Prisms can be categorized into four main types: dispersion prisms, reflection prisms, displacement prisms, and rotation prisms.
A triangular prism is a simple example of a dispersion prism. Due to the change in refractive index with wavelength, according to Snell’s law, different wavelengths of light are refracted by different amounts. For instance, longer red wavelengths (656.3 nm) are refracted less than shorter green (587.6 nm) or blue (486.1 nm) wavelengths. This dispersion separates the colors or wavelengths of light as it passes through the prism. The amount of dispersion a prism provides depends on its index dispersion curve and its geometry. Equilateral prisms are commonly used in spectroscopy and telecommunications.
Displacement prisms maintain the light beam’s direction but change its relation to the normal. An example is the rhomboid prism, which displaces the incident beam without altering its handedness. This makes rhomboid prisms suitable for applications like binoculars, rangefinders, and laser instruments.
Rotation prisms, such as dove prisms, rotate the light beam and consequently the image. They find use in pattern recognition, astronomy, and imaging behind detectors or around corners. For instance, dove prisms rotate the image by 180 degrees, producing a left-handed image. Schmidt prisms deviate the ray by 45 degrees, 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, commonly used in microscopes and telescope eyepieces. Coating the roof faces can deviate the beam by other angles. Half-penta prisms deviate the ray path by 45° and are used in Pechan erector assemblies.
One prism can serve multiple purposes based on its orientation. For example, a 45° – 90° – 45° prism, known as a right-angle prism, can deviate light by 90° if it enters at a right angle through one of the faces adjacent to the 45° angle (a leg face).
When the same prism is oriented for light to enter through the hypotenuse face, it’s called a porro prism. This configuration reverses the ray’s direction, with the light beam entering and exiting from the same face, resulting in a reverted or inverted image depending on the prism’s orientation.
Additionally, a right-angle prism can be used as a dove prism, producing a 180-degree rotation. Again, the image may be reverted or inverted depending on the prism’s orientation.