What is a Prism?

An optical prism, is a piece of glass or other transparent material cut with flat, polished surfaces, useful for reflecting light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as optical prisms. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, acrylic, and fluorite. A dispersive prism can be used to break white light up into its constituent spectral colors the colors of the rainbow as described in the following section. An ordinary triangular prism as shown in the figure above can separate white light into its constituent colors called a spectrum. As a beam of light encounters the surface of a prism, it slows a bit. The slowing changes the angle at which the light moves. The light bends again as it exits the prism. The angle between the two surfaces is called the refracting angle and how much the medium bends the light is its refractive index. As Each colour, or wavelength, making up the white light is bent, or refracted, a different amount; the shorter wavelengths those toward the violet end of the spectrum are bent the most, and the longer wavelengths those toward the red end of the spectrum are bent the least. Prisms of this kind are used in certain spectroscopes, instruments for analyzing light and for determining the identity and structure of materials that emit or absorb light.

Construction of Prism

A prism is a solid object with identical flat faces and the same cross-section all along its length. However, in geometry, a prism is a polyhedron comprising an n-sided polygon base, a second base which is a translated copy rigidly moved without rotation of the first, and n other faces, necessarily all parallelograms, joining corresponding sides of the two bases. There are two types of faces in a prism. The top and bottom faces are identical and are called bases. A prism is named after the shape of these bases. For example, if a prism has a triangular base it is called a triangular prism, a prism with a pentagonal base is called a pentagonal prism. All cross-sections parallel to the bases are translations of the bases. The faces other than the top and bottom of a prism are called lateral faces. Prisms can also reverse the direction of light by internal reflection, and for this purpose, they are useful in binoculars. Here internal reflection means when light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected into the same medium and partly refracted to the second medium.

Types of Prism

Different types of prisms noted below can be used to reflect light, or to split light into components with different polarizations.

Amici prism

The Amici prism consists of two triangular prisms in contact, with the first typically being made from medium-dispersion crown glass, and the second a higher-dispersion flint glass. Crown glass is a type of optical glass used in lenses and other optical components. It has a relatively low refractive index (≈1.52) whereas flint glasses have refractive indices ranging between 1.45 and 2.00. An Amici prism, named for the astronomer Giovanni Battista Amici, is a type of compound dispersive prism used in spectrometers. Light entering the first prism is refracted at the first air-glass interface, refracted again at the interface between the two prisms, and then exits the second prism at near-normal incidence. The prism angles and materials are chosen such that one wavelength color of light, the center wavelength, exits the prism parallel to the entrance beam. The prism assembly is thus a direct-vision prism and is commonly used as such in hand-held spectroscopes.

One can join this type of prism back-to-back with a reflected copy of itself, producing a three-prism arrangement known as a double Amici prism as shown in the above figure. This doubling of the original prism increases the angular dispersion of the assembly, and also has the useful property that the center wavelength is refracted back into the direct line of the entrance beam. The exiting ray of the center wavelength is thus not only undeviated from the incident ray but also experiences no displacement or offset away from the incident ray's path.

Pellin–Broca prism

The prism consists of a four-sided block of glass shaped as a right prism with 90°, 75°, 135°, and 60° angles on the end faces. Light enters the prism through face AB as shown in the figure below, undergoes total internal reflection from face BC, and exits through face AD. Total internal reflection is the optical phenomenon in which waves arriving at the interface from one medium to another are not refracted into the second medium but completely reflected into the first medium. The refraction of the light as it enters and exits the prism is such that one particular wavelength of the light is deviated by exactly 90°. As the prism is rotated around an axis O, the line of intersection of the bisector of ∠BAD and the reflecting face BC, the selected wavelength which is deviated by 90° is changed without changing the geometry or relative positions of the input and output beams.

The prism is commonly used to separate a single required wavelength from a light beam containing multiple wavelengths, such as a particular output line from a multi-line laser due to its ability to separate beams even after they have undergone a non-linear frequency conversion. For this reason, they are also commonly used in optical atomic spectroscopy.

Littrow prism

A Littrow prism or Littrow spectrograph or Littrow mirror devised by Otto von Littrow (1843—1864)is a retro-reflecting dispersing prism, i.e., its surface, material, or device reflects light or other radiation to its source. It is arranged in such a way that an incident light beam that enters at the Brewster angle undergoes minimal deviation and hence maximum dispersion. Brewster's angle also known as the polarization angle is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. Littrow prisms are typically 30°/60°/90° prisms, with a reflective film coating on the surface opposite the 60° angle. Typically Littrow prisms are used in lasers at the end of an optical cavity to offer fine adjustment of the laser's output frequency by altering the angle of incidence.

Abbe prism 

An Abbe prism, named for its inventor, the German physicist Ernst Abbe. The prism consists of a block of glass forming a right prism with 30°–60°–90° triangular. When in use, a beam of light enters face AB, is refracted and undergoes total internal reflection from face BC, and is refracted again on exiting face AC as shown in the figure above. The prism is designed such that one particular wavelength of the light exits the prism at a deviation angle relative to the light's original path of exactly 60°. This is the minimum possible deviation of the prism, with all other wavelengths being deviated by greater angles. By rotating the prism in the plane of the diagram around any point O on the face AB, the wavelength which is deviated by 60° can be selected.

Porro prism 

Porro is a type of reflection prism used in optical instruments to alter the orientation of an image. It consists of a block of material shaped like a right geometric prism with right-angled triangular end faces as shown in the figure below. In operation, light enters the large rectangular face of the prism, undergoes total internal reflection twice from the sloped faces, and exits again through the large rectangular face. When the light enters and therefore exits the glass at normal incidence, the prism is not dispersive. An image traveling through a Porro prism is rotated by 180° and exits in the opposite direction offset from its entry point.

While a single Porro prism can be constructed to work as well as a roof prism, it is seldom used as such. Therefore, to reduce the cost of production for a Porro prism, the edge of the roof is usually left out. Sometimes only one small window as an entrance surface and one window as an exit surface are polished. The distinction between a roof prism and a Porro prism is that for the roof prism the roof edge lies in the same plane as the entrance and exit beam, while for a Porro prism the left-out roof edge is orthogonal to the plane formed by the beams. Furthermore, the roof prism has no displacement and a deviation typically between 45° and 90°, while in a single Porro prism the beam is typically deviated by 180° and displaced by a distance of at least one beam diameter. Porro prisms can reflect light rays that are not parallel to the optical axis in such a manner that they are internally reflected off the hypotenuse of the prism. Such an abaxial ray then emerges from the prism having been reflected a third time, thus introducing non-image-forming stray light and reducing contrast. Abaxial reflections can be eliminated by putting a groove or notch across the width of the hypotenuse face center of the prism, which blocks these detrimental reflections. The Porro prism is used in many optical viewing instruments, such as periscopes, binoculars, and monoculars.

Nicol prism

A Nicol prism is a type of polarizer, an optical device made from calcite crystal used to produce and analyze plane-polarized light, it consists of waves in which the direction of vibration is the same for all waves. The Nicol prism consists of two specially cut calcite prisms bonded together with an adhesive known as Canada balsam. This prism transmits waves vibrating in one direction only and thus produces a plane-polarized beam from ordinary light.

An unpolarized light ray enters through the left face of the crystal, as shown in the diagram above, and is split into two orthogonally polarized, differently directed rays by the birefringence property of calcite. The ordinary ray, or o-ray, experiences a refractive index of no = 1.658 in the calcite and undergoes a total internal reflection at the calcite–glue interface because of its angle of incidence at the glue layer, refractive index n = 1.550, exceeds the critical angle for the interface. It passes out the top side of the upper half of the prism with some refraction, as shown. The extraordinary ray, or e-ray, experiences a lower refractive index (ne = 1.486) in the calcite crystal and is not reflected at the interface because it strikes the interface at a sub-critical angle. The e-ray merely undergoes slight refraction, or bending, as it passes through the interface into the lower half of the prism. It finally leaves the prism as a ray of plane-polarized light, undergoing another refraction, as it exits the far right side of the prism. The two exiting rays have polarizations orthogonal at right angles to each other, but the lower, or e-ray, is the more commonly used for further experimentation because it is again traveling in the original horizontal direction, assuming that the calcite prism angles have been properly cut. The direction of the upper ray, or o-ray, is quite different from its original direction because it alone suffers total internal reflection at the glue interface, as well as final refraction on exit from the upper side of the prism. Nicol prisms are widely used in mineralogical microscopy and polarimetry.

Dove prism

A Dove prism is a type of reflective prism which is used to invert an image. Dove prisms are shaped from a truncated right-angle prism. The shape of this prism is similar to the shape described by a Dovetail joint as shown below.

A beam of light traveling parallel to the longitudinal axis, entering one of the sloped faces of the prism undergoes total internal reflection from the inside of the longest bottom face and emerges from the opposite sloped face. Images passing through the prism are flipped mirrored, and because only one reflection takes place, the image is also inverted but not laterally transposed.

Refraction at the entrance and exit surfaces results in substantial image astigmatism when used in convergent light. Thus, the Dove prism is used almost exclusively for images appearing at infinity. Astigmatism is a defect in a lens caused by a deviation from spherical curvature, which results in distorted images, as light rays are prevented from meeting at a common focus.

Uppendahl prism

An Uppendahl prism is an erecting prism, i.e. a special reflection prism that is used to invert an image (rotation by 180°). The erecting system consists of three partial prisms made of optical glass with a high refractive index cemented together to form a symmetric assembly and is used in microscopy as well as in binocular technology.

The Uppendahl prism system is composed of three cemented prisms, with two glass/air transition surfaces. On its way through the first prism, the bundle of rays (red) is first reflected on a surface that is coated with either a metallic or a dielectric coating (mirroring) and a total internal reflection. The other reflections of the beam take place through loss-free total internal reflection. The second prism is a 90° reflection and shouldn't need a mirroring coating. To achieve a complete reversal of the image, a roof edge is ground into the third prism (green). Furthermore, the beam leaves the inversion system without any axial offset, which is why the Uppendahl prism is counted among the straight-vision roof prisms. The net effect of the six reflections (two reflections are on roof plains). Since the light is reflected an even number of times, this produces a 180° image rotation without changing the image's handedness and allows the use of the prism as an image erecting system to flip the image both vertically and horizontally.

An advantage of this prism system is that the light beam only passes two transitions between air and glass, which minimizes losses in the form of Fresnel reflections. In Fresnel reflection when a light beam (e.g. a laser beam) reaches an interface between two different transparent media, it is partly transmitted into the other medium and partly reflected into the original medium.

Applications of prism

A prism is used to separate different colors of light that travel at different speeds inside the glass. Because the colors of light travel at different speeds, they get bent by different amounts and come out all spread out instead of mixed up. Violet travels the slowest so it is on the bottom and red travels the fastest so is on the top. This is because what is called the index of refraction, (the ratio of the speed of light in a vacuum to the speed of light in a material), is increased for the slower-moving waves (i.e. violet). The higher index of refraction means that violet light is the most bent, red is then the least bent because of its lower index of refraction, and the other colors fall somewhere in between. When the air is full of water, like after a rainstorm, the water droplets act like a prism and can make a rainbow. Rainbows are circular because the prisms (raindrops) that created them are spherical. Prisms of this kind are used in certain spectroscopes, and instruments for analyzing light and for determining the identity and structure of materials that emit or absorb light.