Rainbows are natural optical phenomena that occur when sunlight interacts with water droplets in the atmosphere, creating a spectrum of colors.

Light Enters the Water Droplet: Refraction

Key idea: Sunlight refracts, or bends, as it enters a water droplet, with different colors bending at different angles.

The first step in rainbow formation is refraction. When sunlight, which appears white, enters a water droplet, it slows down and changes direction. This bending of light is called refraction. Different colors of light travel at slightly different speeds through water, so they bend at slightly different angles.

Think of it like a car hitting mud at an angle: one side of the car slows down before the other, causing the car to turn. Similarly, light bends as it enters the water droplet.

“Each tiny water droplet acts as a miniature prism, beginning the separation of white light into its vibrant components.”

Light Bounces Back: Reflection

Key idea: Light reflects off the back inner surface of the water droplet, sending the separated colors back towards the observer.

After entering the water droplet and refracting, the light travels to the back of the droplet. Here, it hits the inner surface and bounces back. This process is called reflection. For a primary rainbow, the light undergoes one internal reflection.

This reflection is what sends the light back towards your eyes, allowing you to see the rainbow. Without this internal bounce, the light would simply pass through the droplet and continue on its way.

Light Exits and Spreads Out: Second Refraction and Dispersion

Key idea: Light refracts again upon exiting the droplet, further dispersing colors and directing them to the observer's eye at specific angles.

Finally, as the light exits the water droplet and re enters the air, it refracts a second time. This second refraction further separates the colors and directs them towards the observer's eye. Because each color bends at a slightly different angle, they emerge from the droplet at different angles.

This separation of white light into its constituent colors is known as dispersion. Red light bends the least, and violet light bends the most. This is why red is always on the outside of a primary rainbow and violet is on the inside.

“The magic of dispersion ensures that each color finds its unique place in the rainbow's arc, from fiery red to deep violet.”

Seeing the Arc: The Angle of View

Key idea: The arc shape of a rainbow is formed by the collective effect of millions of water droplets, each sending specific colors to the observer's eye at precise angles.

The reason we see an arc of colors, rather than just a single point of light, is due to the specific angles at which these colors emerge from the water droplets. For a primary rainbow, red light is most intensely seen at an angle of about 42 degrees relative to the incoming sunlight, and violet light at about 40 degrees.

Millions of water droplets are involved, each refracting, reflecting, and dispersing light. We see the combined effect of all the droplets that are positioned at the correct angle relative to our eyes and the sun. The arc shape is because all the droplets that contribute to the rainbow lie on a cone, with your eye at the apex and the sun's rays forming the axis.