Physics Q.3

With reference to the diagram below:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/April07/Mirage.jpg

Cold air is denser than warm air, and has therefore a greater refractive index. As light passes from colder air to warmer air it bends away from the direction of the temperature gradient (the "normal" in the figure at right); when it passes from hotter to colder, it bends towards the direction of the gradient. The diagram above shows a light ray coming from the sky toward the hot ground. If the air near the ground is warmer than that higher up, the light ray bends in a concave upwards trajectory and eventually, the ray will be reflected back upward again (which is known as the total internal reflection) due to the incident angle greater than the critical angle. Once the ray reaches the viewer's eye, the eye traces it as the line of sight, which is the tangential to the path the ray takes at the point it reaches the eye. The result is that an inferior image for the above sky appears on the ground. The viewer may incorrectly interpret this sight as water reflecting the sky. In the case where the air near the ground is cooler than that higher up, the light rays will of course curve downwards, producing a superior image.

On a hot day, the air close to the ground is heated up faster than that higher up. As the refractive index of hotter air is smaller, there is a continuous change in the refractive index of the air. Light from the sky is gradually refracted as it travels through the air. As a result, we will see a reflection of the sky, which looks like a puddle of water.

http://hk.geocities.com/adrianchan111/d/mirage.bmp
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