Assertion: In displacement method of finding focal length of a convex lens its magnification in one position of lens is `+2,` then magnification in another position of lens should be `-1/2.`
Reason:This method can't be applied for a concave lens.

Options

A. If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. If both Assertion and Reason are true but Reason is not the correct explanation of the Assertion.
C. If both Assertion is true, but the Reason is false.
D. If both Assertion is false but the Reason is true.

Explanation

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 in the displacement method of finding the focal length of a convex lens, if the magnification in one position of the lens is +2, then the magnification in another position of the lens should be -1/2.
- In the displacement method, the relationship between the magnifications (m1 and m2) at two different positions of the lens is given by the equation:
\[ m_1 \times m_2 = -1 \]
- If we let \( m_1 = +2 \), we can find \( m_2 \):
\[ 2 \times m_2 = -1 \implies m_2 = -\frac{1}{2} \]
- Thus, the assertion is true.

2. **Understanding the Reason**:
- The reason states that the displacement method cannot be applied to a concave lens.
- This is true because the displacement method relies on the fact that the magnification can be positive or negative depending on the position of the object and the image. For a concave lens, the image formed is always virtual and upright, leading to a positive magnification. Therefore, the relationship \( m_1 \times m_2 = -1 \) cannot be satisfied for a concave lens.
- Hence, the reason is also true.

3. **Conclusion**:
- Both the assertion and the reason are true.
- However, the reason does not correctly explain the assertion, as the assertion is specifically about the magnification relationship for a convex lens.

### Final Answer:
- Both the assertion and the reason are true, but the reason is not the correct explanation for the assertion.

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