The colour of the sky and of the Sun

Why is the sky blue?

Look up on a clear day. The sky is blue. Why is it not white, or black, or green?

The air is made of molecules that are very small, much smaller than the wavelength of light. These tiny molecules scatter the short wavelengths of sunlight, such as blue and violet, much more than the long wavelengths, such as red. The scattering is proportional to $\dfrac{1}{\lambda^4}$.

When you look at a part of the sky that is away from the Sun, you do not see direct sunlight. You see the light that the air has scattered toward your eyes. This light is mostly blue. So the sky looks blue.

Why not violet?

Violet light has an even shorter wavelength, so it is scattered even more. But the Sun sends out less violet than blue light, the upper air takes in some of the violet, and our eyes are less sensitive to violet. So the mixture of the scattered colours looks blue to us.

Why is the Sun red at sunrise and sunset?

At noon, the sunlight goes through a thin layer of air to reach us. The Sun looks white or yellowish, because only a little of the blue is scattered away.

At sunrise and at sunset, the Sun is near the horizon. The light must travel a much longer way through the air before it reaches us. Most of the blue and the shorter wavelengths are scattered out of the beam on the way. The light that is left, and that reaches our eyes, is mostly red and orange. So the Sun looks red, and the clouds near it are bright orange and red.

The long path is also why the setting Sun looks dimmer, so we can look at it without hurting the eyes.

You should find that at noon most of every colour gets through. Lower the Sun below about $10^\circ$ and the blue bar shrinks fast, while the red bar stays tall: the Sun turns orange, then red.

A worked example

The Sun is $15^\circ$ above the horizon. About how many times as much air does its light cross as at noon overhead?

1. The rule. For a Sun not too close to the horizon, the path is about $\dfrac{1}{\sin h}$ times the overhead path.
2. Put in the number. $\dfrac{1}{\sin 15^\circ} = \dfrac{1}{0.259} = 3.86$.
3. So the light crosses almost four times as much air, and much more of its blue is lost.

Why are danger signals red?

The lights of danger signals, such as the red light at a railway crossing or on a stop sign, are red. Red has the longest wavelength of the visible colours, so it is scattered the least by fog, smoke and dust. A red light can be seen from a long way, even in poor weather. This is also why the lights on the back of a vehicle are red.

What would the sky look like without air?

On the Moon, and in space, there is no atmosphere. There is nothing to scatter light, so the sky looks black, even in the day, and the stars are seen. The astronauts see a very dark sky.

Practise with new numbers

Try it in Python

The program shows how much of the blue light and of the red light reaches the ground when the Sun is at different heights above the horizon.

Remember these

• The sky is blue because the air scatters the short wavelengths of sunlight far more than the long ones, and the scattered blue light comes from all directions.
• At sunrise and sunset, the light goes a long way through the air. The blue is scattered away, and the red reaches the eye.
• Red has the longest wavelength and is scattered the least, so it is used for danger signals.
• Without an atmosphere, the sky is dark.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Violet light is scattered even more than blue. Why is the sky not violet?

2. Why can you look at the setting Sun more easily than at the Sun at noon?

3. The sky close to the horizon looks paler, almost white. Why?

4. The Sun is $30^\circ$ above the horizon. About how many times as much air does its light cross as at noon overhead?