The optical phenomenon responsible for the blue colour of sky is

Correct — D, scattering. Sunlight entering the atmosphere meets air molecules that are far smaller than the wavelength of light, and in that situation the amount of light scattered varies inversely as the fourth power of the wavelength. Short wavelengths are therefore thrown sideways far more strongly than long ones — blue several times more than red. The light that reaches your eye from every part of the sky, rather than straight from the sun's disc, is this scattered light, and it is dominated by blue. The same law explains the reddening of the sun at sunrise and sunset, when the longer path through the atmosphere scatters the blue out of the direct beam.

• (a)dispersion — Dispersion is the splitting of white light into its colours because the refractive index of a medium differs slightly from one wavelength to the next. It gives the prism spectrum and the rainbow, but it does not send blue light towards you from every direction of the sky.
• (b)reflection — Reflection needs a surface, and the open sky has none. What clouds do reflect comes back as white, which is why a cloudy sky is not blue.
• (c)refraction — Refraction is the bending of light as it crosses from one medium into another. It explains a stick that looks bent in water and the flattened shape of the sun at the horizon, not the colour of the sky overhead.

When light meets particles much smaller than its own wavelength, it is scattered in all directions with a strength that falls off very steeply as the wavelength increases — as the inverse fourth power of it. This is Rayleigh scattering, and the molecules of nitrogen and oxygen in the atmosphere are exactly such particles for visible light.

One law accounts for both of the sky's famous colours. Looking away from the sun, you see scattered light, which is blue. Looking at the sun low on the horizon, you see the beam that has survived a long slanting passage through the air with its blue removed, which is red. Two refinements are worth carrying. Violet is scattered even more strongly than blue, but sunlight contains less of it and the human eye is much less sensitive to violet, so the sky reads as blue rather than violet. And scattering by larger particles behaves differently — a colloid scatters all wavelengths fairly evenly, which is the Tyndall effect that makes a beam of light visible in a colloidal solution and makes clouds and fog white rather than blue. An astronaut sees a black sky at noon because above the atmosphere there is nothing left to scatter.

• In Rayleigh scattering the amount scattered varies as the inverse fourth power of the wavelength.
• The scattering particles must be much smaller than the wavelength of light, as air molecules are.
• Blue is scattered several times more strongly than red, which is why the sky is blue.
• The sun looks red at sunrise and sunset because the longer atmospheric path scatters blue out of the direct beam.
• The Tyndall effect is scattering by the larger particles of a colloid, and it makes clouds and fog appear white rather than blue.

• Choosing dispersion because the question mentions a colour, when dispersion separates colours rather than selecting one.
• Explaining the blue sky as reflection from the sea, which is a persistent folk explanation and is wrong.
• Forgetting that violet is scattered even more than blue, and that the eye's sensitivity is what settles the sky's apparent colour.

Asked as a straight identification of the phenomenon, with the three other standard optical effects offered alongside.

• practice — not a real PYQ

  The sun appears reddish at sunrise and sunset chiefly because

  • (a)the sun is actually cooler then
  • (b)blue light is scattered out of the longer atmospheric path
  • (c)the atmosphere reflects red light
  • (d)the sun is nearer the earth at those times

  Answer(b) blue light is scattered out of the longer atmospheric path — leaving the reddened direct beam.

• practice — not a real PYQ

  The sky appears black to an astronaut in orbit because

  • (a)there is no sunlight in space
  • (b)there is no atmosphere to scatter light
  • (c)the spacecraft windows filter out blue
  • (d)space is at a very low temperature

  Answer(b) there is no atmosphere to scatter light — with no scatterers, no light reaches the eye except straight from a source.