What is scattering of light? State Rayleigh's law and use it to explain the blue colour of the sky and the red colour of the Sun at sunset.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define scattering. Tiny particles absorb light and at once send it out again in all directions.
2 · State Rayleigh's law. For particles smaller than the wavelength, the scattered intensity I ∝ 1/λ⁴.
3 · Explain the blue sky. Air molecules scatter blue far more than red, and this blue light reaches us from all over the sky.
4 · Draw the paths. [Earth and atmosphere; observer; short path to the overhead Sun and long path to the setting Sun; blue scattered sideways from the long path]
5 · Explain the red sunset. Light from the Sun near the horizon crosses a long path of air; the blue is scattered away and red reaches the eye.
6 · Real life. Red danger lights are seen far through fog because red is scattered least.
7 · Close in one line. So, the colours of the sky and the setting Sun both come from Rayleigh scattering.
Scoring tip: Write Rayleigh's law with its condition on particle size; many answers lose a mark by leaving the condition out.
