Question

Why must the slit width of a prism monochromator be varied to provide constant effective bandwidths but a nearly constant slit width provides constant bandwidth with a grating monochromator?

Short answer

The output wavelength in the case of a prism monochromator is continuously variable so the slit width also must be varied.

Step-by-step solution

Given information

We have to determine reason for the variation in the slit width of prism monochromator to obtain the constant effective bandwidth.

Explanation

The wavelength, that is the output keeps changing continuously for a considerable spectral range in the case of a prism monochromator while in the case of a grating monochromator, the output wavelength remains the same, as the radiation is dispersed in the component wavelength.

As the output wavelength keeps varying, the slit width is also required to be varied in the case of a prism monochromator in order to keep the effective wavelength constant which is not needed in the case of a grating monochromator.