Question

According to the thin lens formula, which one of the following is true regarding the focal length of the lens?

• A. \( f \) is positive for concave lens
• B. \( f \) is negative for convex lens
• C. \( f \) is positive for a diverging lens
• D. \( f \) is negative for concave lens

Hint:

For concave lenses (diverging lenses), the focal length is negative, while for convex lenses (converging lenses), the focal length is positive.

Answer 1

The Correct Option is D

Step 1: Thin lens formula.
The thin lens formula is given by: \[ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \] where \( f \) is the focal length, \( v \) is the image distance, and \( u \) is the object distance.
Step 2: Sign convention for focal length.
For a concave lens (diverging lens), the focal length is negative.
For a convex lens (converging lens), the focal length is positive.

Step 3: Identify the correct option.
The correct option is (d) since concave lenses have a negative focal length.