Why the sky is blue

The idea

Air molecules and fine particles, smaller than the wavelength of visible light, scatter shorter blue wavelengths more strongly than red; this scattered blue light makes the sky blue, and without an atmosphere the sky would look dark.

Look up

On a clear day the sky overhead has a colour. Look at it.

What colour is the clear sky?

• Blue
• Red
• Black

Sunlight in the atmosphere

Press Next step.

Scattered blue light

The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. They scatter the shorter wavelengths at the blue end more strongly than the red. Red light has a wavelength about 1.8 times that of blue. The scattered blue light enters our eyes, so the sky looks blue. Without an atmosphere the sky would be dark.

Air molecules and fine particles, smaller than the wavelength of visible light, scatter blue (shorter wavelengths) more strongly than red; scattered blue light reaches our eyes, so the clear sky is blue. With no atmosphere the sky would look dark.

Notes

Fine particles in air scatter blue light more strongly than red; the scattered blue light enters our eyes, so the clear sky is blue. Without an atmosphere the sky would look dark.

Check yourself

Which colour is scattered more strongly by the fine particles in air?

Red light has a wavelength about how many times that of blue light?

Answer: 1.8

The red light has a wavelength about 1.8 times greater than blue light.

Why does the clear sky look blue?

If the earth had no atmosphere, the sky would look…

• Blue — correct. Yes!
• Red. The fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red.
• They scatter all colours equally. These particles are more effective in scattering light of shorter wavelengths at the blue end than of longer wavelengths at the red end.
• The sky reflects the colour of the sea. When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour more strongly than red. The scattered blue light enters our eyes.
• The scattered blue light enters our eyes — correct. Yes!
• Blue light is the only colour in sunlight. Sunlight has seven colours. The blue is scattered more strongly and enters our eyes.
• blue. If the earth had no atmosphere, there would not have been any scattering. Then the sky would have looked dark.
• red. Without an atmosphere there is no scattering, and the sky would look dark.
• dark — correct. Yes!