Question

A lens of focal length 10 cm forms an image at 15 cm from the lens . Write name of lens. Also draw ray diagram.

Answer 1

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.