Name the instrument used to measure the optical activity of a substance.

Explanation

To answer the question about the instrument used to measure the optical activity of a substance, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Optical Activity**: Optical activity refers to the ability of a substance to rotate the plane of polarized light. This property is commonly exhibited by chiral compounds.

2. **Plane Polarized Light**: The measurement of optical activity involves the use of plane polarized light. Plane polarized light has its electric and magnetic field vibrations confined to a single plane, which is perpendicular to the direction of propagation.

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