Dispersion (optics) in the context of Optics

Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahertz. The visible band sits adjacent to the infrared (with longer wavelengths and lower frequencies) and the ultraviolet (with shorter wavelengths and higher frequencies), called collectively optical radiation.

In physics, the term "light" may refer more broadly to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. The primary properties of light are intensity, propagation direction, frequency or wavelength spectrum, and polarization. Its speed in vacuum, 299792458 m/s, is one of the fundamental constants of nature. All electromagnetic radiation exhibits some properties of both particles and waves. Single, massless elementary particles, or quanta, of light called photons can be detected with specialized equipment; phenomena like interference are described by waves. Most everyday interactions with light can be understood using geometrical optics; quantum optics, is an important research area in modern physics.

The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light (or simply light).The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well, known collectively as optical radiation.

A typical human eye will respond to wavelengths from about 380 to about 750 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 400–790 terahertz. These boundaries are not sharply defined and may vary per individual. Under optimal conditions, these limits of human perception can extend to 310 nm (ultraviolet) and 1100 nm (near infrared).

In optics, a dispersive prism is an optical prism that is used to disperse light, that is, to separate light into its spectral components (the colors of the rainbow). Different wavelengths (colors) of light will be deflected by the prism at different angles. This is a result of the prism material's index of refraction varying with wavelength (dispersion). Generally, longer wavelengths (red) undergo a smaller deviation than shorter wavelengths (blue). The dispersion of white light into colors by a prism led Sir Isaac Newton to conclude that white light consisted of a mixture of different colors.

Triangular prisms are the most common type of dispersive prism. Other types of dispersive prism exist that have more than two optical interfaces; some of them combine refraction with total internal reflection.

In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction. How much a wave is refracted is determined by the change in wave speed and the initial direction of wave propagation relative to the direction of change in speed.

Optical prisms and lenses use refraction to redirect light, as does the human eye. The refractive index of materials varies with the wavelength of light, and thus the angle of the refraction also varies correspondingly. This is called dispersion and allows prisms and raindrops in rainbows to divide white light into its constituent spectral colors.

A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.

Rainbows can be full circles. However, the observer typically sees only an arc formed by illuminated droplets above the ground, and centred on a line from the Sun to the observer's eye.

Spectroscopy is the field of study that measures and interprets electromagnetic spectra as it interacts with matter. In narrower contexts, spectroscopy is the precise study of color as generalized from radiated visible light to all bands of the electromagnetic spectrum.

Spectroscopy, primarily in the electromagnetic spectrum, is a fundamental exploratory tool in the fields of astronomy, chemistry, materials science, and physics, allowing the composition, physical and electronic structure of matter to be investigated at the atomic, molecular and macro scale, and over astronomical distances.

In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing. Since the focal length of the lens varies with the color of the light, different colors of light are brought to focus at different distances from the lens or with different levels of magnification. Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

In the physical sciences, spectrum describes any continuous range of either frequency or wavelength values. The term initially referred to the range of observed colors as white light is dispersed through a prism — introduced to optics by Isaac Newton in the 17th century.

The concept was later expanded to other waves, such as sound waves and sea waves that also present a variety of frequencies and wavelengths (e.g., noise spectrum, sea wave spectrum). Starting from Fourier analysis, the concept of spectrum expanded to signal theory, where the signal can be graphed as a function of frequency and information can be placed in selected ranges of frequency. Presently, any quantity directly dependent on, and measurable along the range of, a continuous independent variable can be graphed along its range or spectrum. Examples are the range of electron energy in electron spectroscopy or the range of mass-to-charge ratio in mass spectrometry.

The optical properties of a material define how it interacts with light. The optical properties of matter are studied in optical physics (a subfield of optics) and applied in materials science. The optical properties of matter include:

• Refractive index
• Dispersion
• Transmittance and Transmission coefficient
• Absorption
• Scattering
• Turbidity
• Reflectance and Reflectivity (reflection coefficient)
• Albedo
• Perceived color
• Fluorescence
• Phosphorescence
• Photoluminescence
• Optical bistability
• Dichroism
• Birefringence
• Optical activity
• Photosensitivity

A basic distinction is between isotropic materials, which exhibit the same properties regardless of the direction of the light, and anisotropic ones, which exhibit different properties when light passes through them in different directions.

An Amici prism, named for the astronomer Giovanni Battista Amici, is a type of compound dispersive prism used in spectrometers. The Amici prism consists of two triangular prisms in contact, with the first typically being made from a medium-dispersion crown glass, and the second from a higher-dispersion flint glass. 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 (colour) of light, the centre wavelength, exits the prism parallel to (but offset from) the entrance beam. The prism assembly is thus a direct-vision prism and is commonly used as such in hand-held spectroscopes. Other wavelengths are deflected at angles depending on the glass dispersion of the materials. Looking at a light source through the prism thus shows the optical spectrum of the source.

By 1860, Amici realized that 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. This doubling of the original prism increases the angular dispersion of the assembly and also has the useful property that the centre 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 translation (i.e. transverse displacement or offset) away from the incident ray's path.