A rainbow appears curved because sunlight bends, reflects, and spreads inside tiny round raindrops before reaching our eyes. The key reason for the arc shape is simple: raindrops are spherical, and light exits them at a specific angle. When millions of raindrops send light toward your eyes at the same angle, you see a curved rainbow.
Sunlight looks white, but it contains seven colors—VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red). When sunlight enters a raindrop, it bends (refraction), reflects inside the drop, and then bends again as it exits. During this process, the light spreads into different colors (dispersion). Because each color exits at a slightly different angle, we see a colorful arc.
What Is a Rainbow?
A rainbow is an optical phenomenon created when sunlight interacts with water droplets in the atmosphere.
It requires three conditions:
- Sunlight
- Water droplets
- Observer positioned between Sun and rain
Rainbows always appear opposite the Sun in the sky.
Step-by-Step: How a Rainbow Forms
1. Refraction (Light Bends)
Sunlight enters a raindrop and slows down, bending as it moves from air into water.
2. Reflection (Light Bounces)
The light reflects off the inside surface of the drop.
3. Dispersion (Light Splits)
Different colors bend by slightly different amounts.
4. Refraction Again (Light Exits)
The light exits the drop and travels toward your eyes.
Each color leaves the drop at a specific angle—about 42° for red and 40° for violet.
Why Is the Rainbow Curved?
The curved shape happens because:
- Raindrops are round.
- Light exits each drop at a fixed angle.
- The collection of all these angles forms a circular cone.
- We see part of that cone as an arc.
If you were in an airplane or high mountain, you could see a full circular rainbow.
Why We Usually See Only a Half Circle
On the ground:
- The horizon blocks the lower half of the circle.
- Only the upper arc is visible.
From high altitudes:
- No obstruction.
- Full circle becomes visible.
Why Are the Colors Always in the Same Order?
The order is always:
Red
Orange
Yellow
Green
Blue
Indigo
Violet
Red appears on the outside because it exits at a larger angle. Violet exits at a slightly smaller angle, placing it inside.
Primary vs Secondary Rainbow
Primary Rainbow
- One internal reflection
- Brighter
- Red on outside
Secondary Rainbow
- Two internal reflections
- Fainter
- Colors reversed
Between them is a darker region called Alexander’s band.
Why Double Rainbows Happen
Double rainbows occur when light reflects twice inside raindrops. This second reflection reverses the color order.
Why Rainbows Appear After Rain
After rain:
- Many water droplets remain in air.
- Sunlight shines through them.
- Ideal scattering and reflection conditions form rainbows.
Why Rainbows Move When You Move
A rainbow is not a physical object in one location. It depends on:
- Observer’s position
- Angle of sunlight
- Location of droplets
When you move, the angles change. That’s why the rainbow seems to follow you.
Can Two People See the Same Rainbow?
Not exactly.
Each person sees light coming from different droplets at the correct angle. So technically, each observer sees their own rainbow.
Rainbow Formation Table
| Process | What Happens | Result |
|---|---|---|
| Refraction | Light bends entering drop | Slowing of light |
| Reflection | Light bounces inside | Direction change |
| Dispersion | Colors separate | Spectrum forms |
| Exit Refraction | Light bends leaving | Arc visible |
Simple Experiment at Home
Materials:
- Water hose
- Sunny day
Steps:
- Stand with Sun behind you.
- Spray water mist in front.
- Observe rainbow arc.
You’ve created your own rainbow.
Why Rainbows Are Not Always Visible
Rainbows require:
- Sunlight at low angle.
- Water droplets in front.
- Clear view.
If any condition is missing, no rainbow forms.
Common Myths About Rainbows
- You can reach the end of a rainbow – False.
- A pot of gold exists there – Folklore.
- Rainbows are solid objects – No, they are light effects.
- Rainbows are flat – Actually circular.
FAQs
1. Why is a rainbow curved and not straight?
Because raindrops are spherical and light exits them at a specific angle. The combined angles create a circular shape.
2. Why do rainbows appear after rain?
After rain, many droplets remain in the air. Sunlight refracts, reflects, and disperses inside them.
3. Why do we see only half a rainbow?
The ground blocks the lower half of the circle. From airplanes, full circles are visible.
4. What causes double rainbows?
Double rainbows occur when light reflects twice inside droplets, creating a fainter second arc.
5. Why are rainbow colors always in the same order?
Each color exits the drop at a different angle. This fixed geometry creates a consistent order.
6. Can we see a rainbow at night?
Yes, rare moonbows form when moonlight reflects in droplets.
7. Why does the rainbow disappear quickly?
Changes in sunlight angle or droplet movement break the correct viewing angle.
8. Why is red on the outside?
Red light exits at a slightly larger angle than violet.
9. Are rainbows full circles?
Yes, but we usually see only half due to horizon obstruction.
10. Why do rainbows look brighter sometimes?
Brightness depends on droplet size, sunlight intensity, and air clarity.
