We know that light refracts (or bends) when it goes from one medium to another. Now, the atmosphere contains only air but still refraction occurs this is due to the reason that all the air present in the atmosphere is not at same temperature. Some of the air layers are cold whereas others are comparatively warm. The cooler air layers of the atmosphere behave as optically denser medium for the light rays whereas the warmer air layers behave as optically rarer medium.