Why does the sky look red or orange at sunrise and sunset?

In one line

The rule

The reddish or orange colour of the sky at sunrise and sunset is due to the scattering of sunlight by the atmosphere, where shorter wavelengths (blue and violet) are scattered away, and longer wavelengths (red and orange) pass through to our eyes.

Let's understand it simply

Imagine you are playing cricket in the evening. As the sun goes down, you notice the sky turning orange and red. This happens because sunlight has to travel through more air when the sun is low. The air is like a crowd at a railway station. If you try to walk straight through a crowded platform, you get pushed and turned many times. In the same way, sunlight gets scattered (spread out) by tiny particles in the air. Blue light gets scattered the most, so it goes in all directions and doesn't reach your eyes directly. Red and orange light, which are stronger and less easily scattered, make it through the crowd and reach you. That's why you see those colours.

How it works

1. Sunlight is made of many colours: violet, blue, green, yellow, orange, and red.
2. When the sun is overhead (noon), sunlight travels a short path through the air. All colours reach your eyes, so the sky looks blue (because blue scatters the most).
3. At sunrise and sunset, the sun is near the horizon. Sunlight has to travel a much longer path through the atmosphere.
4. On this long journey, the blue and violet light get scattered away in all directions by air molecules and dust.
5. The red and orange light, which scatter less, pass through and reach your eyes, making the sky look red or orange.

The story behind it

This effect is explained by the Rayleigh scattering theory, named after Lord Rayleigh (John William Strutt), a British scientist. In the late 1800s, he wanted to know why the sky is blue and why sunsets are red. He found that tiny particles in the air scatter short-wavelength light (like blue) much more than long-wavelength light (like red). His work helped us understand not just the sky, but also why things look the way they do under different lights. Rayleigh even won the Nobel Prize in Physics in 1904 for his work on gases.

Where you see it in real life

• Evening cricket match: The sky turns orange as the sun sets — blue light is scattered away, red/orange remains.
• Sunrise at a hill station: The first light is often red or pink — same reason, long path through the air.
• Diwali rockets: When you burst a rocket at sunset, the smoke looks reddish against the sky — the background sky is red/orange.
• Dust storms: During a dust storm at sunset, the sky can look deep red — more dust means even more blue light is scattered away.
• Photos from airplane windows: When flying during sunset, you see a band of red/orange near the horizon.
• Village pond at sunrise: The water reflects the orange sky, making the pond look golden.

Why it matters

• Weather prediction: Red sky at night can mean good weather; red sky in morning can mean rain — farmers use this.
• Photography and art: Artists and photographers use sunrise/sunset light for beautiful pictures.
• Astronomy: Astronomers plan observations to avoid times when the sky is too red, as it affects what they see.
• Science exams: This topic is common in school exams and helps you understand light and colour.

Worked example

Let's say the sun is directly overhead at noon and at the horizon at sunset. Why does the sky look blue at noon and red at sunset?

Step 1: At noon, sunlight travels a short path through the air.
Step 2: Blue light scatters in all directions, making the sky look blue.
Step 3: At sunset, sunlight travels a long path through the air.
Step 4: Blue light is scattered away before it reaches your eyes.
Step 5: Red and orange light, which scatter less, reach your eyes.
Step 6: So, the sky looks red or orange at sunset.

Common mistakes

• Thinking the sun itself changes colour: The sun is always white; it's the atmosphere that changes the colour you see.
• Believing pollution is the main cause: Normal sunset colours are due to air and dust, not just pollution. Pollution can make sunsets even redder, but it's not the main reason.
• Forgetting about the path length: The key is the longer path sunlight travels at sunrise/sunset, not just the time of day.
• Mixing up scattering and reflection: The colour change is due to scattering (spreading), not reflection (bouncing off surfaces).

Remember it

At sunrise and sunset, sunlight travels a longer path through the air, so blue light scatters away and red/orange light reaches you — that's why the sky looks red or orange.