Question

In a concave lens of focal length 15 cm forms an image 10 cm from the lens how far is the object placed from the lens?

Answer 1

Given,

focal length, f = -15 cm

Distance of image, v = -10 cm

Distance of object, u =?

We know that,

1/v - 1/u = 1/f

So, 

1/-10 - 1/u = 1/-15

So, -1/10 + 1/15 = 1/u

(-3 + 2)/30 = 1/u

-1/30 = 1/u

Therefore u = -30 cm

Thus object is placed 30 cm away from the concave lens.

Negative sign shows that object is at 30cm in front of the lens.

Answer 2

$\underline {\Huge \mathfrak {Solution ;}}$

Given,

Focal length (f) = - 15 cm

Image distance (v) = - 10 cm

Object distance (u) =?

We know: From Lens Formula,

1/v - 1/u = 1/f

=> - 1/10 - 1/u = -1/15

=> -1/u = - 1/15 + 1/10

=> - 1/u = - 2 +3/30

=> - 1/u = 1/30

=> u = - 30

Hence, the object should be placed 30 cm from the lens.