The distance upto which ray optics holds good is called

Options

A. Fresnel distance
B. Fraunhofer distance
C. Optical distance
D. Wave distance

Answer

To solve the question regarding the distance up to which ray optics holds good, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Ray Optics**:
- Ray optics, also known as geometric optics, is a branch of optics that describes light propagation in terms of rays. It is applicable when the wavelength of light is much smaller than the size of the optical elements involved.

2. **Identifying the Relevant Distance**:
- In optics, there are specific distances that define the limits of ray optics. The question asks for the specific term that describes the distance up to which ray optics is valid.

3. **Options Analysis**:
- We have four options:
1. Fresnel distance
2. Fraunhofer distance
3. Optical distance
4. Wave distance

4. **Correct Definition**:
- The correct term that describes the distance up to which ray optics holds good is known as the **Fresnel distance**. This distance is significant in determining when wave effects become important compared to ray effects.

5. **Mathematical Representation**:
- The Fresnel distance (Fz) can be mathematically represented as:
\[ Fz = \frac{a^2}{\lambda} \]
where:
- \( a \) is the radius of the aperture,
- \( \lambda \) is the wavelength of light.

6. **Conclusion**:
- Based on the analysis, the answer to the question is **Fresnel distance**.

### Final Answer:
The distance up to which ray optics holds good is called **Fresnel distance**.

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