Monochromator anatomy in UV–Vis/IR instruments: which component serves as the dispersing element to separate wavelengths?

Introduction / Context:
Monochromators isolate narrow wavelength bands from broadband sources. Identifying the dispersing element is crucial for understanding spectral resolution and instrument design.

Given Data / Assumptions:

• Optical train includes slits, mirrors/lenses, and a disperser (prism or grating).
• Modern instruments commonly use ruled or holographic diffraction gratings.
• Slits define bandwidth; lenses/mirrors collimate and focus light.

Concept / Approach:
A disperser spatially separates wavelengths by angular dispersion. Gratings obey the grating equation, producing distinct diffraction orders and enabling tunable selection via rotation.

Step-by-Step Solution:
Entrance slit shapes beam; not dispersive.Collimating optics produce a parallel beam for the grating.Diffraction grating disperses wavelengths; exit slit selects the desired band.Therefore, the dispersing element is the diffraction grating.

Verification / Alternative check:
Instrument manuals specify grating groove density (e.g., 1200 lines/mm) as the key dispersion parameter.

Why Other Options Are Wrong:
Slits and lenses are essential but non-dispersive; detectors measure intensity and do not disperse light.

Common Pitfalls:
Confusing bandwidth control (slits) with dispersion (grating/prism).

Final Answer:
The diffraction grating.