The optical instrument with zero power is

Options

A. microscope
B. telescope
C. eyepiece
D. all the above

Answer

To determine the optical instrument with zero power, we need to understand the concept of optical power and how it relates to focal length.

### Step-by-Step Solution:

1. **Understanding Optical Power**:
- Optical power (P) of a lens is defined as the reciprocal of its focal length (f) in meters:
\[ P = \frac{1}{f} \]
- The unit of optical power is diopters (D).

2. **Condition for Zero Power**:
- For an optical instrument to have zero power, its focal length must be infinite:
\[ P = 0 \Rightarrow f = \infty \]
- This means that the lens does not converge or diverge light rays, which is characteristic of a lens with an infinite focal length.

3. **Identifying the Instrument**:
- The only optical instrument that effectively has an infinite focal length is a telescope. Telescopes are designed to observe distant objects, and they use lenses with long focal lengths to achieve this.
- Other instruments like microscopes and eyepieces are designed to have short focal lengths to magnify nearby objects, thus having high optical power.

4. **Evaluating Options**:
- Microscope: Has high power (short focal length).
- Eyepiece: Also has high power (short focal length).
- Telescope: Has low power (long focal length, potentially infinite).
- Therefore, the correct answer is the telescope.

5. **Conclusion**:
- The optical instrument with zero power is a **telescope**.

### Final Answer:
The optical instrument with zero power is a **telescope**.