In spectrophotometers, which dispersive element provides linear wavelength dispersion?

• A. Prism
• B. Diffraction grating
• C. Interference filters
• D. Glass filters

Answer: (B)

In spectrophotometers, spreading light into its component wavelengths in a way that maps wavelengths to position on the detector with a nearly constant scale is essential. A diffraction grating achieves this because of the grating equation mλ = d sin θ. Different wavelengths are diffracted at different angles, and over a practical angular range the change in angle with wavelength translates into an almost uniform change in position on the detector. This makes the wavelength axis effectively linear, so calibration and measurement are straightforward.

Prisms, while they separate colors, do so with a dispersion that varies nonlinearly with wavelength because the refractive index changes with wavelength in a nonstraightforward way. Interference filters and glass filters don’t spread light into a spectrum at all; they are used to pass or block specific wavelength ranges, not to create a dispersed spectrum.