What does the passage set out to explain?

How a blue sky and an orange sunset arise from the same scattering of light. (The opening promises that both facts come from one process, and the passage then traces that single process through both cases.)

According to the passage, why does sunset light lose its blue?

Its light crosses far more air, so the blue is scattered away before it arrives. (The last paragraph says the low sun's light crosses far more air, and the blue is scattered away long before it reaches you.)

Based on the passage, how would the daytime sky look from a place with far thinner air?

Darker, because less air means less scattered light to see. (The sky's blue is scattered light. The sunset paragraph shows that more air means more scattering, so less air means less scattered light and a darker sky.)

The passage calls scattering 'biased'. Here that means the process:

affects some wavelengths far more than others (The sentence explains itself: the shorter the wavelength, the more it is scattered. The effect is uneven across colours.)

The Colour Of Air: Level 4 Reading Comprehension Questions

Air has no colour of its own, yet the sky overhead is blue and the same sky turns orange at the end of the day. Both facts come from one process, and that process depends on size.

Sunlight is a mixture of wavelengths, and the molecules of nitrogen and oxygen in the air are far smaller than any of them. When a wave meets a particle much smaller than itself, the particle does not block the wave; it takes in a little energy and sends it out again in every direction. This redirection is called scattering, and it is biased: the shorter the wavelength, the more it is scattered. Blue light, being short, is thrown sideways across the whole sky, while red light mostly continues straight ahead. Looking anywhere except at the sun, you see the light that was scattered, so you see blue.

At sunset the geometry changes. The sun sits low, so its light crosses far more air before it reaches you, and the blue is scattered away long before it arrives. What survives the journey is the long end of the spectrum. The orange of a sunset is not something added to the light; it is what is left after the blue has been taken out.

Comprehension questions