Question

What is a Rainbow?How is it formed?

Answer 1

A spectrum is the band of seven colors which a white light splits into in the visible region wavelength when it gets diffracted due to whatever reason. A rainbow is a sort of natural spectrum formed on the sky due to diffraction of white light (sun light) in the raindrops which are sometimes suspended in air. When a light ray enters a droplet, an internal reflection occurs and the light rays bounce back out of the raindrop (some refraction also occurs due to difference in optical index of air and liquid in the drop) thus diffracting and forming the rainbow which is a band of violet, indigo, blue, green, yellow, orange and red from outer to inner. They get separated due to different wavelengths of different colors of light. ( The total. internal reflection of the light Ray inside the drops result in rainbows always occurring opposite to direction of sun).

Answer 2

First of all, let me clarify the difference between a rainbow and a spectrum. A spectrum is formed when white light emerges out of transparent and gets spilit in into seven components. In this process, ONLY DISPERSION OCCURS. However , the rainbow is a combination of DISPERSION, REFRACTION AND REFLECTION OF LIGHT.
When sunlight enters the water droplets after rain, they REFRACT AND DISPERSE the incident sunlight , after which the droplet REFLECTS IT INTERNALLY inside and finally gets REFRACTED AGAIN.