Assertion: Intensity of magnetic field due to a solenoid near its ends is only half of its magnitude inside the solenoid.
Reason: If solenoid is made very long then magnetic field inside the solenoid becomes uniform.

Options

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

Answer

To analyze the given assertion and reason, we will break down the concepts step by step.

### Step 1: Understanding the Assertion
The assertion states that the intensity of the magnetic field due to a solenoid near its ends is only half of its magnitude inside the solenoid.

- **Explanation**: Inside a long solenoid, the magnetic field is uniform and has a constant value, denoted as \( B \). Near the ends of the solenoid, the magnetic field lines begin to spread out, and thus the intensity of the magnetic field decreases. The assertion claims that this intensity at the ends is half of that inside the solenoid.

### Step 2: Understanding the Reason
The reason provided states that if a solenoid is made very long, then the magnetic field inside the solenoid becomes uniform.

- **Explanation**: This is a well-known fact in electromagnetism. For an ideal long solenoid, the magnetic field inside is uniform and is given by the formula:
\[ B = \mu_0 n I \]
where \( \mu_0 \) is the permeability of free space, \( n \) is the number of turns per unit length, and \( I \) is the current flowing through the solenoid. The uniformity of the magnetic field is a result of the solenoid being long enough that the end effects are negligible.

### Step 3: Evaluating the Assertion and Reason
Now, we need to evaluate whether the assertion is correct and whether the reason correctly explains the assertion.

- **Assertion Evaluation**: The assertion that the intensity of the magnetic field near the ends of the solenoid is half of that inside is not universally true. The intensity can vary depending on the length of the solenoid and its configuration. Thus, the assertion is **incorrect**.

- **Reason Evaluation**: The reason is correct; a long solenoid does produce a uniform magnetic field inside. However, it does not provide a valid explanation for the assertion since the assertion itself is incorrect.

### Conclusion
Based on the evaluations:
- The assertion is **incorrect**.
- The reason is **correct** but does not explain the assertion.

Thus, the correct answer is **D**: The assertion is incorrect, and the reason is correct.