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Penetration depth is the depth in a material to which electromagnetic radiation can penetrate. A number of things can influence penetration depth, including properties of the material itself, the intensity and frequency of the radiation, and various environmental factors. There are a number of applications for depth testing, and a number of different formulas can be used to find penetration depth for various substances. Specialized equipment for testing is available from some manufacturers of scientific equipment as well as companies which develop tools for use in materials testing.
When electromagnetic radiation hits an object, some of it bounces off the object, but some of it is absorbed. As the electromagnetic radiation is absorbed, it also interacts with the material it passes through, which can be important to know about in some applications. Imagine a person sitting in the sunshine; some of the sun's radiation bounces off, and some of it is absorbed, making the sitter feel warm or even uncomfortably hot. People may also note that the strength or weakness of the sunlight can cause variations in experience; someone sitting in the winter sun may not fully warm up, for example, while someone out at noon on a summer day will get quite hot.
In testing conducted to determine penetration depth. Electromagnetic radiation of a known wavelength and intensity is either used or generated to see how deeply it penetrates. The penetration depth is the point at which the electromagnetic radiation retains 37% of the value it has at the surface. Knowing penetration depth can provide interesting information about new or unknown materials, but it also has other applications.
It can be used, for example, in materials testing, to confirm that materials are consistent and that they contain what they are supposed to contain. For example, the penetration depth of a metal alloy can be tested. Penetration depth can also be useful when designing systems which utilize electromagnetic radiation to function, and for researching materials which might be useful in such systems. Studies on penetration depth can be utilized in the design of electromagnetic shields, as well, to make sure that a shielding system will work as intended.
People see examples of penetration depth every day, and it can be used in some surprising ways. Light, a form of electromagnetic radiation, is used in a process called transillumination in the medical field, in which light is shone at or through a body part to collect diagnostic information. Changes in the way the light travels through the material can indicate problems with the tissue the light is passing through.