Which instrument is used to measure refractive index in diagnostic work?

• A. Refractometer
• B. Thermometer
• C. Hydrometer
• D. Spectrometer

Answer: (A)

Refractive index is a measure of how much light bends as it passes into a liquid, and in diagnostic work you need a quick, direct read of that property to infer concentration, such as urine specific gravity or plasma proteins. A refractometer is designed for this purpose: you place a drop of the sample on the measuring surface, close the cover, and the device uses a prism and light to compare the angle of refraction to a calibrated scale. The readout directly reflects the refractive index (and is often correlated to urine specific gravity or other concentration measures), making it the ideal tool for this task.

The other instruments serve different purposes: a thermometer measures temperature, which is important because refractive index depends on temperature but isn’t the measurement itself; a hydrometer assesses density by buoyancy, not optical refraction; a spectrometer analyzes the spectrum of light to identify components rather than providing a direct refractive-index value. Since the goal is to measure how light bends in the sample, the refractometer is the best choice.