In optics, a dispersive prism is a prism, normally having the shape of a geometrical triangular prism. It is the most widely-known type of optical prism, although perhaps not the most common in actual use. Triangular prisms are used to disperse light, that is to break light...If two prisms can be arranged so that the face through which light enters is parallel to the face through the light is split , then the combination will work like a glass slab and no dispersion may take place.
Also if a light with single wavelength is incident, no dispersion will take place.
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