Question

an object is placed at a distance x from a convex lens when a real image if formed at a distance of 15 cm from the lens if the focal length of the lens is 10 cm find x

Answer 1

Answer= -30

By applying lens formula which is 1/v - 1/u = 1/f

Answer 2

Given : object distance =x

Image distance=15 cm

Focal length= 10cm

To find : value if object distance x

Solution:

We know that,

Object distance is negative for convex lens

Object distance is negative for convex lensImage distance and focal length for convex lens is positive.

Here ,f = focal length

v= Image distance

u= object distance

Now ,

Using formula for convex lens:

I/f= 1/v - 1/ u

1/10= 1/15-1/x

Now transposing :

1/x= 1/15 -1/10

1/x= 2-3/30 (LCM OF 15 and 10 is 30)

1/x= -1/30

Reciprocate both sides:

x= -30 cm

Answer : object distance x for convex lens is = -30 cm