Rainbows are one of nature's most spectacular optical phenomena. They occur when sunlight encounters tiny water droplets suspended in the air, typically after rain or near waterfalls. The sun acts as a light source, sending white light toward these droplets, which then create the beautiful arc of colors we see in the sky.
The first step in rainbow formation is refraction. When white sunlight hits a water droplet, it enters the denser medium and slows down, causing it to bend. Crucially, different wavelengths of light bend by different amounts. Red light, with the longest wavelength, bends the least, while violet light, with the shortest wavelength, bends the most. This separation of white light into its component colors is called dispersion.
Once inside the droplet, the separated colors travel through the water until they reach the back surface. Here, they undergo total internal reflection, bouncing off the curved back wall of the droplet. The light then travels back toward the front of the droplet. As it exits, it refracts once more, further separating the colors and sending them out at specific angles toward the observer.
For a rainbow to be visible, the observer must be positioned correctly. The sun needs to be behind the observer, and water droplets must be in front. Each color of light exits the droplets at a specific angle relative to the incoming sunlight - red at about 42 degrees and violet at about 40 degrees. Since this angle is constant for each color, the observer sees an arc of color as they look at the many droplets reflecting light at these specific angles toward their eyes.
In summary, rainbow formation is a beautiful demonstration of optical physics. It requires sunlight as a white light source, countless water droplets suspended in the atmosphere, and an observer positioned with the sun behind them. The process involves refraction as light enters each droplet, dispersion that separates white light into colors, internal reflection at the back of the droplet, and a second refraction as light exits. This creates the magnificent arc of colors we call a rainbow - a perfect example of how simple physical principles can create extraordinary natural beauty.