If a lens can converge the sun rays at a point 20 cm away from its optical centre, the power of this lens is (a) +2 D (b) -2 D (c) +5 D (d) -5D

🔍 Physics MCQ – Lens and Power

Question:
If a lens can converge the sun rays at a point 20 cm away from its optical centre, the power of this lens is:

• +2 D
• -2 D
• +5 D
• -5 D

✅ Correct Answer: (c) +5 D

🧠 Explanation:

The lens converges sunlight, which consists of parallel rays. This means it must be a convex lens. The point where it converges the rays is the focus, so:

• Focal length (f) = 20 cm = 0.20 m
• Power (P) of a lens is given by:
  P = 100 / f (in cm) or P = 1 / f (in meters)

Calculation:

Since the lens is converging, the power is positive.