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Spectra, Refractive Indices, and Wave-Lengths

A beam of light generally consists of a combination of differently-coloured sets of rays; the result of the decomposition of a compound beam into its constituents is called a spectrum. If the beam be derived from an illuminated aperture, and the spectrum consist of a series of distinct images of the aperture, one for each constituent set of rays of the compound light, the spectrum is said to be pure.

A spectroscope is generally employed to obtain a pure spectrum. The following method of projecting a pure spectrum upon a screen by means of a slit, lens, and prism, illustrates the optical principles involved. The apparatus is arranged in the following manner.

The lamp is placed at L, fig. 34, with its flame edgewise to the slit; then the slit s and the lens M are so adjusted as to give a distinct image of the slit at S' on the screen AB; the length of the slit should be set vertical. The prism PQR is then placed with its edge vertical to receive the rays after passing through the lens. All the rays from the lens should fall on the front face of the prism, which should be as near to the lens as is consistent with this condition. The rays will be refracted by the prism, and will form a spectrum A'B' at about the same distance from the prism as the direct image S'. Move the screen to receive this spectrum, keeping it at the same distance from the prism as before, and turn the prism about until the spectrum formed is as near as possible to the position of S', the original image of the slit; that is, until the deviation is a minimum. The spectrum thus formed is a pure one, since it contains an image of the slit for every different kind of light contained in the incident beam.

Last Update: 2011-03-15