A prism splits light because the refractive index of its material depends on wavelength, causing different wavelengths to change direction by different amounts as they enter and leave the prism. This wavelength dependent refraction is called dispersion. When white light enters a transparent prism, the light slows relative to its propagation in air or vacuum, and the change in speed causes the direction of propagation to change according to Snells law.
Because violet and blue wavelengths generally experience a slightly higher refractive index in ordinary transparent materials than red wavelengths, they are deviated more strongly. The different wavelengths therefore emerge from the prism travelling in different directions, producing a visible spectrum when the light is projected onto a screen or observed against a suitable background. The prism does not create new colours from nothing. It spatially separates wavelength components that were already present in the incident white light.
The refractive index of a material is a measure related to how electromagnetic waves propagate through that material and is commonly defined as the ratio of the speed of light in vacuum to its phase velocity in the material. For normal transparent materials in the visible region, refractive index generally decreases as wavelength increases, so shorter wavelength visible light is refracted more strongly. This relationship is called normal dispersion.
The amount of bending at an interface is determined by Snells law, n1 sin theta1 equals n2 sin theta2, where n1 and n2 are the refractive indices and theta1 and theta2 are the angles measured relative to the normal. A prism has two refracting surfaces, so the total angular deviation results from refraction at entry and refraction at exit. The geometry of the prism determines how strongly the separated colours spread for a given material and wavelength range.
A triangular glass prism is particularly effective at producing visible dispersion because its two non parallel faces cause the beam to undergo a net angular deviation. If a beam entered a flat parallel glass plate at an angle, the exit ray would generally become parallel to the original direction after the two refractions, although it would be laterally displaced. In a prism, the surfaces are angled relative to each other, so the wavelength dependent changes in direction do not cancel.
Red light usually emerges with the smallest deviation and violet with the largest deviation in ordinary crown glass. Between them lie orange, yellow, green and blue components, producing a continuous visible spectrum rather than six physically discrete beams. The apparent division into named colours is partly a matter of human classification because the electromagnetic spectrum varies continuously in wavelength.
The separation of colours depends on both refraction and dispersion. Refraction by itself does not necessarily produce a visible spectrum if all wavelengths have exactly the same refractive index. For example, if a hypothetical material had a wavelength independent refractive index in the relevant range, every wavelength would bend by the same amount and the beam would remain spectrally overlapped. Dispersion means that n changes with wavelength.
The magnitude of dispersion is described using quantities such as the Abbe number for optical glasses, while more detailed descriptions use dispersion equations such as the Sellmeier equation. Optical engineers select materials according to refractive index, dispersion, transmission, absorption and mechanical properties. Crown and flint glasses have historically been used in combinations to control chromatic effects in lenses, illustrating how the same phenomenon that creates a spectrum in a prism can produce unwanted colour fringing in optical instruments.
A prism can demonstrate that white light contains a range of visible wavelengths, but it is important to distinguish this from the idea that the prism colours the light. If the separated spectrum is passed through a second prism oriented appropriately, the colours can be recombined into approximately white light. This demonstrates that the first prism has not permanently transformed the light into fundamentally different coloured substances. The prism has changed the spatial direction of each wavelength component. Newtons classic prism experiments were historically important because they challenged the idea that prisms simply manufacture colour. The experiment helped establish that white light is spectrally heterogeneous and that the visible colours correspond to different components with different refractive behaviour.
Prism dispersion has numerous practical applications beyond classroom demonstrations. Spectroscopes and spectrometers can use dispersive optical elements to separate wavelengths so that their intensities can be measured individually. This enables identification of chemical elements and molecules through their spectral signatures, analysis of stars and planets, monitoring of industrial processes and study of lasers and materials. Prisms have also been used in binoculars, cameras, optical instruments and beam steering systems. Modern spectrometers may use diffraction gratings instead of prisms because gratings can offer different combinations of resolution, dispersion and efficiency. The choice between a prism and grating depends on the application, wavelength range, optical design and required performance.
The degree of separation produced by a prism depends on prism angle, material refractive index, wavelength range, incidence angle and optical geometry. At certain configurations known as minimum deviation conditions, the optical path through a prism has useful symmetry and the refractive index can be determined accurately from measured prism angle and minimum deviation angle. Prism spectroscopy can therefore be used to determine refractive index as a function of wavelength.
Real optical materials also absorb some wavelengths, so the transmitted spectrum may not be equally intense across all colours. At ultraviolet or infrared wavelengths, the material may become opaque or show strong dispersion or absorption features. Consequently, prism behaviour is not identical across the entire electromagnetic spectrum.
The physical explanation of prism dispersion ultimately follows from the interaction of electromagnetic waves with matter. The electric field of light drives bound charges within the material, and their response depends on frequency. This frequency dependent response determines the materials complex refractive index, with its real component governing phase propagation and its imaginary component describing absorption. Dispersion is therefore connected to the microscopic electronic and vibrational structure of the material.
Near absorption resonances, the refractive index can change rapidly with wavelength and may exhibit anomalous dispersion. In ordinary visible light transmission through common optical glass, normal dispersion dominates and shorter wavelengths are deviated more strongly. A prism therefore provides a simple visible demonstration of a deeper principle of electromagnetic material response: different frequencies interact differently with the same substance, producing different propagation speeds and consequently different refraction angles.
