(O Level Phy) How're rainbows formed?

How is the rainbow formed? You may liken the water droplet to the prism to explain the formation of the rainbow. That's close but only part of the picture. (If you like to know the nature of light, go to this link that tells the story of its discoveries.) 

The following diagram shows only two raindrops. Their size is exaggerated for the purpose of this explanation. The top raindrop is responsible for sending red light to your eye. The bottom one is responsible for sending violet light to you. Between these two are the many, many raindrops that send different colors in the spectrum to your eye. And that's only one set of raindrops. There are also many, many other sets of raindrops causing you to see the rainbow as a semicircle.

Millions of raindrops send different colours from different positions to your eyes... 

This video will give you a better picture of what I am saying. https://www.youtube.com/watch?v=uRmdZVvzMzQ

Some people thought that the coloured lights undergo total internal reflection at the back of the raindrop. But the angle of incidence inside the raindrop is smaller than the critical angle. Therefore, total internal reflection can't happen inside the raindrop.

Refraction and reflection, not total internal reflection, happen in the raindrops...

Try these questions before you check with the answers below.
1. (a) Does light bend towards or away from the normal in the rain drop? Why does this happen?
(b) Why does white light disperse into a spectrum when it passes through the water?
(c) The greater the refractive index of colour, the more it bends. Which colour in the spectrum has the greatest refractive index in water? Which has the smallest refractive index water?
2. What is total internal reflection? How does it occur?
3. According to the diagram that shows the formation of the rainbow, the Sun is:
A in front of the observer
B behind the observer
C above the observer
D below the observer
4. Why are the Sun's rays drawn parallel?

Answers: 1. (a) Light bends towards the normal in the raindrop because it passes from the optically less dense air to the optically denser water. (b)The different colours in white light have different refractive indices in the water and so bend differently when the white light passes from air into water. OR Different colours of light have different wavelengths and so travel in different speeds in water. That is why they bend differently in water. (c) Red has the smallest refractive index. Violet has the greatest refractive index. 
2. Total internal reflection is the reflection at a boundary within the denser medium without refraction into the less dense medium. It occurs when the angle of incidence at the boundary is greater than the critical angle. 
3. B behind the observer 
4. The Sun is very far away. We can approximate the rays from a far away light source to be parallel.