Four lenses A, B, C and D of power `+ 100 D , -50D ", 20 D and " 5D`. Which lerses will you use to design a compound microscope for best magnification ?

Options

A. A and C
B. B and D
C. C and D
D. A and B

Answer

To determine which lenses to use for designing a compound microscope with the best magnification, we need to analyze the powers of the given lenses and find the combination that yields the highest total power.

### Step-by-Step Solution:

1. **List the Powers of the Lenses:**
- Lens A: +100 D
- Lens B: -50 D
- Lens C: +20 D
- Lens D: +5 D

2. **Identify Usable Lenses:**
- Since the compound microscope requires converging lenses, we will only consider lenses with positive power. Therefore, we will exclude Lens B (-50 D).

3. **Calculate Combinations of Lenses:**
- We will check the combinations of the remaining lenses (A, C, and D):
- **Combination 1:** A and C
- Total Power = Power of A + Power of C = 100 D + 20 D = 120 D
- **Combination 2:** A and D
- Total Power = Power of A + Power of D = 100 D + 5 D = 105 D
- **Combination 3:** C and D
- Total Power = Power of C + Power of D = 20 D + 5 D = 25 D

4. **Compare the Total Powers:**
- Combination 1 (A and C) gives us 120 D.
- Combination 2 (A and D) gives us 105 D.
- Combination 3 (C and D) gives us 25 D.

5. **Determine the Best Combination:**
- The combination with the highest total power is A and C, which gives us 120 D. This combination will provide the best magnification for the compound microscope.

### Conclusion:
The best combination of lenses to design a compound microscope for maximum magnification is **Lens A (+100 D) and Lens C (+20 D)**.