A: The oil-drops of Millikan's experiment should be microscopic size.
R: For larger drops the electric fields needed in the experiment will be impractically high.

Options

A. If both Assertion & Reason are true and the reason is the correct explantion of the assertion , then mark (1)
B. if both Assertion & Reason are true but the reason is not the correct explantion of the assertion , then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the assertion and reason question regarding Millikan's oil drop experiment, we will analyze both the assertion (A) and the reason (R) step by step.

### Step 1: Understand the Assertion (A)
The assertion states that "The oil-drops of Millikan's experiment should be microscopic in size."

**Explanation**:
In Millikan's experiment, tiny oil droplets are used to measure the charge of the electron. These droplets must be small enough to be influenced by electric fields without falling due to gravity. If the droplets are too large, they will not remain suspended in the electric field, making it impossible to measure their charge accurately.

### Step 2: Understand the Reason (R)
The reason states that "For larger drops, the electric fields needed in the experiment will be impractically high."

**Explanation**:
The force acting on the oil droplet due to the electric field (E) is given by \( F = Q \cdot E \), where Q is the charge on the droplet. The weight of the droplet is given by \( W = m \cdot g \), where m is the mass of the droplet and g is the acceleration due to gravity. For larger droplets, the mass (and hence weight) increases, which means a larger electric field would be required to balance the weight of the droplet. This electric field can become impractically high for larger droplets, making it difficult to conduct the experiment.

### Step 3: Conclusion
Both the assertion and reason are correct. The assertion is true because microscopic oil droplets are necessary for the experiment to work effectively. The reason is also true because larger droplets would require impractically high electric fields to balance their weight, which is not feasible in the experimental setup.

### Final Answer
Both the assertion (A) and reason (R) are correct, and the reason (R) is a correct explanation of the assertion (A).

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