Telescopes and spectrographs

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# 2.1.1 Prisms and the refraction of light

The simplest way to disperse light is to use a prism. When light enters a prism, it is no longer travelling in a vacuum, and its speed decreases. If the incident wavefront is travelling at an angle to the surface of the prism, which is easy to arrange because of its angled faces, then the propagation of the part of the wavefront in the prism is retarded, thus bending the wavefront and changing its direction of propagation through the prism (Figure 15). This phenomenon is referred to as refraction.

Figure 15 The wavefront AC is incident upon the surface AC. In the time that it takes the wavefront to travel the distance CC in material 1, the wavefront has travelled a shorter distance AA′ in material 2, thus changing the direction of propagation. The distance AA′ depends on the speed of light in material 2, which depends on the frequency of the radiation, and hence the amount of refraction also depends on frequency

The speed of light in most materials depends on frequency, so the change in direction also depends on frequency, and hence different colours become separated. Figure 16 illustrates the situation when a beam of white light (i.e. a mixture of all colours) encounters a triangular glass prism. The white light is dispersed at the air-glass boundary and, because of the shape of the prism, the different colours undergo further dispersion at the glass-air boundary as they leave the prism.

Figure 16 A beam of white light enters a triangular prism as shown. Red light has a lower frequency (longer wavelength) than violet light and the direction of the red beam is altered less than the direction of the violet beam. Consequently the white light spreads into its constituent colours within the prism. The different colours are further dispersed on leaving the prism
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