The Sun is seen little before it rises and for a short while after it sets. This is because of
- A.total internal reflection
- B.atmospheric refraction
- C.apparent shift in the direction of Sun
- D.dispersion
Correct answer
Explanation
The correct answer is B, atmospheric refraction. Air near the ground is denser than air higher up, so sunlight bends as it passes through the atmosphere and reaches us while the Sun is still just below the horizon.
Refraction is the bending of light when it moves from one medium into another of different optical density. The Earth's atmosphere grows steadily denser towards the surface, so rays from the Sun bend downward as they come in. When the Sun is slightly below the horizon, its bent rays still reach the observer, and the Sun appears above the horizon. As a result we see the Sun about two minutes before actual sunrise and about two minutes after actual sunset, which lengthens the day by roughly four minutes.
A is wrong because total internal reflection explains mirages, optical fibres and the sparkle of diamonds, not early sunrise. C is wrong because the apparent shift is the result, not the cause; refraction produces it. D is wrong because dispersion splits white light into colours, as in a rainbow.
Exam tip: advance sunrise and delayed sunset, about 2 minutes each – atmospheric refraction; twinkling stars too.
Practice Questions
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- B.100 mm of Hg
- C.120 mm of Hg
- D.140 mm of Hg
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- C.Palaeontologists
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- C.Zinc and H2
- D.Nickel and H2
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- A.Gibberellins
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- C.Auxins
- D.Thyroxine
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