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Curiosity · Light and colour

Why is the sunset red?

Why does the Sun, shining white at midday, look like an orange ball in the evening? Find out how blue gets left behind as light's path through the air grows longer, and why dust and volcanic ash colour the sunset.

  • 15 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

During the day, the Sun gives off a nearly white light, too bright to look at. In the evening, though, the Sun sinking towards the horizon turns into an orange, even red disc, and the clouds around it turn pink. Does the Sun cool down in the evenings? Or is something else changing?

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No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Colour and light

The light of the setting Sun

On the scene, you can change the Sun's height and compare how the light particles are scattered along the way. Go back to the height given in the task, look at the direct light meter, and set each spotlight to off, half or full.

Prediction

Predict first

As the Sun sets, the path its light takes through the air gets longer. How does this change the light reaching your eye directly?

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Ordering

The Sun's colours through the day

Drag the cards by their handles or move them up and down with the arrows.

Prediction

Predict first

What do you think is the main reason the Sun changes colour during the day?

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Lab

Light’s path through the air

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Frequently asked

5 questions

Does the Sun turn red in the evening because it cools down?

No. The Sun's surface temperature is about 5,500 °C and does not change during the day. What changes is the path its light takes through the air to reach you: as the Sun nears the horizon, this path gets much longer and blue light is scattered away along the way.

At sunset, is everything red because red light scatters more?

It is the other way round. Blue light scatters much more than red and is removed from the direct beam. In the light that is left and reaches your eye, red and orange dominate.

How much longer is the light's path through the air at sunset?

When the Sun is about 2° above the horizon, its light travels through about 20 times as much air as when the Sun is overhead; right at the horizon this rises to about 38 times. Because the losses multiply layer by layer, the blue is almost completely used up.

Where does the blue light scattered along the way go?

It is not lost; it colours the sky blue in the places the light passes over. While you watch the sunset, someone further west, under the air the light crossed, can see a blue afternoon sky lit by that same scattered light.

How do dust and volcanic ash affect sunsets?

Dust, sea salt or volcanic ash in the air scatter extra light and can deepen sunset colours. After the Krakatoa eruption in 1883, unusually red sunsets were seen in many parts of the world for months.