Atmospheric refraction causes advance sunrise and delayed sunset. By how much time is:
a) Sunrise advanced?
b) Sunset delayed?

Hint:In this question, use the concept of refraction that is when the light travels in the different medium, then due to the difference of the two concerned mediums, the light tends to bend towards a particular direction. This incident is termed as refraction.

Complete step by step solution:
a) During the Sunrise:
As we know the sunlight travels from less dense air to more dense air, so in more dense medium the light tends to bend downwards when passing through the atmospheric air .So, while the sun is actually beyond the horizon during sunrise, we can observe it much earlier than it really appear in the horizon (approximately 2 minutes earlier than actual sunrise). So, when the sun is actually below the horizon, it appears above the horizon due to the refraction of sunlight.
b) During the sunset:
As we know that the similar phenomenon happens during sunset, the medium of travel of sunlight changes from rarer medium(lighter medium)to denser medium , as a result the atmospheric refraction of sunlight occurs, and while the sun has actually set beyond the horizon, it appears in the horizon for approximately for 2 minutes into our vision. So the duration of sunrise and sunset is extended for 4 minutes than normal time.
Note: As we know that that when the ray of light travels from rarer medium to the denser medium the light refract towards the normal and if the ray of light travel from denser medium to the rarer medium then it refract away from the normal.