What is meant by atmospheric refraction? Use this phenomenon in order to explain the twinkling of stars.

Hint: The air just above an object like a radiator is hotter than the air farther up. Since the hotter air is having less density than the cooler air above it, it is having less refractive index than cooler air. This is because of the atmospheric refraction. That is the gradient of less denser air and more denser air is the reason for atmospheric refraction. Stars feel like twinkling due to these phenomena.

Complete answer:
The atmospheric refraction is explained as the phenomenon of refraction of light due to variation in the refractive index at different levels in the atmosphere. Light gets refracted at each layer of the atmosphere. The layers of atmosphere are having different refractive indices. As the physical conditions of the atmosphere are not similar, the apparent position of the body fluctuates through hot air. This is called atmospheric refraction. There are many examples for this. Twinkling of stars is one of them. Twinkling of stars is because of the atmospheric refraction of star light. The light from the star after entering the earth's atmosphere will undergo refraction in a continuous manner before it arrives at the Earth. The atmospheric refraction is caused because of the variation in the refractive index at different levels of atmosphere. The star light will get bent towards the normal, hence the apparent position will be different from the actual position of the star. Since the atmosphere is not at rest and keeps varying, the path of rays of light coming from the star changes slightly and also the apparent position of a star also changes slightly and the amount of light entering the eye will be flickered. Often it is brighter and sometimes the star is viewed fainter. This is how a star twinkles.

Note:
The actual sunrise and sunset happens before or after the visible one because of the phenomenon of atmospheric refraction. The Sun is seen to us before two minutes in sunrise and two minutes after the sunset. The original sunrise occurs when the sun crosses the horizon.