In Fraunhofer diffraction experiment, L is the distance between screen and the obstacle, b is the size of obstacle and `lambda` is wavelength of incident light. The general condition for the applicability of Fraunhofer diffraction is :

Options

A. `(b^(2))/(L lambda) gt gt 1`
B. `(b^(2))/(L lambda)=1`
C. `(b^(2))/(L lambda)lt lt1`
D. `(b^(2))/(L lambda) ne 1`

Answer

To determine the general condition for the applicability of Fraunhofer diffraction, we need to analyze the relationship between the size of the obstacle (b), the distance from the obstacle to the screen (L), and the wavelength of the incident light (λ).

### Step-by-Step Solution:

1. **Understanding Fraunhofer Diffraction**:
- Fraunhofer diffraction occurs when both the source of light and the observation screen are at a large distance from the diffracting obstacle. This means that the distances involved (from the source to the obstacle and from the obstacle to the screen) are effectively infinite.

2. **Identifying the Parameters**:
- Let:
- \( b \) = size of the obstacle
- \( L \) = distance from the obstacle to the screen
- \( \lambda \) = wavelength of the incident light

3. **Condition for Fraunhofer Diffraction**:
- The condition for Fraunhofer diffraction can be expressed in terms of the ratio:
\[ \frac{b^2}{L \lambda} \]
- For Fraunhofer diffraction to be applicable, this ratio must be very small. This is because as \( L \) approaches infinity, the term \( L \lambda \) becomes very large, making the ratio \( \frac{b^2}{L \lambda} \) approach zero.

4. **Conclusion**:
- Therefore, the general condition for the applicability of Fraunhofer diffraction is:
\[ \frac{b^2}{L \lambda} \ll 1 \]
- This means that \( b^2 \) must be much less than \( L \lambda \).

### Final Answer:
The general condition for the applicability of Fraunhofer diffraction is:
\[ \frac{b^2}{L \lambda} \ll 1 \]