Why is the sky blue? The story of scattered light
Sunlight looks white, yet the sky above us is blue. The explanation is one of the most elegant ideas in everyday physics.
White light is a mix
Sunlight contains all the colours of the spectrum, and each colour has a different wavelength. Blue and violet have short wavelengths; red has a long one.
Tiny molecules, big effect
When sunlight enters the atmosphere it meets nitrogen and oxygen molecules that are far smaller than the wavelength of light. These molecules scatter short wavelengths much more strongly than long ones. The effect is called Rayleigh scattering, after the British physicist Lord Rayleigh, and it sends blue light bouncing across the whole sky, so blue reaches our eyes from every direction.
Why not violet?
Violet is scattered even more than blue, but sunlight contains less violet to begin with, some of it is absorbed high in the atmosphere, and our eyes are more sensitive to blue. The combination gives us a blue sky.
Why sunsets are red
At sunrise and sunset, light travels through a much longer path of air before reaching us. Most of the blue is scattered out along the way, leaving orange and red to colour the horizon.
Try it at home
Fill a clear glass with water, stir in a few drops of milk and shine a torch through it in a dark room. From the side the water looks faintly blue; looking through it toward the torch, the light looks orange. The small particles do what the atmosphere does.
Original Vemetto explainer in English, adapted from our Arabic page. Last reviewed: 9 November 2026.
Questions and practical notes
Photography tips
- The richest sunset colours often appear a few minutes after the Sun disappears.
- Use manual exposure so the sky is not washed out.
- Never look at the Sun directly through a camera or binoculars.
Frequently asked questions
Is the sea blue because it reflects the sky?
Partly, but water itself absorbs red light more than blue, so deep water looks blue even on cloudy days.
Why are clouds white?
Cloud droplets are much larger than air molecules and scatter all colours roughly equally, so clouds look white or grey.
What colour is the sky on Mars?
Mars has a thin, dusty atmosphere: its daytime sky looks butterscotch, and its sunsets look bluish around the Sun.