Twinkling of Stars and Mirage
Heat energy radiated by the earth changes the density of the atmospheric layers continuously. This changing density of the air layers near the ground affects its refractive index. Due to the refraction of light rays from the star, path of these rays goes on varying. Hence the eye some times receives more light with the result that the star appears brighter and sometimes it receives only a few rays or no rays which make the star appear fainter. The brighter and fainter appearance of the star with varying time is called the twinkling of the stars.
A mirage is an optical illusion observed in deserts or over hot extended surfaces like a coal tarred road. During hot days the lower layers of air near the earth’s surface are hotter and lighter than the upper layers away from the earth’s surface. Cold air is more dense than warm air and has therefore a greater refractive index. As light passes from colder air across a sharp boundary to significantly warmer air, the light rays bend away from the direction of the temperature gradient. When light rays pass from hotter to cooler, they bend toward the direction of the gradient. If the air near the ground is warmer than that higher up, the light ray bends in a concave, upward trajectory. Hence light from an object (say the top of a tree) undergoes a series of refraction and total internal reflections and bends upwards. Once the rays reach the viewer’s eye, the visual cortex interprets it as if it traces back along a perfectly straight “line of sight”. This line is however at a tangent to the path the ray takes at the point it reaches the eye. The result is that an “inferior image” of the sky above appears on the ground. The viewer may incorrectly interpret this sight as water which is reflecting the sky, which is, to the brain, a more reasonable and common occurrence. In the case where the air near the ground is cooler than that higher up, the light rays curve downward, producing a “superior image”. Superior Image is common in polar areas, which is known as Looming.
Due to the mirage, a traveller sees shimmering pond of water some distance ahead of him. This optical illusion is called mirage. Thus, Mirage is due to the combination of Refraction as well as Total Internal Reflection of light.