Rainbow — Definition

Definition

Imagine a sunny day, just after or during a light rain shower. If you stand with your back to the sun and look towards the rain, you might see a magnificent arc of colors stretching across the sky – that's a rainbow! But how does this beautiful spectacle form? It's all thanks to a fascinating interplay of light and tiny water droplets suspended in the air.

At its heart, a rainbow is a natural demonstration of how white sunlight, which is actually a mixture of all colors, can be split into its individual components. This splitting of light is called 'dispersion'.

When sunlight enters a spherical raindrop, it first undergoes 'refraction', meaning it bends as it passes from air into water. Because different colors of light (like red, orange, yellow, green, blue, indigo, and violet) travel at slightly different speeds in water, they bend by slightly different amounts.

Violet light bends the most, and red light bends the least. This is the first step in separating the colors.

Once inside the raindrop, this dispersed light travels to the back inner surface of the droplet. Here, most of the light undergoes 'total internal reflection' (TIR). Instead of passing out of the droplet, it bounces off the back surface, much like a ball bouncing off a wall, and heads back towards the front of the droplet. This reflection is crucial for sending the light back towards your eyes.

Finally, as the light reaches the front surface of the raindrop again, it undergoes another 'refraction' as it exits the water and re-enters the air. This second bending further separates the colors and directs them towards your eyes. Because each color emerges at a slightly different angle, and because millions of raindrops are involved, you perceive a continuous spectrum of colors arranged in an arc.

There are two main types of rainbows you might see: the primary rainbow and the secondary rainbow. The primary rainbow is the brighter and more common one, with red on the outside (top) and violet on the inside (bottom).

It forms due to one total internal reflection inside each raindrop. The secondary rainbow, which is fainter and appears outside the primary one, has its colors reversed (violet on top, red on bottom) and forms due to two total internal reflections inside each raindrop.

Both are stunning examples of light's behavior in nature.