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Why Are Rainbows Refraction?

Published in Light Refraction 4 mins read

Rainbows are a spectacular natural phenomenon primarily caused by the refraction and internal reflection of sunlight within countless water droplets suspended in the atmosphere. The bending of light as it passes through these droplets is what allows us to see the vibrant spectrum of colors.

Understanding Refraction: The Core Principle

Refraction is the phenomenon where light changes direction as it passes from one medium to another. This occurs because light changes speed when moving between materials of different densities.

  • Change in Speed: When light, for instance, travels from air into a denser medium like water (or glass, as an example), it slows down.
  • Bending of Light: This change in speed causes the ray of light to bend, or change its path. This bending is precisely what is called refraction.

How Refraction Creates a Rainbow

Water droplets act like tiny prisms, demonstrating the principles of refraction and dispersion (the separation of light into its constituent colors).

  1. Light Enters the Droplet: When a ray of white sunlight enters a spherical raindrop, it first undergoes refraction as it moves from air into the water.
  2. Dispersion of Colors: Crucially, white sunlight is composed of various colors, each with a slightly different wavelength. Because of these different wavelengths, each color in a ray is bent at a different angle when it refracts. For example:
    • Red light bends the least.
    • Violet light bends the most.
      This differential bending is known as dispersion, and it causes the white light to spread out into its individual colors.
  3. Internal Reflection: After entering the droplet and dispersing, the light then travels to the back inner surface of the raindrop, where it undergoes total internal reflection. This sends the light back towards the front of the droplet.
  4. Light Exits the Droplet: Finally, the separated colors refract once more as they exit the water droplet and re-enter the air, further spreading out and becoming visible to our eyes as distinct bands of color.

This entire process, heavily reliant on light changing speed and bending (refraction) within the water droplets, is what allows the "seven colors to be seen," forming the beautiful arc we identify as a rainbow.

Key Factors for Rainbow Formation

For a rainbow to be visible, specific conditions related to light and water droplets must be met:

  • Sunlight and Rain: You need sunlight shining from behind you onto water droplets (rain, mist, or spray) in front of you.
  • Optimal Angle: Each color of light is reflected out of the raindrop at a slightly different angle. The primary rainbow is seen at an angle of approximately 42 degrees from the observer's line of sight to the sun.
  • Individual Droplets: Each raindrop contributes to the rainbow by refracting and reflecting light. The collective effect of millions of droplets creates the full arc.
Phenomenon Description Role in Rainbows
Refraction Bending of light as it passes from one medium to another (e.g., air to water), due to change in speed. Essential; separates white light into its constituent colors (dispersion).
Dispersion The separation of light into its component colors due to each color bending at a different angle upon refraction. Essential; creates the spectrum of colors (red, orange, yellow, green, blue, indigo, violet).
Reflection Bouncing of light off a surface. Internal reflection within the raindrop redirects the dispersed light back towards the observer.

Why Different Colors Are Seen

As mentioned, the critical aspect of refraction in rainbows is that different wavelengths (colors) of light bend at slightly different angles. This is why you see the colors in a specific order, typically from red on the outside to violet on the inside of the primary rainbow. Each color's unique refractive angle contributes to its position in the visible spectrum.