How are Rainbows Formed? | Don't Memorise

Rainbows, often seen on rainy days, can also appear on sunny days near waterfalls or fountains. Their formation requires two key elements: white light and tiny water droplets. During the rainy season, abundant atmospheric water droplets make rainbows more common.

A rainbow forms through the interplay of refraction and total internal reflection. When light enters a raindrop, it bends (refraction) due to the change in medium from air to water. Inside the drop, light reflects off its inner surface (total internal reflection), then refracts again as it exits back into air. This process separates white sunlight into its constituent colors, creating a spectrum visible as a rainbow.

Rainbows form during rainy seasons due to sunlight interacting with water droplets in the atmosphere. The colors appear in a specific order—red, orange, yellow, green, blue, indigo (often faint), and violet—from top to bottom. Light from each color travels directly to our eyes; for instance, red light at the upper part reaches us because it refracts within individual droplets after white sunlight strikes them at precise angles.

White sunlight, composed of seven primary colors (VIBGYOR), disperses when passing through a water droplet acting as a prism. This dispersion occurs at the entry point, splitting light into its constituent colors. The dispersed light then hits the rear side of the droplet and undergoes refraction again but in reverse direction due to total internal reflection—a phenomenon where light traveling from denser to rarer medium bends away from normal yet reflects back within if conditions are met.

As the angle of incidence increases, so does the angle of refraction. However, at a specific point known as the critical angle, something unique occurs: beyond this threshold, instead of refracting outwards into another medium, light undergoes total internal reflection within its original medium.

Rainbows form due to the refraction, reflection, and dispersion of light within water droplets. When sunlight enters a droplet at an angle, it refracts (bends), reflects internally off the back surface, then exits while bending again. This process separates white light into its constituent colors because each color bends by a different amount. The red portion is visible from certain angles as only red rays reach our eyes; other colors are bent too far away to be seen from that position.