What happens when the sunlight travels through the Earth's atmosphere?
- (a)The blue colour is scattered more compared to the red colour
- (b)The red colour is scattered more compared to the blue colour
- (c)Both the blue and the red colours are scattered equally
- (d)The blue colour is not scattered but the red colour is scattered the most
Correct — A, the blue colour is scattered more compared to the red colour. As sunlight passes through the atmosphere, its shorter-wavelength components (blue and violet) are scattered by air molecules far more than the longer-wavelength red, because Rayleigh scattering is proportional to 1/lambda^4. This preferential scattering of blue light is why the daytime sky looks blue.
- (b)The red colour is scattered more compared to the blue colour — This is the reverse of the truth — red (longer wavelength) is scattered least, which is why it survives the long path to give red sunrises and sunsets.
- (c)Both the blue and the red colours are scattered equally — Scattering depends strongly on wavelength (1/lambda^4), so blue and red are not scattered equally.
- (d)The blue colour is not scattered but the red colour is scattered the most — Incorrect — blue is scattered the most, not the least, and red is scattered the least.
When sunlight travels through the atmosphere, small air molecules scatter it by Rayleigh scattering, whose intensity varies as 1/lambda^4. Blue light (short wavelength) is scattered several times more than red light (long wavelength). Scattered blue light reaching us from all directions makes the sky look blue; at sunrise and sunset the long path removes the blue and leaves reddish transmitted light.
The controlling fact is 'shorter wavelength scatters more' (1/lambda^4). This immediately picks blue over red and rejects the 'equal' and 'red-more' options. The same principle explains the reddish Sun near the horizon and the use of red for danger signals.
- Rayleigh scattering intensity is proportional to 1/lambda^4, so shorter-wavelength blue scatters most.
- Scattered blue light makes the clear daytime sky appear blue.
- At sunrise and sunset the long atmospheric path scatters blue away, so the Sun and sky look red.
- Red is scattered least, one reason it is used for danger and stop signals.
- Shorter-wavelength blue scatters MORE than red — do not reverse it.
- Scattering is strongly wavelength-dependent (1/lambda^4), so it is not equal for all colours.
Asks why the sky is blue or the sunset is red, or which colour scatters most, testing Rayleigh scattering (1/lambda^4).
Consider the following statements: 1. Light of longer wavelength is scattered much more than light of shorter wavelength. 2. The speed of visible light in water is 0.95 times its speed in vacuum. 3. Radio waves are produced by rapidly oscillating electrical currents. 4. To detect over-speeding vehicles, police use the Doppler effect of reflected short radio waves. Which of these statements are correct?
- (a) 1 and 2
- (b) 1 and 3
- (c) 2 and 4
- (d) 3 and 4
Answer(d) 3 and 4 — statement 1 is false because shorter-wavelength light is scattered more (1/lambda^4), and statement 2 misstates the speed of light in water.
UPSC prelims tested the same scattering principle — that scattering depends on wavelength — as a statement to evaluate (its statement 1 is the exact inverse of this NDA fact).
- practice — not a real PYQ
The sky appears blue because blue light is
- (a)absorbed by the atmosphere
- (b)scattered more than the other colours
- (c)reflected by clouds
- (d)refracted by water vapour
Answer(b) scattered more — Rayleigh scattering favours short wavelengths.
- practice — not a real PYQ
The Sun appears reddish at sunrise and sunset because
- (a)the Sun is actually red then
- (b)blue light is scattered away over the long atmospheric path
- (c)the Sun is closer to the Earth
- (d)clouds turn the light red
Answer(b) blue light is scattered away over the long path, leaving mostly red.