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

Options

A. Refractometer
B. Photometer
C. Voltmeter
D. Polarimeter

Answer

To solve the question regarding the instrument used to measure optical activity, specifically specific rotation, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Optical Activity**:
- Optical activity refers to the ability of a substance to rotate the plane of polarized light. This property is often measured in terms of specific rotation, which is a standardized measure of this rotation.

2. **Identify the Instruments Listed**:
- The question provides four options:
- A) Refractometer
- B) Photometer
- C) Voltmeter
- D) Polarimeter

3. **Analyze Each Instrument**:
- **Refractometer**: This instrument measures the refractive index of liquids and gases. It is not used for measuring optical activity.
- **Photometer**: This device measures the intensity of light or electromagnetic radiation. It does not measure optical activity.
- **Voltmeter**: This instrument measures the voltage (potential difference) between two points in an electrical circuit. It is unrelated to optical activity.
- **Polarimeter**: This is the instrument specifically designed to measure the angle of rotation caused by passing polarized light through an optically active substance. It directly measures specific rotation.

4. **Select the Correct Answer**:
- Based on the analysis, the correct instrument used to measure optical activity (specific rotation) is the **Polarimeter**.

### Final Answer:
The instrument which can be used to measure optical activity, i.e., specific rotation, is the **Polarimeter**.

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