The index of refraction of a volatile oil can be determined by which instrument?

• A. Spectrophotometer
• B. Pycnometer
• C. Refractometer
• D. Polarimeter

Answer: (C)

Determining how light bends in the oil—the index of refraction—requires an instrument that directly measures refraction. A refractometer does just that: it passes light into the oil and assesses the angle at which the light is refracted (or the related critical angle) at the boundary with a reference prism. From that measurement, the refractive index is calculated. This method is quick and uses only a small sample, which is ideal for volatile liquids.

Other instruments measure different properties. A spectrophotometer analyzes how much light is absorbed at various wavelengths, which relates to concentration or color rather than bending of light. A pycnometer determines density by comparing masses of a known volume, which is a separate physical property. A polarimeter measures how much the plane of polarized light is rotated as it passes through a chiral substance, giving optical activity rather than refractive index.