Our Activities

The rainbow is a familiar sight when the sun is shining and rain is falling. It can also be seen in the spray from sprinklers and car-washes and in the spray above waterfalls (see lower photograph). To see a rainbow, you must have the sun behind you and the rain or spray in front of you.

How does a rainbow form?

When sunlight shines on raindrops, most of it passes through the drops. If, however, the light shines at a certain angle (the critical angle), some of it is refracted internally and reflected back towards us. The light is refracted when it enters the raindrop and reflected from the far side of the dropb. The critical angle is different for different colours of light (as different colours have different wavelengths), so the various colours are spread out and form a spectrum of light, which we see as a rainbow. For red light, the angle is 42°. For violet light, it is 40°. This spreading out of light at different wavelengths is called dispersion. Because we see only one colour from each raindrop, a great many drops must be present for us to see a rainbow.

Sometimes, as in the picture above, two rainbows can be seen, a brighter primary rainbow and a fainter secondary rainbow. The latter occurs when sunlight is reflected twice inside each drop (see Figure 2). The additional internal reflection makes the secondary bow dimmer than the primary.

The colour on the outside of a primary rainbow is red and the colour on the inside violet. The opposite is true for secondary rainbows, which have red on the inside, violet on the outside.

The rainbow that you see is not the same one that a person standing near you sees. The reason is that only one ray of light is able to reach your eye from each drop. 'Your' rainbow moves with you when you move.

Demonstrations and Experiments

Make your own rainbow

• What you need
• Spherical glass flask
• Water
• Screen
• Torch

The demonstration

Fill the flask with water and place it in front of the screen. Shine the light from the torch through the water towards the screen. A faint rainbow-like image should appear on the screen. The image takes the form of a closed circle displaying the colours of the rainbow. Which colour is on the outer ring of the circle, red or violet? Can your students explain?

Studying internal refraction

• What you need
• Ray box or optical lamp and slit or narrow beam of white light
• Protractor
• Semi circular glass block
• Pencil and Paper

Procedure

Place the glass block on the paper and draw around it. Darken the room and shine the light as shown in the diagram (right). The incident beam should be 1 or 2 mm in width. Mark the incident and emergent beams with small crosses along their paths.

Change the angle of the incident ray and see what happens to the emergent ray

What happens to the emergent ray:-
a) at small angles;
b) at large angles?

At what angle is there a faint ray produced parallel to the flat side of the glass block? This is called the critical angle and it should be approximately 42°.