Which instrument measures the optical activity of a sample?

• A. Refractometer
• B. Spectrometer
• C. Polarimeter
• D. Flame photometer

Answer: (C)

Optical activity is the property of certain chiral substances to rotate the plane of polarized light. A polarimeter is the instrument used to measure this rotation. It sends plane-polarized light through a sample and then uses an analyzer to determine how far the plane has rotated, giving the rotation angle. This observed rotation depends on the path length, the sample concentration, and the substance’s specific rotation at a given wavelength and temperature, summarized by the relation α = [α] l c. This is why polarimetry is essential for assessing enantiomeric purity or identifying a chiral compound.

Other instruments don’t measure this rotation: a refractometer gauges refractive index, which relates to how light bends in a substance but not its polarization rotation; a spectrometer analyzes light’s spectrum, not its polarization; a flame photometer measures elemental emission in a flame.