Rainbows appear so large because of how each raindrop scatters light. Each raindrop acts like a tiny projector, scattering light across a circular disc in the sky. This individual scattering combines from millions of raindrops to create the large, arching rainbow we see.
How Raindrops Create the Rainbow's Size
The Scattering Effect
- Individual Raindrop Contribution: Each raindrop, being a sphere, acts as a tiny prism, bending and reflecting sunlight. This process doesn't just send light in one direction; it spreads the light over an entire circular disc in the sky.
- Varying Angle of Dispersion: The size of this disc is determined by the wavelength of the light. Red light, with its longer wavelength, is scattered over a wider angle compared to blue light, which has a shorter wavelength. This difference in scattering angles is why we see the distinct colours of a rainbow, with red always at the outer arc and blue on the inner one.
- Combined Effect: The huge size of a rainbow is not because each raindrop is enormous; instead, it's the cumulative effect of light from countless raindrops. Imagine thousands of individual spotlights each projecting a small disc of light. When combined, these small discs blend into a large, beautiful arc across the sky.
The Angle of Rainbow Formation
The specific angle at which rainbows form is about 42 degrees away from the direction opposite the sun for the primary rainbow, with the red light being on the outside and violet/blue light on the inside. For a double rainbow, the secondary arc appears due to two internal reflections inside a raindrop, with the colours in the reverse order and typically less intense. This angle is constant, which is why rainbows always appear to be the same size, regardless of the distance.
Brightness of the Bow
It's interesting that the inside of the rainbow often appears brighter than the sky outside it. This is because the light scattered by each raindrop creates this circular disc effect, concentrating the light within the arc. This is a crucial factor that makes the rainbow visible and large.
Key Takeaways
- Raindrops act as light projectors: Each drop scatters light over a disc in the sky.
- Angle of dispersion varies by colour: Red light is scattered at a wider angle, resulting in the rainbow's arc.
- Millions of raindrops work together: The combined scattering from many drops creates the large size of a rainbow.
- The light inside the rainbow is brighter: Because of the way the light is scattered, the sky inside the rainbow is brighter than the sky outside it.
| Feature | Explanation |
|---|---|
| Raindrop Shape | Spherical, acting like a tiny lens and prism. |
| Light Scattering | Each raindrop scatters light over a circular area. |
| Colour Separation | Different wavelengths of light (colours) are scattered at varying angles. |
| Cumulative Effect | The large size of the rainbow is the result of the light combined from millions of raindrops. |
