An optical prism is a transparent optical element with nonparallel faces that redirect light by refraction or total internal reflection. For a dispersing prism, the wavelength dependence of refractive index makes different wavelengths leave in different directions.
An entrance slit, collimator, prism and viewing screen or detector form a simple spectroscope. A continuous source gives a spread of colors, whereas a narrow emission line gives a narrow image of the slit. Rotating the prism changes the deviation; replacing the source while holding the geometry fixed distinguishes a source optical spectrum from the prism's dispersive response.
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