Assertion : Two particles of de Broglie wavelength `lamda_1 and lamda_2` combine to form a third particle of wavelength `lamda_3` . Then , `lamda_1 +lamda_2 =lamda_3`.
Reason : If `p_1,p_2 and p_3` are momenta of the three particles , respectively, then `p_1+p_2=p_3`.

Options

A. If both assertion and reason are correct and reason is a correct explanation of the assertion.
B. If both assertion and reason are correct but reason is not the correct explanation of assertion.
C. If assertion is correct but reason is incorrect
D. If assertion is incorrect but reason is correct.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that when two particles with de Broglie wavelengths \( \lambda_1 \) and \( \lambda_2 \) combine to form a third particle with wavelength \( \lambda_3 \), then \( \lambda_1 + \lambda_2 = \lambda_3 \).

2. **Analyzing the Reason**:
The reason given states that if \( p_1, p_2, \) and \( p_3 \) are the momenta of the three particles, then \( p_1 + p_2 = p_3 \). This is a statement of conservation of momentum.

3. **Relating Wavelength and Momentum**:
The de Broglie wavelength \( \lambda \) is related to momentum \( p \) by the equation:
\[ p = \frac{h}{\lambda} \]
where \( h \) is Planck's constant.

4. **Applying Conservation of Momentum**:
From the conservation of momentum:
\[ p_1 + p_2 = p_3 \]
Substituting the expression for momentum in terms of wavelength:
\[ \frac{h}{\lambda_1} + \frac{h}{\lambda_2} = \frac{h}{\lambda_3} \]

5. **Simplifying the Equation**:
Dividing through by \( h \):
\[ \frac{1}{\lambda_1} + \frac{1}{\lambda_2} = \frac{1}{\lambda_3} \]
Rearranging gives:
\[ \lambda_3 = \frac{\lambda_1 \lambda_2}{\lambda_1 + \lambda_2} \]

6. **Conclusion**:
The assertion \( \lambda_1 + \lambda_2 = \lambda_3 \) is incorrect. The correct relationship is \( \lambda_3 = \frac{\lambda_1 \lambda_2}{\lambda_1 + \lambda_2} \). However, the reason about the conservation of momentum is correct.

### Final Answer:
- The assertion is incorrect, but the reason is correct. Thus, the correct option is: "Assertion is incorrect or reason is correct."