In a two-monochromator spectroscopic setup, what is the role of the second monochromator?

• A. Selects the wavelength
• B. Amplifies the signal
• C. Prevents stray light from reaching the detector
• D. Stabilizes the light source

Answer: (C)

The key idea is that a second monochromator mainly cleans up the light that reaches the detector. In a two-monochromator setup, the first stage selects a rough band of wavelengths, but some unwanted light—such as wavelengths outside the target window or higher-order diffraction products—can still get through. The second monochromator acts as a second, independent filtering step. Its own dispersion and narrow slit help reject that stray light and preserve only the precise wavelength of interest, dramatically improving spectral purity and reducing background noise at the detector. It doesn’t amplify the signal or stabilize the light source; those roles come from other components.