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Why is the sky blue?
The sky is blue because tiny molecules in the air scatter the blue part of sunlight more than any other colour.
Sunlight looks white, but it is really hiding all the colours of the rainbow: red, orange, yellow, green, blue, and purple. As this light comes into our sky, it bumps into tiny gas molecules in the air.
Blue and purple light have small, tight waves. Because of this, air molecules scatter them in every direction much more than the other colours. This is called scattering.
Our eyes are not as good at seeing purple as they are at seeing blue, so we see the sky as blue instead of purple. When the sun sets, its light has to pass through a thicker layer of air, so the blue light scatters away completely and the sky turns red and orange.
Quick facts
- Sunlight carries every colour of the rainbow inside it.
- Tiny molecules in the air scatter blue light more than any other colour.
- This is called the scattering of light.
- Our eyes are more sensitive to blue than to purple, so the sky looks blue.
- At sunset, light passes through a thicker layer of air, which turns the sky red.
- The sky on the Moon is always black because there is no atmosphere there.
Check what you remember
3 questions — all of them come from the quick facts above. There is no score and no wrong turn.
1. What makes the sky look blue?
2. Why does the sky turn red when the sun sets?
3. Why does the sky look black on the Moon?
Where this comes from
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1 spaceplace.nasa.gov 90/100NASA Space Place - Why Is the Sky Blue?
A page made by NASA for kids, with pictures and simple explanations.
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2 britannica.com 75/100Britannica - Why is the sky blue?
A page from a trusted encyclopedia explaining the colour of the sky.
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