Why do rainbows always appear as an arc or a circle?

In one line

The rule

A rainbow forms as an arc or circle because sunlight is refracted (bent), reflected (bounced), and dispersed (spread into colors) inside round raindrops, and we only see the colored light at a certain angle (about 42°) from the direction opposite the sun.

Let's understand it simply

Imagine you are playing cricket after rain. The sun comes out, and you see a rainbow. Why does it look like a curved bow and not a straight line? Think of each raindrop as a tiny glass ball. When sunlight enters a raindrop, it bends, bounces inside, and comes out at a special angle. Only the light that leaves the drop at this angle reaches your eyes as a rainbow. Since all the drops around you are round and the angle is always the same, the light forms a circle. But because the ground blocks the bottom half, you usually see only an arc.

How it works

1. Sunlight enters a round raindrop and bends (refracts) because water slows down light.
2. Inside the drop, the light bounces (reflects) off the back of the drop.
3. The light bends again as it leaves the drop.
4. The process spreads the light into different colors (dispersion), because each color bends a bit differently.
5. Only the light that comes out at about 42° from the direction opposite the sun reaches your eyes as a rainbow.
6. All the drops at this angle together form a circle, but the ground usually hides the lower part, so you see an arc.

The story behind it

The first scientific explanation of rainbows was given by René Descartes in 1637. He used glass balls to study how light bends and reflects, just like in a raindrop. Later, Isaac Newton showed that white sunlight is made of many colors, which spread out when bent. These discoveries helped us understand why rainbows are curved and colorful.

Where you see it in real life

• After rain, when the sun is low and shining behind you → you see a rainbow arc in the sky.
• Watering plants with a hose on a sunny day → a mini rainbow arc appears in the spray.
• Mist from waterfalls or fountains → rainbow arcs form in the mist.
• Looking from an airplane above the clouds → you may see a full circular rainbow.
• Oil spills on wet roads → colorful arcs, due to a similar bending and spreading of light.
• Soap bubbles → curved surfaces show rainbow-like colors by bending and spreading light.

Why it matters

• Shows how light behaves — bending, bouncing, and spreading.
• Helps in understanding optics (the science of light), used in cameras, glasses, and microscopes.
• Explains natural beauty and weather patterns.
• Used in art, photography, and festivals (like Holi colors).

Worked example

Let's see why the angle is always about 42°:

Given: Sunlight enters a round raindrop.
Step 1: Light bends (refracts) as it enters water.
Step 2: Light reflects inside the drop.
Step 3: Light bends again as it exits.
Step 4: Red light comes out at about 42° from the direction opposite the sun.
Step 5: All drops at this angle together form a circle around the 'anti-solar point' (the spot directly opposite the sun).

Common mistakes

• Thinking a rainbow is a flat thing in the sky — it is actually a circle of light.
• Believing everyone sees the same rainbow — each person sees their own, from their own angle.
• Thinking rainbows only happen after rain — any water droplets in the air can make one.
• Forgetting that the ground hides the bottom half, so we see only an arc from the ground.

Remember it

A rainbow is a circle of colors made by light bending, bouncing, and spreading in raindrops — we see only the arc because the ground blocks the rest.