Scattering of light - blue sky and red sunset — practice questions

Question 1 · difficulty L1 · recall

The blue colour of a clear daytime sky is mainly due to

• A. greater absorption of blue by air
• B. reflection of oceans only
• C. stronger scattering of shorter visible wavelengths by the atmosphere
• D. gamma-ray emission from molecules

Question 2 · difficulty L1 · understanding

Rayleigh scattering intensity varies approximately as

• A. λ⁴
• B. 1/λ
• C. λ
• D. 1/λ⁴

Question 3 · difficulty L2 · understanding

The Sun often looks reddish near sunset because

• A. shorter wavelengths are scattered out of the long atmospheric path
• B. red light travels slower in vacuum
• C. the Sun emits only red light in the evening
• D. blue wavelengths are converted into infrared

Question 4 · difficulty L2 · application

Why is the sky not normally violet even though violet scatters more strongly than blue?

• A. Violet has a longer wavelength than red
• B. Human visual sensitivity, solar spectrum and atmospheric absorption make blue dominate perceived sky colour
• C. Violet is not electromagnetic radiation
• D. Rayleigh scattering excludes violet completely

Question 5 · difficulty L3 · application

If wavelength doubles in the Rayleigh regime, scattered intensity becomes approximately

• A. 2 times
• B. 4 times
• C. 1/16 as large
• D. 16 times

Question 6 · difficulty L3 · application

Which colour is scattered more strongly by very small atmospheric particles?

• A. Red
• B. Both equally
• C. Infrared more than blue by Rayleigh law
• D. Blue

Question 7 · difficulty L4 · analysis

Astronauts on the Moon see a dark sky even in daylight mainly because

• A. the Moon lacks a substantial atmosphere to scatter sunlight
• B. sunlight cannot reach the Moon
• C. visible light is absorbed by vacuum
• D. the Moon emits only infrared

Question 8 · difficulty L4 · analysis

If atmospheric molecules were much larger relative to visible wavelengths, the simple 1/λ⁴ colour dependence would

• A. become exact for all particle sizes
• B. be less applicable
• C. force every scattered photon to be blue
• D. eliminate all scattering

Question 9 · difficulty L5 · analysis

A distant mountain appears bluish mainly because

• A. the mountain emits blue thermal radiation
• B. red light cannot propagate through air
• C. scattered skylight enters the line of sight between observer and mountain
• D. gravity shifts all colours to blue

Question 10 · difficulty L5 · analysis

Which observation most directly supports stronger scattering of shorter wavelengths?

• A. A plane mirror forms a virtual image
• B. A convex lens focuses parallel rays
• C. A resistor heats when current flows
• D. The sky is blue while direct sunlight near sunset is redder