. Now bend the tube and look through. The flame is not visible. This implies that light always travels in a straight line.

SHADOWS

• Some materials allow light to pass through them completely, some allow only partially light to pass through them and some do not allow the passage of light all. Based on this, materials are categorised in three ways:

(i)   Transparent materials

(ii)   Translucent materials

(iii) Opaque materials

(i) Transparent materials: Those materials that allow light to pass through them are called transparent materials. For example, we can look at a lighted candle through a clear glass or some kinds of plastic sheets. These allow light to pass through them.

(ii) Translucent materials: Those materials that allow light to pass through them only partially are called translucent materials. For example, we can see some dim light when we look at the candle through a sheet of butter paper. This sheet of butter paper allows only partial passage of light through it.

• Opaque materials: Those materials that do not allow light to pass through them are called opaque materials. For example, if a notebook is placed between our eyes and a candle flame, we will not be able to see even the blur of the candle. It will stop the light of the candle flame from reaching our eyes.
• Shadows are produced when light is stopped by objects. An opaque object casts a dark shadow, a translucent object a weak one and a transparent object casts no shadow at all.
• Our shadow is very long in the morning when the Sun rises. Then its length keeps decreasing till noon. Our shadow is the shortest at noon. Its length again increases in the afternoon and becomes very long just before sunset.

MAIN CONCLUSION ABOUT SHADOW FORMATION

• The shadow of an object is formed in the direction opposite to the side of a light source.
• When the source of the light or the object moves, the shadow also shifts accordingly.
• The length of the shadow formed by sunlight changes with time. This is because the angle between the source, the object and the ground changes.

THE ECLIPSES

• We see shadows being cast on the ground every day. Similarly, the Earth, the Moon and the planets also cast their shadows in space. However, we cannot see these shadows unless they fall on some surface or object.
• Sometimes, on a full moon day, the Moon passes through the shadow of the Earth. When this happens, we cannot see the Moon's disc until it comes out of the Earth's shadow. A lunar eclipse occurs in this way.
• If the entire disc of the Moon passes through the shadow, we observe a total lunar eclipse. If only a part of it covered by the shadow, we call it a partial lunar eclipse.
• On the New Moon day, if the Sun, the Moon and the Earth are in a straight line, the shadow of the Moon can fall on the Earth. If we are in this shadow, we will not be able to see that part of the solar disc, which is covered by the Moon. That is what happens in a solar eclipse.
• The eclipses are simply a play of shadows. Knowing the movements of the Sun, the Earth and the Moon, we can predict the time and the date of an eclipse.
• We can watch the Moon and a lunar eclipse directly with the naked eye. However we must never watch the Sun without protecting our eyes. This is because direct rays of the Sun are very strong, even in a solar eclipse.