Question

What is the distance $PB$ called?

• A. Efficient length
• B. Radius of curvature
• C. Equivalent length
• D. Focal length.

Answer 1

Answer: (D)

Focal length.

In case of concave mirror, the position of image and object will coincide if and only if the object is placed at center of curvature of lens.
If B is the center of PA, then PB will be equal to focal length.
Radius of curvature is twice the focal length.