Light and colour in the atmosphere

Question 1

what particles in the atmosphere cause the blue sky

Answer 1

air molecules

Question 2

what particles in the atmosphere cause a halo

Answer 2

ice crystals

Question 3

what is the main reason the sky is blue and sunsets are red

Answer 3

Short wavelengths (blue) are scattered by air molecules about a factor of seven more than long wavelengths (red), so the scattered light from the sky is enhanced in blue light and the direct beam from the setting sun is enhanced in red light (because the blue light was scattered away).

Question 4

why is guinness froth almost white when the beer itself is almost black?

Answer 4

the froth scatters white light very efficiently because it has a high surface area. the veeer is black because light penetrates into the beer