An optical prism is a transparent optical device that is engineered to bend light by utilising flat, polished surfaces. To qualify as a prism, it must have at least one angled surface, as prisms with two parallel surfaces are not considered prisms. The triangular prism is the most recognisable type of optical prism, with triangular sides and a rectangular base. While some prisms follow geometric principles, not all geometric prisms qualify as optical prisms. These prisms can be constructed from any transparent material that is capable of transmitting the wavelengths for which they are intended, such as acrylic, glass and fluorite.
A normal dispersive prism is capable of separating white light into its various spectral colours, also known as the colours of the rainbow. Other prism types can be utilised for reflecting light or dividing it into elements with diverse polarisations. The refractive index is dependent on wavelength; hence, dispersive prisms are utilised to break down light into its constituent spectral colours. As white light is a combination of different wavelengths, each wavelength is bent in a different manner as it enters the prism. Blue light is slowed down more than red light and hence experiences greater bending compared to red light.
Prisms that reflect light are utilised to perform various functions such as flipping, inverting, rotating, deviating or displacing the light beam. One common application is erecting the image in single-lens reflex cameras or binoculars. Without these prisms, the image would appear upside down to the viewer. Reflective prisms employ total internal reflection to achieve an almost perfect reflection of light when the facets are struck at an oblique angle. They are typically crafted from optical glass and are coupled with an anti-reflective coating of input and output facets to minimise light loss, resulting in significantly lower losses compared to metallic mirrors.
Practical Applications of Prisms
Prisms are integral components in the design of various optical equipment due to their ability to alter and manipulate light. One example of a prism used in such applications is the Porro prism, which is composed of two prisms in a single unit. These prisms were first developed in 1850 by their namesake, Ignazio Porro. The Porro prism operates by reflecting light back along its incoming path while simultaneously inverting the light vertically and horizontally.
Optical instruments such as microscopes, telescopes, submarine periscopes and cameras employ prisms to manipulate light in various ways. Telescopes, in particular, use multiple prisms combined into a single unit to help light travel vast distances and reach the eye. The Wollaston prism, for instance, is capable of separating unpolarised and randomly polarised light into linearly polarised light. When light intersects with the two triangular prisms, it splits into two beams, the ordinary and the extraordinary, which bend away from each other. Prisms of this type are also crucial components in CD players, polarisation microscopy and rotational mounts.
Prisms are also utilised by astronauts to determine the distance between the Earth and the Moon during round-trip missions. The Lunar Laser Ranging RetroReflector (LRRR) array, a set of one hundred corner cubes, is an example of a retroreflector used in the Lunar Laser Ranging experiment conducted during the Apollo 14, 15 and 11 missions. These corner cubes function as prisms that reflect light back towards their source, allowing precise measurements of the distance between the Moon and the Earth. Astronauts positioned the LRRR array on the Moon to facilitate this process.
Prisms that manipulate light are widely utilised in architectural designs, making them an essential element in the planning and construction of buildings. Prisms, which are shapes with three or more flat sides and identical cross-sections, are commonly incorporated into architectural structures. In some countries, for instance, triangular prisms are frequently used to construct buildings with roofs designed to prevent snow from accumulating by encouraging it to shed off. In the past, rectangular prisms were the primary shape used in the construction of skyscrapers. Modern architectural projects use various prisms, including hexagonal, rectangular and triangular, as seen in structures like the Burj Khalifa in Dubai.
Wedge prisms are optical devices that deflect monochromatic light at a set angle and can be utilised for beam steering. A Risley prism pair is the most prevalent approach, which involves rotating the prisms to control the beam’s direction within a cone-shaped “field of regard”. Additionally, transparent windows in vacuum chambers or cuvettes can be slightly angled (around 10′ − 1°) to inhibit Fabry-Pérot interferences that would otherwise alter their transmission spectrum, even though it doesn’t reduce reflection.
Eye care professionals use prisms, as well as off-axis lenses, to treat a variety of orthoptics problems, including negative and positive fusion problems, diplopia (double vision) and other connected issues. A corrective lens can displace images, similar to how a prism does, by shifting them off-axis. Prism spectacles, which consist of a single prism, can achieve a relative displacement of the two eyes, thereby correcting hyper-, exo-, eso-, or hypotropia. On the other hand, spectacles with prisms of identical power for both eyes, known as yoked prisms (performance glasses, conjugate prisms or ambient lenses), shift the visual field of both eyes to the same extent.