Assertion : Photosensitivity of a metal is high if its work function is small.
Reason : Work function `= hf_(0)` where `f_(0)` is the threshold frequency.

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 the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To analyze the given assertion and reason, let's break it down step by step.

### Step 1: Understanding the Assertion
The assertion states that "Photosensitivity of a metal is high if its work function is small."
- **Photosensitivity** refers to the ability of a material to emit electrons when exposed to light (photons).
- **Work function** is the minimum energy required to remove an electron from the surface of a metal.

### Step 2: Analyzing the Work Function
The work function is defined as:
\[ \text{Work Function} (W) = hf_0 \]
where \( h \) is Planck's constant and \( f_0 \) is the threshold frequency.
- A smaller work function means that less energy is required to eject an electron from the metal surface.

### Step 3: Relating Work Function to Photosensitivity
When the work function is small:
- It requires less energy (or a lower frequency of light) to eject electrons.
- Therefore, the metal can be sensitive to lower energy photons, leading to higher photosensitivity.

### Step 4: Understanding the Reason
The reason provided states that "Work function = hf_0 where f_0 is the threshold frequency."
- This is a correct statement as it relates the work function to the threshold frequency.
- However, it does not directly explain why a smaller work function leads to higher photosensitivity.

### Step 5: Conclusion
Both the assertion and the reason are true:
- The assertion correctly states that a smaller work function leads to higher photosensitivity.
- The reason correctly defines the work function but does not adequately explain the assertion.

Thus, the conclusion is:
- **Assertion is true.**
- **Reason is true, but it is not the correct explanation of the assertion.**

### Final Answer
The correct choice is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

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