MAK LABS — Std 7 General Science Chapter 17 Effects of Light

1Fill in the blanks.

(a) When the beams from the headlights of a car fall on an object in the night, the shadows called umbra and penumbra can be seen.

(b) During a lunar eclipse the shadow of the earth falls on the moon.

(c) During a solar eclipse the shadow of the moon falls on the earth.

(d) Various shades of colour are seen in the sky at sunrise and sunset due to scattering of light.

2Give reasons.

(a) Space beyond the earth’s atmosphere appears dark.

The sky looks bright because the particles in the air scatter sunlight. Beyond the atmosphere, there is no air. So there are no particles to scatter light. Hence space appears dark.

(b) We are able to read while sitting in the shade.

Direct sunlight does not reach the shade. But sunlight is scattered by particles in the air. It is also reflected by walls and objects around. This light reaches the shade. So we can read there.

(c) We should not observe the solar eclipse with naked eyes.

The sun is very bright even during an eclipse. Its harmful rays can damage the eyes. We may even lose our eyesight. So a solar eclipse should be seen only with special goggles.

3Give some example of scattering of light that we come across in day-to-day life.

The sky looks blue during the day.
The sky looks red and orange at sunrise and sunset.
Dust particles are seen in a beam of sunlight entering a room.
The beams of car headlights are seen in fog.
The beam of a torch is seen in a dusty or smoky room.

4Why is the shadow of a bird flying high not seen on the earth?

The sun is an extended source of light. It is much bigger than a bird. A bird flying high is very far from the ground. So its umbra does not reach the earth. Only a very faint penumbra reaches the ground. It cannot be seen. So the shadow of a bird flying high is not seen.

5Why is a penumbra not obtained from a point source?

Shadow of a ball formed by (a) a point source and (b) an extended source.

A point source gives light from only one point. The rays from it either reach the screen or are fully blocked by the object. So only a dark shadow (umbra) is formed. A penumbra is formed only when light comes from many points, as in an extended source. So no penumbra is formed from a point source.

6Answer the following questions in your own words.

(a) What is meant by scattering of light?

When light falls on tiny particles of dust, smoke or gases in the air, it spreads in all directions. This is called scattering of light. Due to scattering, the sky looks blue.

(b) Does the shadow really vanish in the zero shadow condition?

No. On a zero shadow day, the sun is exactly overhead at noon. So the shadow falls just below the object. It is hidden under the object. So it looks as if the shadow has vanished.

(c) Will the laser beam be seen if it passes through a glass box which contains a lighted incense stick?

Yes. The incense stick fills the box with smoke. The smoke particles scatter the laser light. So the path of the beam becomes visible.

7Discuss and write.

(a) Write a science based paragraph on ‘What if the sun did not rise?’

If the sun did not rise, there would be darkness everywhere. The sun gives us heat. Without it, the earth would become freezing cold. Rivers and seas would freeze. Plants could not make food without sunlight. So plants would die. Animals and human beings depend on plants for food. So they would also die. There would be no rain and no wind. Thus, life on earth would not be possible without the sun.

(b) What efforts will you make to remove the misconceptions about eclipses?

I will tell people that an eclipse is a natural event.
I will show how an eclipse occurs using a torch and balls.
I will tell them that food does not get spoiled during an eclipse.
I will arrange safe viewing of an eclipse with special goggles.
I will put up posters with correct scientific information.

(c) Various eclipses and the conditions during that period.

Solar eclipse:

The moon comes between the sun and the earth.
The shadow of the moon falls on the earth.
It occurs only on a new moon day.
It can be total or partial.
It becomes dark during the day. It must not be seen with naked eyes.

Lunar eclipse:

The earth comes between the sun and the moon.
The shadow of the earth falls on the moon.
It occurs only on a full moon night.
It can be total or partial.
The moon looks dark or reddish. It can be seen with naked eyes.

8Explain the difference:

(a) Point sources and extended sources of light.

Point source	Extended source
Light comes from a single point.	Light comes from many points.
Only umbra is formed.	Both umbra and penumbra are formed.
Example: light from a pinhole.	Example: the sun, a tubelight.

(b) Umbra and penumbra.

Umbra	Penumbra
It is the dark part of a shadow.	It is the faint part of a shadow.
No light reaches this part.	Some light reaches this part.
It is formed by point and extended sources.	It is formed only by an extended source.

★Project: Obtain information about the special goggles used to watch a solar eclipse.

Solar eclipse goggles have special dark filters.
They allow only a very small amount of sunlight to pass.
They block harmful ultraviolet and infrared rays.
Ordinary sunglasses and coloured glass are not safe.
Goggles should not be scratched or torn.
A safe method is to see the image of the sun through a pinhole on a white paper.

17. Effects of Light

Exercises