What is a mirage?
A mirage is an optical phenomenon which often occurs naturally. An inferior mirage occurs due to the inverted image lying below the upright one. This inferior mirage is the most common and is produced by the refraction of light when it passes into a layer of warm air lying close to a heated ground surface. This is known as gradient index optics.
Gradient index optics is the branch of optics that involves optical effects produced by a gradual variation of the refractive index of a material. An example of this is the "pool of water" we see as we drive along the road on a hot day. This pool is actually an image of the sky which is apparent on the road due to light rays being refracted from their normal straight path. This happens because the air at the surface of the road is hotter and less dense than the air above it. This makes the refractive indes increase with heights and light rays are refracted from the sky and bent into the eye of the viewer making the road look like it is covered with water.
The refractive index of a material is defined as the factor by which electromagnetic radiation is slowed down when it travels inside the material. Therefore it is the factor of difference in temperature and also density in the hot air at the road's surface and the air above it that causes the electromagnetic radiation to slow down. This in turn causes the bending of light rays and gives us the illusion of water.
As stated above, refraction plays the big role in a mirage. Here is another example of refraction creating an optical illusion. Water has a refractive index of about 1.33. This means that light rays are bent slightly as they enter the water from the air. One way to experience this phenomenon is to place a pencil at a slant into some water. The pencil appears to bend as it enters the water because the light rays are being refracted by the 1.33 refractive index of water.
There are four types of mirages that have specific names:
1. Looming- appearance of objects that are normally hidden below the horizon. This usally occurs over water surfaces when air thickness decreases and altitude is heightened.
2. Sinking- This is the reverse effect of the looming phenomenon. This happens when air thickness increases and vessels such as boats which are normally seen on the horizon seem to sink and become invisible.
3. Towering- This type occurs when there is irregular refraction. Light rays tend to curve downward with the top of the object curving more than the lower ones. Most times you will see objects lifted up higher than usual and enlarged in the vertical direction.
4. Stooping- This happens when the light rays of the object curve downward less than the bottom rays. When this occurs objects on the horizon can be observed with the rising or setting of the sun or moon. There is a famous stooping mirage between Calabria and Sicily and it is known as fata morgana.
Here are some links to webpages where you can see pictures of mirages
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