A : Resolving power of a microscope can be increased by choosing a medium of higher refractive index between object and objective lens.
R : To increase resolving power of microscope, usually on oil having R.I colse to that of objective glass is used.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true and R is not the correct explanation of A
C. A is true and R is false
D. Both A and R are false

Answer

To solve the question, we need to analyze the two statements provided: the assertion (A) and the reason (R).

### Step 1: Understand the Assertion (A)
The assertion states that "the resolving power of a microscope can be increased by choosing a medium of higher refractive index between the object and the objective lens."

**Explanation**:
- The resolving power (RP) of a microscope is defined as the ability to distinguish between two closely spaced objects. It is inversely related to the least distance (d) that can be resolved.
- The formula for resolving power can be expressed as:
\[ RP = \frac{1}{d} = \frac{2n \sin \theta}{1.22 \lambda} \]
where:
- \( n \) is the refractive index of the medium,
- \( \theta \) is the half-angle of the maximum cone of light that can enter the objective lens,
- \( \lambda \) is the wavelength of light used.
- From this formula, we see that as the refractive index \( n \) increases, the resolving power \( RP \) also increases. Thus, the assertion is true.

### Step 2: Understand the Reason (R)
The reason states that "to increase the resolving power of a microscope, usually an oil having refractive index close to that of the objective glass is used."

**Explanation**:
- It is common practice to use immersion oil with a refractive index close to that of the glass of the objective lens to minimize light refraction and maximize the amount of light entering the lens.
- While this statement is true, it does not directly explain why the resolving power increases with a higher refractive index. Instead, it describes a method (oil immersion) that is used to achieve this increase.

### Step 3: Conclusion
- Both statements A and R are true.
- However, R does not correctly explain A because the assertion is about the general principle of increasing resolving power with a higher refractive index, while the reason describes a specific application (using oil) that does not explain the principle itself.

### Final Answer
Both A and R are true, but R is not the correct explanation of A.