Why does the sky change colour at sunrise and sunset?

In one line

The rule

The colour of the sky is due to the scattering of sunlight by the atmosphere, and at sunrise and sunset, the light travels a longer path through the air, causing more scattering of shorter wavelengths and letting longer wavelengths (like red and orange) reach our eyes.

Let's understand it simply

Imagine you are looking at a cricket ball under a white tube light. The ball looks red because it reflects red light to your eyes. Now, think of sunlight as a mix of all colours, like a box of crayons. When the sun is high, the light comes straight down, and the sky looks blue because blue light gets scattered the most by the tiny air particles. But at sunrise and sunset, the sun is low, and its light has to travel sideways through more air. The blue and green colours get scattered away before reaching you, so you mostly see the reds and oranges left behind.

It’s like when you shine a torch through a thick glass of lassi—the light looks different at the edge than at the top, because it travels through more of the drink.

How it works

1. Sunlight is made of many colours (like a rainbow).
2. When the sun is overhead, sunlight travels a short path through the air.
3. Tiny particles in the air scatter blue light more than red light (this is called Rayleigh scattering).
4. At sunrise and sunset, the sun is near the horizon, so sunlight travels a much longer path through the atmosphere.
5. Most of the blue and green light gets scattered out of your line of sight.
6. The remaining light is mostly red, orange, and yellow, which reach your eyes and make the sky look colourful.

The story behind it

The explanation for the blue sky and red sunsets was first described by Lord Rayleigh (John William Strutt) in the 19th century. He was a British scientist who wanted to understand why the sky is blue. He found that small particles in the air scatter short wavelength light (like blue) more than long wavelength light (like red). His work is now called Rayleigh scattering.

Where you see it in real life

• Evening walks: The sky turns orange and red as the sun sets, especially after a hot day.
• Sunrise at the beach: The sky glows pink and purple before the sun rises above the water.
• From an airplane: You can see a band of red or gold near the horizon during take-off or landing.
• During Diwali: Smoke in the air can make sunsets look even redder, because more particles scatter the light.
• After rain: The sky may look extra clear and blue, because rain washes away dust, changing the scattering.
• On a dusty road: The sky near the horizon can look yellowish or reddish, especially at sunset, due to extra dust scattering the light.

Why it matters

• Weather prediction: Red skies at sunrise or sunset can signal changes in weather, which helps farmers and fishermen.
• Photography and art: Understanding sky colours helps artists and photographers capture beautiful scenes.
• Air pollution studies: Unusual sky colours can show if there is more dust or smoke in the air.
• Astronomy: Knowing about scattering helps astronomers plan when to observe stars (best when the sky is least scattered).

Worked example

Let’s say sunlight travels through 10 km of air at noon, but 40 km at sunset.

Given: Path at noon = 10 km, Path at sunset = 40 km
Step 1: More air = more scattering
Step 2: Blue light scatters away first
Step 3: Only red/orange light left to reach your eyes
Step 4: Sky looks red/orange at sunset

Common mistakes

• Thinking the sun itself changes colour: The sun stays the same; it’s the air that changes what you see.
• Believing pollution always causes red sunsets: Pollution can make sunsets redder, but even clean air scatters light to make red skies.
• Forgetting about the path length: The longer path at sunrise/sunset is the main reason for the colour change, not just the time of day.
• Mixing up scattering and reflection: Scattering is spreading of light by tiny particles, not bouncing off surfaces.

Remember it

At sunrise and sunset, sunlight travels through more air, so blue light scatters away and red/orange light colours the sky.