Prism (optics)
A prism in optics is a transparent object that refracts and bends light waves as they pass through it, commonly taking the form of a three-sided piece of glass with a triangular cross-section. When light enters a prism, it changes speed and direction, resulting in the phenomenon of refraction. This bending effect is most famously illustrated by the dispersion of white light into a spectrum of colors, creating a rainbow effect. Different types of prisms serve various purposes; dispersive prisms spread light into its constituent colors, while deflecting prisms redirect beams at specific angles, and reflective prisms are used to invert images.
Historically, significant work on prisms was conducted by Sir Isaac Newton, who demonstrated that white light contains various colors rather than them being introduced by the prism itself. Prisms have practical applications in everyday life, particularly in optical devices such as eyeglasses, binoculars, and microscopes, where they help to correct vision or redirect light. Additionally, specialized prisms, like wedge prisms, are utilized in fields such as forestry for practical measurements. The refractive properties of a prism depend on the material's characteristics and the angle at which light enters, influencing how the light behaves as it passes through.
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Subject Terms
Prism (optics)
A prism is an object that slows and bends, or refracts, light waves as they pass through it. The most familiar example of a prism is a piece of cut glass shaped like a three-sided rod with a triangular cross section. Other materials can also act as prisms. An example of this is the rainbow effect created by drops of water around a waterfall.
In abstract terms, a prism is a geometrical figure. Its edges are planes, with two parallel planes in the shape of identical polygons forming the bases. The sides of the prism are parallelograms. Prisms can be described as triangular, rectangular, and so on, depending on the shape of the two parallel planes.
Background
When rays of light move from one medium to another, as happens when they go from passing through air to passing through glass, two things happen. First, the speed of the light changes. At the same time, the direction of the light changes, causing the light to appear to bend. This change in direction is known as refraction.
Visible light contains light of varying wavelengths. Each wavelength carries a different amount of energy and, when viewed in isolation, appears as a different color of the visible spectrum: red, orange, yellow, green, blue, or violet. When a beam of visible light enters a prism, each wavelength of light refracts at a slightly different angle, making it appear that the beam has been split into a rainbow composed of bands of differently colored light. The type of prism that produces this effect is called a dispersing or dispersive prism, because it disperses, or spreads out, the light. Sometimes the effect of dispersion can be problematic. This may happen when one is trying to design a lens to focus light in a particular direction but the material used to make the lens causes the light to disperse in unintended ways, requiring the designer to compensate for the effect.
There are several other types of prisms. Deflecting or deviating prisms can bounce rays of light at specific angles, making it possible to “steer” beams around obstacles. Reflective prisms operate similarly and are usually used to invert images, as in binoculars. A displacing prism, such as a rhomboid prism, can laterally displace a beam of light without inverting it. Polarizing prisms are specially adapted to split light by using materials of different densities.
Overview
Some of the earliest work on prisms was performed by Sir Isaac Newton (1642–1727) starting in 1664, when he was still an undergraduate at Trinity College, Cambridge. Scientists had previously believed that visible light had no color of its own and that the prisms themselves were responsible for introducing extra colors. Newton tested this theory by passing visible light through two prisms. The first split the light into the colors of the spectrum. Newton then caused the red light emerging from the first prism to pass through a second prism, from which it emerged unchanged. This enabled Newton to conclude that the different colors of light were already present in the apparently noncolored light—that is, white light—rather than having been introduced by the prism. In another experiment, he refracted light through one prism to create a spectrum, then passed the refracted light through another prism, causing the different wavelengths to reconverge into white light.
Prisms are used in many different areas of life. The application most people are familiar with is that of eyeglasses or contact lenses. Both of these vision aids function by bending light before it enters the eye, thus compensating for misshapen retinas that would otherwise produce faulty vision. The light enters the denser material of the eyeglass or contact lens and refracts, then exits the lens and passes into the eye, whereupon it refracts again. Optometrists and ophthalmologists are able to craft lenses of exactly the right thickness to correct defects in individual patients’ eyes. Similarly, optical instruments such as binoculars, telescopes, and microscopes frequently use prisms to redirect light inside the device.
One unusual type of prism is used in forestry. One of the duties of foresters is to count how many mature trees are within a particular area. To do this, the forester must have a standard indication of what size a tree must be to count as mature. It would be impractical to measure the circumference of each tree within a large area, so a type of wedge prism is sometimes used instead. A wedge prism is designed to refract light at a particular angle, causing the image it produces to be displaced (moved) by an amount that varies according to the diameter of the tree. By observing how far the image is displaced, foresters viewing trees through a wedge prism can immediately tell whether or not the tree is large enough to be included in the count.
The refractive effect of prisms varies according to the properties of the material. Scientists have developed a measure called the refractive index in order to compare the refractive properties of different materials. The refractive index of a material also depends on the wavelength of the light passing through it. A pane of glass could have a different refractive index for blue light than it does for yellow light, for example.
Of particular relevance to a prism’s function is the angle at which light enters it, known as the angle of incidence. This angle determines in which direction the ray of light will refract. At certain angles, instead of passing through the prism, the ray will reflect inside the prism and return along the same path it arrived, as if the prism were a mirror. When opticians create lenses by grinding glass into different shapes, they are altering the surface shape of the lens in order to change the angle of incidence at which light will enter it.
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