Rainbows form through a combination of sunlight and water droplets in the atmosphere, involving the processes of refraction, reflection, and dispersion of light.
How Rainbows Form
- Sunlight enters a raindrop : When sunlight hits a spherical raindrop, it passes from air (a less dense medium) into water (a denser medium). This causes the light to slow down and bend, a process called refraction.
- Dispersion of light : Sunlight is white light composed of many colors with different wavelengths. As light refracts entering the droplet, the different wavelengths bend by different amounts. Shorter wavelengths (blue and violet) bend more than longer wavelengths (red), causing the white light to split into its component colors.
- Internal reflection : The light then reflects off the inside surface of the raindrop.
- Refraction again as light exits : The reflected light exits the droplet, refracting again as it moves from water back to air, further separating the colors.
- Observer's perspective : For a rainbow to be seen, the observer must have the sun behind them and water droplets in front. The light dispersed by many droplets reaches the observer’s eyes, creating the colorful arc of a rainbow. The angle between the incoming sunlight and the observer’s line of sight to the rainbow is typically less than 42 degrees for the primary rainbow.
- Color order and appearance : The colors appear in order from red on the outer edge to violet on the inner edge. The red light comes from droplets higher in the sky than the violet light, due to the difference in refraction angles.
Conditions for Seeing a Rainbow
- The sun must be low in the sky, usually less than 42° above the horizon.
- There must be water droplets (rain, mist, or fog) in front of the observer.
- The sun must be behind the observer.
This combination of light behavior and geometry creates the familiar multi- colored arc known as a rainbow
. In summary, rainbows are optical phenomena caused by sunlight refracting, dispersing, reflecting inside water droplets, and refracting again as it exits, with the observer positioned such that the sun is behind them and water droplets are in front
