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Optical Prisms

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An optical prism is a solid, transparent optic with flat surfaces which refract light. Light enters through one surface, and may be rotated, deviated, or dispersed before leaving through another surface. These surfaces are highly polished, and the angles between them, the number of surfaces, and the position of them determine the function of the prism.

Since optical prisms redirect light at a designated angle, they may be used to reflect light, to bend it within a system, to change the orientation of an image, or to break light up into it component wavelengths or polarizations. Isaac Newton used a simple prism to disperse a beam of white light into the full spectrum of component colors, proving that white light was in fact nothing but a combination of colors. This same dispersion technique is still used in spectrographs and refractometers. Optical prisms can also be used, in reverse, to combine optical beams into one.

For more information on Prisms read this article on our resource page.

4 Main Types of Optical Prisms

Prisms may be divided into four main types: dispersion prisms, reflection prisms, displacement prisms and rotation prisms.

A simple triangular prism is an example of a dispersion prism. Since the refractive index of light changes as the wavelength changes, Snell’s law tells us that the deflection angle of the light will change proportionally. For instance, red wavelengths (656.3 nm) are longer and will be refracted less than shorter wavelengths, such as green(587.6nm) or blue(486.1nm). This separates the colors/wavelengths of light as it travels through the prism. The dispersion a prism provides depends on its index dispersion curve and on the geometry of the prism. For spectroscopy and telecommunications, equilateral prisms are often used.

Displacement prisms maintain the direction of the light beam, but adjust its relation to the normal. An example of a displacement prism is the simple rhomboid prism. This prism displaces the incident beam without changing handedness. The direction remains the same, so rhomboid prisms are ideal for binoculars, rangefinders, and laser instrumentation.

Rotation prisms, such as dove prisms, rotate the beam of light and hence the image. Rotation prisms are used in pattern recognition, astronomy, and for imaging behind detectors or around corners. One type of rotation prism is the dove prism, which rotate the image by 180 degrees while creating a left handed image. Schmidt prisms deviate the ray by 45 degrees while creating a right handed image.

Deviation prisms include half-penta prisms and Amici roof prisms. Amici roof prisms deviate the light path by ninety degrees while inverting the image, and are used in microscopes and telescope eyepieces. If the roof faces of the prism are coated the beam can be deviated by other angles as well. Half-penta prisms deviate the ray path by 45° and are used in Pechan erector assemblies.

It is important to note that one prism can serve many different purposes depending on orientation. For instance, take a 45° – 90° – 45° prism, commonly referred to as a right angle prism. If light enters at a right angle through one of the faces adjacent to the 45° angle (a leg face), there will be a single reflection and the incident ray will be deviated by 90°.

Now take the same prism, and orient it so that the light enters through the hypotenuse face.  This configuration has a special name; the porro prism. The ray’s direction will be reversed and the light beam will enter and exit from the same face. The image will be reverted or inverted, depending on whether the prism was upright or flat.

But that’s not all. A right angle can also be oriented so as to be used as a dove prism, which produces a rotation of 180 degrees. Again, the image may be reverted or inverted depending on how the prism is oriented.

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