Comment on the below statement
Bragg reflection can occur only when `gamma le 2d`.

Explanation

To comment on the statement "Bragg reflection can occur only when \( \gamma \leq 2d \)", we need to analyze the conditions under which Bragg's law is satisfied.

### Step-by-Step Solution:

1. **Understanding Bragg's Law**:
Bragg's law is given by the equation:
\[ n\lambda = 2d \sin \theta \]
where:
- \( n \) is the order of reflection (an integer),
- \( \lambda \) is the wavelength of the incident wave,
- \( d \) is the distance between the crystal planes,
- \( \theta \) is the angle of incidence.

2. **Analyzing the Condition**:
For Bragg reflection to occur, the sine function must yield a valid output. The sine function, \( \sin \theta \), can only take values between -1 and 1. Therefore, we can derive the following condition from Bragg's law:
\[ n\lambda = 2d \sin \theta \implies \sin \theta = \frac{n\lambda}{2d} \]
This implies that:
\[ -1 \leq \sin \theta \leq 1 \]
Thus, we need:
\[ -1 \leq \frac{n\lambda}{2d} \leq 1 \]

3. **Deriving the Wavelength Condition**:
From the above inequality, we can derive:
\[ -2d \leq n\lambda \leq 2d \]
Since \( n \) is a positive integer (for the first order reflection, \( n = 1 \)), we can simplify this to:
\[ 0 \leq n\lambda \leq 2d \]
This leads us to conclude that:
\[ \lambda \leq 2d \]

4. **Conclusion**:
Therefore, the statement "Bragg reflection can occur only when \( \gamma \leq 2d \)" is indeed true, as \( \gamma \) represents the wavelength \( \lambda \). The condition \( \lambda \leq 2d \) is necessary for Bragg reflection to occur.

### Final Comment:
The statement is true because Bragg reflection is only possible when the wavelength of the incident wave is less than or equal to twice the distance between the crystal planes.