Question

If an optician prescribes a corrective lens of power -2.0 D, the required lens

• A. is a concave lens.
• B. is a convex lens.
• C. has a focal length of +50 cm.
• D. has a focal length of -50 cm.

Hint:

A negative power corresponds to a concave lens, and a positive power corresponds to a convex lens.

Answer 1

The Correct Option is A, D

Step 1: Understanding the power of a lens. 
The power \( P \) of a lens is related to its focal length \( f \) by the formula: \[ P = \frac{1}{f} \] where \( f \) is in meters and \( P \) is in diopters (D).

Step 2: Analyzing the given power. 
For a lens with power \( P = -2.0 \, D \), we can calculate the focal length \( f \): \[ f = \frac{1}{P} = \frac{1}{-2.0} = -0.5 \, \text{m} = -50 \, \text{cm} \] The negative sign indicates that the lens is a concave lens.

Step 3: Conclusion. 
The correct answer is: 

(A) is a concave lens

(D) has a focal length of -50 cm.