Why are the colours in a rainbow always in the same order?

In one line

The rule

When white sunlight enters a raindrop, it bends (refracts), splits into its colors (disperses), and each color bends by a different amount, so they always appear in the same order: red, orange, yellow, green, blue, indigo, violet.

There is no single formula for the rainbow, but the bending of light is described by Snell's Law: n₁ × sin(θ₁) = n₂ × sin(θ₂) where:

• n₁ = refractive index of air
• n₂ = refractive index of water
• θ₁ = angle of incoming light
• θ₂ = angle of bent light

Let's understand it simply

Imagine you are playing cricket and the ball rolls from the road onto wet mud. The ball slows down and changes direction. Light does something similar when it enters a raindrop from the air—it slows down and bends. But white sunlight is not just one color. It is made of many colors, like a box of crayons. Each color bends by a different amount when it enters and leaves the raindrop. Red bends the least, violet bends the most. So, when sunlight passes through millions of raindrops, the colors spread out and always line up in the same order.

How it works

1. Sunlight (white light) enters a raindrop in the sky.
2. The light bends (refracts) as it enters the water, splitting into its colors (dispersion).
3. Inside the drop, the light reflects off the back surface (like a mirror).
4. The light bends again as it leaves the drop, spreading the colors even more.
5. Each color comes out at a slightly different angle: red at the top, violet at the bottom.
6. Your eyes see these colors as a rainbow, always in the same order.

The story behind it

The first person to carefully study rainbows and explain their colors was Isaac Newton in the 17th century, in England. He used a glass prism to show that white light splits into many colors. Before Newton, people saw rainbows as magic or signs from gods. Newton's experiments with prisms helped scientists understand that light is made of many colors, and each bends differently.

Where you see it in real life

• After rain on a sunny day: Sunlight shines through raindrops → rainbow forms → colors always in the same order.
• Spraying water with a hose: Tiny drops act like raindrops → sunlight splits → mini-rainbow appears.
• Glass prism in school lab: White light passes through prism → splits into colors → same order as rainbow.
• CD or DVD surface: Light reflects off tiny grooves → colors spread out → order matches rainbow.
• Soap bubbles: Thin film bends light → colors appear in rainbow order.
• Oil on water: Oil film bends light → rainbow-like colors in same order.

Why it matters

• Science of light: Helps us understand how light works and how we see colors.
• Optics in technology: Used in making lenses, cameras, and glasses.
• Weather prediction: Seeing a rainbow tells you sunlight and rain are both present.
• Art and design: Knowing color order helps in painting, printing, and making displays.
• Exams: Questions on rainbows, refraction, and color order appear in science exams.

Worked example

Let's see why red is always on top and violet at the bottom:

Given: White sunlight enters a raindrop.
Step 1: Red light bends least (about 42° from the original direction).
Step 2: Violet light bends most (about 40° from the original direction).
Step 3: The light exits the drop.
Step 4: To your eyes, red appears higher in the sky, violet lower.
Step 5: So, the rainbow always shows red on top, violet at the bottom.

Common mistakes

• Thinking the order can change: The order is always the same because the bending depends on the color's wavelength.
• Believing there are only 7 colors: The rainbow is a smooth blend; we name 7 main colors, but there are many more in between.
• Forgetting about reflection: Both bending (refraction) and bouncing (reflection) inside the drop are needed for a rainbow.
• Assuming all rainbows look the same: The brightness and width can change, but the color order does not.

Remember it

Remember "VIBGYOR"—Violet, Indigo, Blue, Green, Yellow, Orange, Red—from bottom to top of the rainbow.