The instrument which can be used to measure optical activity, i.e., specific rotation:

Polarimeter is used to measure Optical activity.

A polarimeter is the instrument used to measure optical activity, which refers to the ability of a substance to rotate the plane of polarized light. This property is observed in chiral (optically active) compounds, which can be naturally occurring substances like sugars, amino acids, or synthetic molecules. The specific rotation is a measure of how much a substance can rotate light, and it depends on factors like concentration, path length, and temperature.

Here’s how a polarimeter works:

1. The instrument passes polarized light through a sample of the substance.
2. If the substance is optically active, it will rotate the plane of polarization.
3. The degree of rotation is measured, and from this, the specific rotation of the compound can be determined.

By using a polarimeter, scientists and chemists can determine the concentration and purity of optically active substances, making it a vital tool in fields like pharmaceuticals and food chemistry.

In comparison:

• Refractometer measures the refractive index of a substance, not optical activity.
• Photometer measures the intensity of light.
• Voltmeter is used to measure electric potential (voltage), not optical properties.

Thus, the polarimeter is the correct and specialized instrument for measuring optical activity.