Ned Ludd
Silver $$ Contributor
Mirage is caused by differences in air density, typically due to thermal effects. Light passes through air currents of varying density,which cause it to refract differently. The mathematics of light incidence and refraction due to mediums of differing density are described by Snell's Law. It's no different than the old example where a pen standing in a half-full glass of water looks as if it's bent. Mirage can affect your shot in at least two ways. The first and most obvious effect is that many use spotting scopes to visualize the air currents (i.e. mirage), which can be used to estimate wind speed and direction. An incorrect estimate of wind speed based on mirage will put a shot out just the same as an incorrect estimate based on wind flags will.
The second effect of mirage is far more sinister. In very strong mirage, the point you think you're aiming at on the target may not actually be where you're aiming. In the example of the pen in the glass of water, our brain tells us that the pen isn't really bent as it appears, largely based on the top segment of the pen that is observed through air only. From that segment, which functions somewhat like a "3-dimensional anchor", or point of reference, our brain can make a very good estimate where the submerged part of the pen is really located. When looking through a rifle scope in bad mirage at a target at 1000 yd, there may not be such an anchor, no common point of reference, etc. It can be very difficult to know that you are actually aiming the rifle at what you're seeing in the scope. One approach that can help with this is to pick your point of aim during your sighting shots and see how/where the mirage is actually displacing your shots, The difficulty in doing that is that you have to be able to distinguish between the visual effect of the mirage (i.e displacement), and what the wind was doing to the shot.
The second effect of mirage is far more sinister. In very strong mirage, the point you think you're aiming at on the target may not actually be where you're aiming. In the example of the pen in the glass of water, our brain tells us that the pen isn't really bent as it appears, largely based on the top segment of the pen that is observed through air only. From that segment, which functions somewhat like a "3-dimensional anchor", or point of reference, our brain can make a very good estimate where the submerged part of the pen is really located. When looking through a rifle scope in bad mirage at a target at 1000 yd, there may not be such an anchor, no common point of reference, etc. It can be very difficult to know that you are actually aiming the rifle at what you're seeing in the scope. One approach that can help with this is to pick your point of aim during your sighting shots and see how/where the mirage is actually displacing your shots, The difficulty in doing that is that you have to be able to distinguish between the visual effect of the mirage (i.e displacement), and what the wind was doing to the shot.
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