Maharashtra Board Class 7 Science Chapter 17 Effects of Light PDF Download

17. Effects Of Light

When sunrays pass through a glass prism, what are the colours in the band of light seen on the other side of the prism?

You have learnt that light is composed of several colours. You must have also seen the dust particles in a beam of sunlight entering the house through a small window. We switch on the head lamps of a car when we drive through a thick fog. You might have seen the beams of those lamps. What do we really see, when we see a beam of light? We see tiny dust particles floating in the beam. That is why we are able to see the beam of light. We see a variety of shades of colour in the early morning and evening sky. In the photographs taken from space by satellites the earth appears to be bluish. What is the cause of all these effects?

Scattering Of Light

All the experiments using laser rays should be done under the supervision of the teacher.

Apparatus: Glass beaker, diode laser (laser pointer), water, milk or milk powder, spoon, dropper, etc.

Procedure: Take clear water in a glass beaker. Pass a beam of laser rays through it. Check whether the beam is seen in the water.

Now use the dropper to add a few drops of milk to the water and stir. The water will be seen to have become slightly turbid. Now, pass the laser beam into it again. A light beam will show the existence of light rays.

A light beam is not seen in plain water, but is clearly seen in slightly turbid water. The light rays hit the tiny particles of milk and get scattered. If these scattered rays enter our eyes, we perceive the light.

Teacher's Note

When you see a beam of light in a dark cinema hall, you see it because of dust particles in the air. These particles scatter the light, just like milk particles scatter light in water.

Exam Trick

Remember: Light needs something to bounce off to be seen. Without dust or milk particles, light beams are invisible. This is why laser beams are not seen in clean, pure water.

Points To Remember

Light travels in straight lines
Scattering happens when light hits small particles
We see light because particles scatter it into our eyes
Pure water does not scatter light
Turbid water with milk particles scatters light

Instead of using milk, mix salt, sugar and detergent powder in separate beakers of water and observe whether a laser beam is seen in them.

When the sun rises our surroundings appear illuminated. The entire sky appears bright. This happens because of the dust and other tiny particles in the air. This is the scattering of sunlight by the tiny particles of the various constituents of air. Had there been no atmosphere, the sky would have appeared dark during the day and of course, the sun would be directly seen. This has been verified by observations from the rockets and satellites which go out of the earth's atmosphere.

Apparatus: A table lamp with a 60 or 100 W milky bulb (LED will not do), thick black paper, sticking tape, a packing needle, 100/200 ml glass beaker, milk or milk powder, dropper, spoon, etc.

Procedure: Cover the mouth of the lampshade properly with black paper, using sticking tape. Prick a hole of 1 to 2 mm diameter in the center of the paper with the help of the packing needle. Take clear water in the beaker. Light the bulb and place the beaker in contact with the hole. Observe from the front and at an angle of 90°. Now add 2-3 drops of milk to the water and stir. Observe again.

A few more drops of milk may have to be added to make the water turbid. A blue tinge is seen when observed along the 90° angle. This is the scattered blue light. Because the blue light is scattered, a red-yellow light is seen from the front, and the hole appears reddish.

Important: This experiment should be done in a dark room and by small groups of students.

If a few more drops of milk are added, the reddish colour seen from the front becomes an intense red. However, if many more drops are added, the reddish colour is not seen. Why is this so?

Sunlight is scattered by the molecules of gases like nitrogen, oxygen in the atmosphere. The blue colour in the sunlight is scattered the most, and, therefore, the sky appears blue.

Sunlight reaches us through the layer of the atmosphere. At sunset, the light reaching us travels a greater distance through the atmosphere. Due to the greater distance, there is more scattering of the blue colour. As a result the red-yellow light reaches us directly and the sun appears red. Red light is scattered less than blue light.

Teacher's Note

During sunset in India, you can see the sun turn red because light travels through more dust and air. This is why sunsets look so beautiful and orange-red in cities.

Exam Trick

Remember: More milk drops = intense red. Less milk drops = pale red. This is like the sky at sunset - the thicker the atmosphere, the redder the sun looks.

Points To Remember

Blue light scatters more than red light
The sky appears blue because blue light scatters most
At sunset, light travels more distance through air
More scattering of blue light makes the sun appear red
Gas molecules in air scatter sunlight

What is a shadow? How is a shadow formed?

The shadow obtained from a point source and extended source.

Apparatus: Candle/torch, cardboard, screen, small ball, big ball, etc.

Take either a candle or torch as a light source. In front of it, set up a cardboard with a tiny hole (O) as shown in the figure. Now the light is seen to emerge from the hole on the cardboard. Such a light source is called point source. Place a screen vertically at a distance of one metre beyond the cardboard. Hang the big ball between the screen and the cardboard. Observe the shadow AB of the ball.

The rays OP and OQ starting from the point source just pass by the ball and fall on the points A and B on the screen, respectively. However, since no rays reach the screen between the points A and B, that part remains unlit. This is the dark shadow or the umbra.

What happens if the cardboard with the pinhole is removed? Now, we do not have a point source. This source is called an extended source. What is the nature of the shadow formed by an extended source?

Due to the extended source, we see two parts in the shadow of the ball on the screen – one faint and one dark. The faint part BC is called the penumbra and the dark part AD, is called the umbra.

Let us see what happens in the following activity when the ball is bigger than the extended source. Keeping the distance between the extended source and the ball constant, move the screen further and further away and observe the shadow. As the screen moves further, the umbra and the penumbra in the shadow of the ball are seen to become bigger and bigger.

Now replace the big ball with one that is smaller in size than the light source. Observe its shadow on the screen. We see the umbra and penumbra of the ball on it. Now, without moving the light source and the ball, move the screen further away from the ball and observe its shadow. As the screen moves further, the umbra becomes smaller and smaller and at a certain point it disappears.

Teacher's Note

When you stand in bright sunlight, you see your shadow on the ground. The shadow is sharp and dark because the sun is very far away and acts like a point source for us.

Exam Trick

Remember: Point source = dark sharp shadow (umbra only). Extended source = shadow with two parts - dark (umbra) and light (penumbra). Like a candle versus a bulb.

Points To Remember

A point source makes a sharp dark shadow only
An extended source makes two parts - umbra and penumbra
Umbra is the dark part of the shadow
Penumbra is the faint light part of the shadow
As screen moves away, shadow size changes

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