How Is a Rainbow Formed?

A rainbow is an optical and meteorological phenomenon that occurs when sunlight is refracted, reflected, and dispersed within tiny water droplets in the atmosphere, often appearing after a rain shower. This process creates a distinctive band of colors in the sky. Here's how a rainbow is formed:

• 1. Steps in the Formation of a Rainbow
• 2. Order of Colors
• 3. Position and Conditions to Observe a Rainbow
• 4. Double Rainbows
• 5. Why Is a Rainbow an Arc?
• Summary

1. Steps in the Formation of a Rainbow

White sunlight from the Sun: Sunlight, which appears white, is actually a combination of different colors, ranging from red, orange, yellow, green, blue, indigo, to violet.

Water droplets in the air: Raindrops or mist act like tiny prisms that interact with sunlight.

Refraction

• When sunlight enters a water droplet, it bends (refracts) because light changes speed as it moves from air to water. Each color in the light bends at a slightly different angle, causing the light to split into its constituent colors (dispersion).

Internal reflection

• Inside the droplet, light is reflected off its inner surface. This reflection directs the light back toward the observer.

Second refraction

• As the light exits the droplet, it bends again, separating the colors even more distinctly.

2. Order of Colors

Rainbows display colors in the following order from the outer edge to the inner edge: red, orange, yellow, green, blue, indigo, and violet.

• Red appears on the outer edge because it is refracted the least.
• Violet appears on the inner edge because it is refracted the most.

3. Position and Conditions to Observe a Rainbow

• Viewing angle: A rainbow is visible only when the Sun is behind the observer and is at a low angle (below 42° from the horizon).
• Rain or mist: Water droplets in the air are essential for refracting and dispersing the sunlight.
• Sunlight: Rainbows are seen during the day when sunlight interacts with the water droplets.

4. Double Rainbows

In some cases, a secondary rainbow may appear outside the primary rainbow. This occurs when sunlight undergoes two reflections inside the water droplets.

• The secondary rainbow is fainter and has reversed colors compared to the primary rainbow.

5. Why Is a Rainbow an Arc?

Rainbows are circular because light is dispersed in all directions inside water droplets. However, observers on the ground typically see only part of the circle, forming an arc.

From an airplane or a high vantage point, it is sometimes possible to see a full circular rainbow.

Summary

Rainbows result from the interaction between sunlight and water droplets in the atmosphere. This beautiful phenomenon not only delights the eyes but also demonstrates how light behaves in nature.