A prism is a piece of any solid transparent material that has been constructed with precise angles and flat surfaces. It is used to analyse and reflect light. A common triangular prism that can split white light into its component colours is known as a spectrum. Each colour, or wavelength, of the white light, bends or refracts at a different angle, with the shorter wavelengths (toward the violet terminal of the spectrum) bending the most and the longer wavelengths (toward the red terminal of the spectrum) bending the least.
When light moves from one medium to another medium, its speed changes, causing the light to be refracted and enter the new medium at a different angle. The degree of bending of the refracted light is calculated by the angle at which the light strikes the medium’s surface and the ratio of refractive indices. The refractive index of various things, such as glass, can change when different wavelengths of light are used. This is why different colours or wavelengths of white light are refracted at different angles.
When a narrow white light beam is shone onto a prism, the advancing light is observed to be made up of seven significant colours: red, orange, yellow, green, blue, indigo and violet. The colour that has the shortest wavelength, violet, bent the most, and red, the colour that has the longest wavelength, bent the least. This effect of separating pure light (white colour) into its individual colours is known as dispersion.
Prisms are specialised optical instruments used to bend and control light beams. They can do more than just disperse light rays; they can invert, rotate, reverse or orient images. Prisms have a variety of uses, including in fields such as ophthalmology, optical instruments, and architecture. They are commonly found in telescopes, binoculars, submarine periscopes and microscopes.
The ability of prisms to bend and shape light makes them crucial in the construction of various optical instruments. Porro prisms are a single unit of two prisms. The Porro prism, invented in 1850 and named after its creator Ignazio Porro, is an optical device that inverts light horizontally and vertically as it retraces its path. Prisms are commonly used in a variety of optical equipment, including microscopes, telescopes, submarine periscopes and cameras. For instance, telescopes use multiple prisms in one unit to manage light travelling long distances to the observer’s eye.
The Wollaston prism is employed to divide unpolarised and random polarised light into linearly polarised light. The meeting point of the two triangular prisms divides light into extraordinary and ordinary beams which diverge from each other. This prism is commonly used in CD players, rotation mounts and polarising microscopy. Astronauts also use prisms to measure the distance between the Earth and the Moon by round trip. One example of this is the Lunar Laser Ranging RetroReflector (LRRR) array, which was used in the Lunar Laser Ranging experiment during Apollo 11, 14 and 15 missions. The array, consisting of 100 corner cubes, was deployed on the Moon by the astronauts. Corner cubes are prisms that reflect light back to their source.
Uses in Ophthalmology
Since the 1800s, ophthalmologists have employed prisms to diagnose and treat conditions such as nystagmus, esotropia, exotropia and amblyopia. They use prisms to bend light to examine different parts of the eye, which helps in identifying problems and detecting eye deficits or disorders. Prisms used in treatment aim to aid the patient’s vision by guiding light into the eye. A corrective lens is made by prisms for diagnosing people with certain vision problems or eye illnesses. Prisms are often utilised in ophthalmic devices as light reflectors. They are commonly found in instruments such as:
- Indirect ophthalmoscope
- Direct ophthalmoscope
- Slit lamp microscope
- Operating microscope
- Goldmann applanation tonometer
- Haidinger brushes