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Chapter 9:
Prism Spectrometers
Prism spectrometers are the oldest spectrometers known to man. Seneca, during the first century A.D., and the Chinese even earlier, made observations on the generation of colors by prisms. Newton made some very crude spectral measurements but did not use the prism as a spectrometer. He was investigating refraction (refrangibility, as he said). In 1752 Melvill used a “circular slit” and prism to examine the spectra of burning spirits in a flame. Prisms make use of the fact that the refractive index of all materials changes with wavelength and that light is refracted differently by different refractive indices. Prisms, in the sense of the word used here, are triangular, as shown in Fig. 9.1. It is not required that the prism be triangular, but it is the simplest shape with the fewest surfaces that produces dispersion. Perhaps the earliest prism spectrometer was the rainbow. It had no slit, but the raindrops performed the refractive dispersion, and the eye of the beholder performed the functions of the camera lens and detector. 9.1 Prism Deviation A prism deviates a ray that is incident on it. It is clear from the geometry of Fig. 9.1 that the total deviation of the beam δ is given by δ=(θ ′ 1 −θ 1 )+(θ 2 −θ ′ 2 ). Since each normal is perpendicular to a side of the prism angle α, the external angle of the prism is also equal to α, and the sum of the other interior angles is equal to α. Therefore δ=(θ 1 −θ ′ 1 )+(θ 2 −θ ′ 2 )=(θ 2 +θ 1 )−(θ ′ 1 +θ ′ 2 )=θ 2 +θ 1 −α. This is the general expression for the deviation.
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