The source is at some distance from an obstacle. Distance between obstacle and the point of observation is .b. and wavelength of light is `.lambda.`. Then the distance of nth Fresnel Zone will be at a distance……….from the point of observation.

Options

A. `(b n l)/(2)`
B. `b-(n lambda)/(2)`
C. `b+( n lambda)/(2)`
D. `b-n lambda`

Answer

To find the distance of the nth Fresnel Zone from the point of observation, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Fresnel Zones**:
Fresnel zones are regions in the wavefront that contribute to the interference pattern at a point of observation. Each zone is defined by a specific distance from the source.

2. **Identify Given Parameters**:
- Distance between the obstacle and the point of observation = \( B \)
- Wavelength of light = \( \lambda \)

3. **Determine the Distance for the First Fresnel Zone**:
The first Fresnel zone is located at a distance of:
\[ B + \frac{\lambda}{2} \]
This is because the first zone starts at half the wavelength from the point of observation.

4. **Generalize for the nth Fresnel Zone**:
Each subsequent Fresnel zone is spaced by an additional half wavelength. Therefore, the distance for the nth Fresnel zone can be expressed as:
\[ \text{Distance of nth Fresnel Zone} = B + n \cdot \frac{\lambda}{2} \]
where \( n \) is the zone number (1 for the first zone, 2 for the second, etc.).

5. **Final Expression**:
Thus, the distance of the nth Fresnel Zone from the point of observation is:
\[ B + \frac{n \lambda}{2} \]

### Final Answer:
The distance of the nth Fresnel Zone from the point of observation is \( B + \frac{n \lambda}{2} \).

---