Question

Question #6dc3b

Answer 1

i don't know if you are asking for mirror or a lens. So i will answer them both

Explanation:

in case of mirror:
mirror formula is `1/f = 1/v + 1/u`
since f is equal to v,that means
`1/f = 1/f + 1/u`.
adding `-1/f` to both sides you get
`0 = 0 + 1/u` or,
`u = 1/0 =oo` , which implies the object was kept at infinity to from image distance equal to focal length.

in case of lens:
lens formula is `1/f = 1/v - 1/u`
since f is equal to v,that means
`1/f = 1/f - 1/u`.
adding `-1/f` to both sides you get
`0 = 0 - 1/u` or,
`u = -1/0 = oo` , which again implies that the object was kept at infinity to from image distance equal to focal length.

So value of focal length is equal to image distance when object was kept at infinity