Assertion Focal length of a lens depends on the wavelength of light used.
Reason The more the wavelengths, lesser is the focal length.

Options

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

Explanation

To solve the assertion and reason question, we will analyze both the assertion and the reason step by step.

### Step 1: Analyze the Assertion
The assertion states that "the focal length of a lens depends on the wavelength of light used."

**Explanation:**
From the lens maker's formula, we know that the focal length (f) of a lens is related to the refractive index (n) of the material of the lens and the radii of curvature (R1 and R2) of the lens surfaces:
\[ \frac{1}{f} = (n - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]

The refractive index (n) is known to depend on the wavelength (λ) of light. Specifically, as the wavelength increases, the refractive index decreases for most materials. This means that:
\[ n \propto \frac{1}{\lambda} \]
Consequently, if the refractive index decreases with increasing wavelength, the focal length increases with increasing wavelength. Thus, the assertion is true.

### Step 2: Analyze the Reason
The reason states that "the more the wavelength, the lesser is the focal length."

**Explanation:**
This statement is incorrect. As established earlier, the focal length is directly proportional to the wavelength. Therefore, if the wavelength increases, the focal length also increases. Thus, the reason provided is false.

### Conclusion
Based on the analysis:
- The assertion is true: The focal length of a lens does depend on the wavelength of light used.
- The reason is false: The more the wavelength, the greater (not lesser) is the focal length.

### Final Answer
The correct option is: **Assertion is true but reason is false.** (Option C)

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