2.4 Dispersing Element

2.4 Dispersing Element

An array of light arranged by order of wavelength is called a spectrum. Spectroscopy is defined as the study of the dispersion of light into its spectrum. There are two types of dispersing elements, the prism and the diffraction grating.

Figure 2.4.1 shows the types of dispersing elements. The optical mechanism of prisms and diffraction gratings are shown in Figure 2.4.2 and Figure 2.4.3 respectively.

(1) Prism
A prism designed for spectroscopy is called a dispersing prism, which is based on the theory that refractive index is different depending on the wavelength, as shown in Figure 2.4.4. The spectral resolution of a prism is much lower than that of a diffraction grating. If higher spectral resolution is required, a layer prism should be produced. This can be a problem, because it is rather difficult to prepare homogeneous material and to keep the weight low.

(2) Diffraction grating
A diffraction grating is a diffraction element which utilizes the theory that incident light to a grating is dispersed in multiple different directions depending on the difference of light path length or phase difference between two neighboring gratings. Multiple spectra are generated in integer order direction in which multiplication by the wavelength corresponds to the light path difference as shown in Figure 2.4.5. Most diffraction gratings are reflection type rather than transparency type. Though the specular diffraction gives the maximum intensity as 0 order diffraction, it cannot be utilized because 0 order diffraction does not produce a spectrum. Therefore a reflecting plane is adjusted to have a proper angle to obtain a strong enough spectrum at a certain order. Such an adjusted grating is called a blazed grating.