Question

State true or False.
At dawn or dusk, the sun appears to be above the horizon for three or four minutes longer than it is.
A) True
B) False

Answer 1

Hint: The bending of light from its direction of propagation through the atmosphere is called refraction of light through the atmosphere or also called atmospheric refraction of light. Atmospheric refraction of light leads to an increase in the day (i.e. lengthening of the day) etc.

Complete step by step answer:
We know that the sun is visible to an observer after the actual sunset (i.e. actual passing of the horizon by the sun) because of refraction of light due to the denser atmosphere near the earth. Due to the refraction of light, the apparent shift in the direction of the sun is about ${\dfrac{1}{2}^\circ }$ leading to delay about 2 minutes in actual and apparent sunsets. Earth completes one rotation (i.e. $360^\circ $) about its axis in 24 hours.

Therefore, the time is taken for $360^\circ $ the shift $ = 24{\rm{hr}}$.
Hence time is taken for ${\dfrac{1}{2}^\circ }$ shift $ = \;\dfrac{{24}}{{360}} \times \dfrac{1}{2}\; = \,\dfrac{1}{{30}}{\rm{h}}$
$ \Rightarrow \;\dfrac{1}{{30}}\; \times 60 = \;2{\;\rm{minutes}}$
For the same reason, the sun appears to rise about 2 minutes before the actual sunrise. Hence, the day is lengthened by 4 minutes. That is why at dawn or dusk, the sun appears to be above the horizon for three or four minutes longer than it is true.

Hence the correct option is (A).

Note: Here, the meaning dawn refers to the first light of the sun in morning and dusk is referring to evening light. It's all about refraction. If we observe both advanced sunrise and delayed sunset, the light enters from vacuum to earth's atmosphere; it enters from rarer to denser medium and bends towards the horizon.