Question

a concave mirror has focal length of 15 cm. What will be its radius of curvature ?

Answer 1

radius of curvature = 2* focal length
.....so
r=30 cm

Answer 2

If object is at focus then image formed infinity , real and inverted and highly magnified than the object 2 × Focal length is the formula of radius of curvature so , 2 × 15 = 30