For a spherical mirror, the focal length is equal to the radius of curvature of the mirror.
a.

one-fourth

c.

one-half

b.

one-third

d.

the square of

The focal length is equal to one-half the radius of curvature of the mirror.

  concave means having an outline or surface that curves inward like the interior of a circle or sphere. so the mirror curves inward.

The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.