Question

the image of a distant object is formed at 20cm from a concave mirror what is the focal length of the mirror ? draw a ray diagram for a convex mirror when the object is placed at infinity?

Answer 1

Concave mirror:

   1/v + 1/v = 1/f.
  If u = -∞  then  v = f

So given v = -20 cm. So  f = -20 cm.

A point Image is formed at the focus.
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Convex mirror:

    1/u + 1/v = 1/f
    given u = -∞
    1/v = 1/f
    of v= f.   f is positive.  Image is behind the mirror.

When object is at infinity, then a point image (erect) at the focus is formed.

Answer 2

The image of a distant object is formed at 30 cm from a concave mirror what is the focal length of the mirror