Question

a concave lens of focal 15 cm forms an image 15 cm from the lens how far is the object placed from the lens​

Answer 1

Explanation:

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Answer 2

Focal length of the lens(f) = -15cm

(focal length of concave lens is negative ).

Image distance(v) = -15cm

(the object is kept on the lest side of the lens).

Object distance(u) = ?

So, from the lens formula,

1/f = 1/v - 1/u.

1/-15 = 1/-15 - 1/u.

-1/15 = -1/15 - 1/u.

1/u = -1/15 +1/15.

(Transposing)

1/u = 0.

u = 1/0 .

So, here the value of u cannot be defined and hence the object is kept at infinity.