Question

At what distance will the rays coming from the object at infinity converge in a concave mirror of radius of curvature 40 cm?

Answer 1

we know , if light rays coming from infinity converge at a point in principal axis of concave mirror is called focus and it's distance from pole of mirror , parallel to principal axis is called focal length . it is denoted by f .

we also know,
focal length = 1/2 × radius of curvature
= 1/2 × 40 = 20 cm
hence, answer is 20 cm

Answer 2

The Answer is 20cm.

Here we go,
  When the object is at infinity, the rays are parallel to the principal axis. So, the rays parallel to principal axis will converge at the principal focus.
 The distance between pole and principal focus is the focal length.
Focal length = Radius of curvature/2
                       = 20cm

.'. The rays will converge at the distance 20cm
 
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