Question

Why is a diffraction grating better than a prism?

Answer 1

Prisms can spread light spectra into many colors for analysis. This is often good enough. A diffraction grating does very much the same thing. However, a diffraction grating is less sensitive to the color of the light and can be made to spread colors over a larger angle than a prism.

The glass in a prism is clear to visible light, but it absorbs and blocks light in the infrared and ultraviolet part of the spectrum. Chemical analysis often depends on identifying particular colors from the spectrum of the sample which are beyond the visible range.

The angle of the first-order deflection (`theta`) can be described in terms of the distance between lines (`d`) and the wavelength (`lambda`).
`tan(theta) = (lambda)/d`
A diffraction grating with a few hundred lines per inch can deflect light in the middle of the visible spectrum by at least 20 degrees. The deflection angle of a glass prism is generally much smaller than this.