What is Refraction and How It Starts the Rainbow Process
Refraction occurs when light passes from one medium to another with a different density, causing the light to bend due to a change in its speed. In rainbows, sunlight enters spherical raindrops in the atmosphere. As white sunlight—composed of all visible wavelengths—refracts at the air-water boundary, it bends, separating the colors because shorter wavelengths (violet) bend more than longer ones (red). This initial refraction disperses the light into its spectrum.
The Role of Internal Reflection and Further Refraction
Inside the raindrop, the dispersed light travels and reflects off the inner surface of the drop, undergoing total internal reflection. This keeps the light trapped briefly before it exits the drop. Upon exiting, the light refracts again at the water-air interface, bending further and enhancing the color separation. For a primary rainbow, this process results in light emerging at specific angles, with red light at about 42 degrees and violet at 40 degrees from the observer's line of sight.
A Practical Example: Observing a Rainbow After Rain
Imagine a rainy afternoon with the sun breaking through clouds. Sunlight hits suspended raindrops acting as tiny prisms. A ray of sunlight enters a drop, refracts to spread colors, reflects internally once, and refracts out, directing red light towards your eyes while violet bends slightly more. You see the rainbow arc because drops at different positions send different colors at the optimal angle, forming the familiar banded spectrum from red to violet.
Why Rainbows Matter: Applications in Science and Nature
Understanding refraction in rainbows reveals principles used in optics, such as prism spectrometers for analyzing light composition in astronomy. It also explains natural phenomena like mirages and highlights atmospheric science's role in weather patterns. This knowledge aids in fields like photography (using filters to mimic effects) and education, debunking myths that rainbows are 'pot of gold' illusions rather than physics in action.