Different instruments are designed to measure different physical properties. This question simply asks what property an Abbe refractometer is built to measure — a clue is hidden in its very name.
Step 1: What is the Abbe refractometer? The Abbe refractometer is a standard laboratory instrument used to measure how much light bends when it passes from one medium into another. This bending ability is called the refractive index.
Step 2: Define refractive index. Refractive index (n) = speed of light in vacuum / speed of light in the substance. It is a physical constant unique to each substance and depends on temperature and wavelength of light used.
Step 3: Rule out the other options. Optical rotation is measured by a polarimeter. Absorbance is measured by a spectrophotometer. Conductance is measured by a conductance meter.
Step 4: Pharmaceutical significance. Refractive index is used in the pharmacopoeial identification and purity testing of oils, fats, and liquid excipients. A pure substance has a characteristic refractive index.
Step 5: Confirm. The Abbe refractometer is specifically designed to measure refractive index with high precision and ease.