Basically, the light is dispersed through the droplet of water which acts as a prism, and is reflected into your eyes. You might ask, what is refraction? What is total internal reflection? How does refraction work in a water droplet? How do the colours: Red, Orange, Yellow, Green, Blue, Indigo, Violet appear? We will discuss these questions below.
Refraction is the bending of a light. When light passes through a different material of a different optical density, the light will bend at an angle. The speed of light would be slower in a more dense material and faster in a less dense material.
As the speed of light is reduced, the light will bend away from the normal, which is an imaginary line drawn perpendicular to the surface it hits. At the same time, the wavelength is shortened proportionately but the frequency remains the same.
After the light ray enters the denser medium and undergoes internal refraction, light is dispersed. The 7 different colours in the white light from the sun bend at a different angle because they have different wavelengths. We will go into further details below.
Lastly, total internal reflection occurs before it is refracted out of the denser medium. When the incident angle of light is more than the critical angle, then the light ray gets reflected back rather than being refracted. (The critical angle is the angle of incidence that provides an angle of refraction of 90-degrees from the normal, parallel to the surface.)
In the case of rainbow formation, the water droplet is more dense compared to the air around it, hence the speed of light slows down as it enters the water droplet and the light is refracted.
Due to the decrease in speed, the light is refracted and bends towards the normal.
The refracted ray, which can also be the incident ray when it hits the back of the droplet, which is called the point of incidence.
Then, the ray is reflected at the same angle where it hits the point of incidence.
This is called internal reflection.
Finally, the light is refracted away from the normal out of the water droplet into our eyes.
But how are the colours formed?
The colours of the rainbow: red, orange, yellow, green, blue, indigo, and violet make up the Sun’s white light.
Firstly, white light is made is made up of wave lengths of different colours of light which are the colours of the rainbow.
When white light is shown through a prism, these colours are arranged in the order as seen below based on their wavelengths with the different angles of deviation.
Red: lowest wavelengths, lowest frequency
Violet: shortest wavelengths, highest frequency
Violet: shortest wavelengths, highest frequency
The colours of a rainbow are produced by dispersion. Hence, when white light passes through a water droplet, the different colours get scattered according to their wavelengths and frequencies, showing a continuous band of colors (a spectrum) as it travels from the water droplet to the air.
As blue has a shorter wavelength compared to red, its angle of deviation is 40 degrees while the angle of deviation for red is 42 degrees. Between these two degrees, the concentration of light rays are the greatest, thus we are able to see the dispersed light as colours of the rainbow.
Also, since the red light is refracted at a steeper angle compared to the blue light, red is the outermost colour of the rainbow, with blue, indigo and violet being the innermost colours.
