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Why does the sky turn red and orange at sunset?

Quick answer from Eartha

At sunset, sunlight has to travel through much more air to reach your eyes than it does at midday. Along the way, most of the blue light gets scattered away and lost. That leaves the red, orange and pink light to shine through, painting the sky in warm sunset colours.

The longer story

During the day, sunlight only travels through a short stretch of air to reach you, because the sun is high above your head. Blue light scatters a little, which is why the sky looks blue.

At sunset, the sun is low near the horizon, so its light has to travel through a much longer stretch of our atmosphere to reach your eyes. That's like the difference between looking straight up through a window versus looking through a whole tunnel of glass.

Travelling through all that extra air scatters away almost all the blue light long before it gets to you. Mostly the red, orange and pink light survives the journey, so that's what paints the sky.

Dust, tiny water droplets or smoke in the air can make sunsets even more colourful, sometimes turning the whole sky a deep, glowing orange. It's the exact same scattering trick that makes the daytime sky blue, just happening at a different angle.

Try this at home: milky glass sunset

  1. Fill a clear glass with water and stir in a couple of drops of milk.
  2. Shine a torch through the long side of the glass so the light travels through more liquid.
  3. Look at the light coming out the other end and see if it looks more orange.

Words to know

horizon
The line far away where the sky seems to meet the ground or sea.
atmosphere
The blanket of air around our planet that sunlight travels through.
scatter
When light bounces off in lots of directions.

For grown-ups: what is my child learning here?

Age band: Ages 5-8

Curriculum link: Australian Curriculum v9 Science, Foundation to Year 2, Chemical Sciences strand: exploring everyday phenomena involving light and colour.

The concept: The child is learning that the colour of the sky changes because of how far sunlight travels through the air.

More classroom ideas are on our teachers page, and safety notes for families are on for parents.

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