IIT JEE , AIEEE Forum - Physics , Optics

Mr.IITIAN007

A spherical lens of focal length f produces a virtual erect image n times the size of the object.Then what is the the distance of the object from lens if the lens is :------
i)Concave;
ii)Convex.

titun

I think the virtual erect image should be 'n' times the size of the object. Assuming that I'm correct I'm giving the solution as follows:

I'm using the old sign convention i.e the direction opposite to the incident light rays is positive.

So, according to my convention the focal length of the convex lens is negative & that of concave lens is positive.

Let the object distance be u. Since linear magnification of the object is n, the image distance is nu ( Here both the object and image distance is positive by my sign convention )

So, for concave lens, the equation involving u is,

1/nu - 1/u = 1/f

u = f (1-n) / n

and for convex lens,

1/nu - 1/u = - 1 / f

u = f (n-1) / n

The above two values of u are ur respective answers.

Note that for concave lens n < 1

and for convex lens n > 1 in case virtual image is produced in both the cases.

Rate me !!

cvramana

In the case of convex lens, 1/v - 1/u = - 1/f and n = -v/u, solve.

In the case of concave lens 1/v - 1/u = 1/f and n = v/u, solve.

Mr.IITIAN007

Thanks.

Ask & Discuss Questions with Community & Experts