1. Formation of Rainbow

2. What is Rainbow?
An arch of colors formed in the sky in certain circumstances, caused by the refraction and dispersion of the sun's light by rain or other water droplets in the atmosphere

3. How is Rainbow formed?
Rainbow is formed when White light separates into different colours on entering the raindrop due to dispersion, causing red light to be refracted less than blue light.
The formation of a rainbow involves reflection, refraction, dispersion and total internal reflection.   

5. BASIC 7 STEPS OF HOW RAINBOW IS FORMED:
Light from sun strikes raindrop.
Some of the light is reflected. (Reflection)
The rest of the light is refracted. (Refraction)
White light splits into component colours. (Dispersion)
Light is reflected at rear of raindrop. (Reflection)
Light is refracted again as it leaves raindrop. (Refraction)
Colours are further dispersed (Dispersion)

6. Videos on how rainbows are formed

7. Introduction to light dispersion
is the phenomenon where the colours of the white light separates when different wavelengths of the different spectrum of colours of the White light travel at different speeds and when they encounter a change to medium that is more dense or less dense, the speeds are effected by different amounts 

8. Reflection of light in water droplet
At the rear of the raindrop, the light hits the water-to-air interface. If the angle of incidence is greater than the critical angle, Total Internal Reflection will occur and thus causing rainbow to be seen

9. Refraction of light in water droplet
Refraction happens when the light changed speed again as it leaves. Here, the light is moving from a denser medium (water) to a less dense medium (air). As it does so, it speeds up and its path bends. In this case the path of the light rays bends away from the normal line

10. Maximum angle at which light should leave droplet to see the rainbow
Primary rainbow and is produced by one internal reflection; the secondary rainbow arises from two internal reflections and the rays exit the drop at an angle of 50 degrees° for the secondary rainbow and the rays exit the drop at an angle of 42 degrees° for the primary rainbow

11. Rainbow as a 3D cone
Every raindrop 42° of the antisolar line is reflecting red light back to our eyes. If we look at every raindrop that is 42° from this line, we would see a circle of raindrops with the centre on the line. Raindrops that contribute to your rainbow all lay on a cone with its apex at your eye. So when you are looking at a rainbow, you are looking at the collected light from many raindrops which, for a fleeting moment, collectively produce a 3D cone with its apex at your eye