FT-IR optics — functional role of the Michelson interferometer In Fourier-transform infrared (FT-IR) spectroscopy, what is the primary function of the Michelson interferometer within the instrument?

Introduction / Context:
FT-IR instruments differ from dispersive spectrometers by encoding spectral information in the time domain. The Michelson interferometer creates an interferogram by varying the optical path difference between two arms, translating high optical frequencies into a manageable modulation pattern that detectors can measure with high signal-to-noise ratio.

Given Data / Assumptions:

• The IR source is broadband (polychromatic).
• Moving mirror changes path length continuously.
• Fourier transformation reconstructs the spectrum from the interferogram.

Concept / Approach:

The beam is split by a beamsplitter, reflected by fixed and moving mirrors, and recombined at the detector. As the moving mirror scans, constructive/destructive interference generates an intensity vs. time (or mirror position) signal known as the interferogram. After passing through the sample, the interferogram encodes the sample’s spectral absorptions, which are retrieved by a Fourier transform to yield the frequency-domain spectrum.

Step-by-Step Solution:

Verification / Alternative check:

Interferogram features (central burst at zero path difference) and throughput advantages (Jacquinot/Fellgett) confirm modulation and multiplex benefits compared to dispersive devices.

Why Other Options Are Wrong:

Option A describes prisms/gratings, not interferometers. Option B is filtering, not modulation. Option E is an optical alignment function but not the defining role. “None of the above” ignores the central FT-IR principle.

Common Pitfalls:

Equating interferometers with wavelength splitters; forgetting that spectral resolution comes from maximum path difference, not slit width.

Final Answer:

Modulate the broadband IR signal into a low-frequency interferogram (time-domain signal) detectable by the sensor