Question

Please answer this question?

Answer 1

See below.

Explanation:

For a biconvex lens we generally take `f=R/2` (by generally i'm actually refering to a 10th gradier in India like me and ujjwal who has asked this question)

Now, using lens maker formula

`1/f = (n-1)(1/R_1 -1/R_2)`

In case `R_1 =R_2`
The formula becomes

`1/f=(n-1)(2/R)`

Generally, we are too ignorant about n (refractive index) and we eliminate `(n-1)` as 1, this is what we have to do here.

Here `n-1=1` , you have to accept it for time being because of absence of proper details.

`1/f=(1/R_1 -1/R_2)` `" "` (`R_2 =-oo`)

`1/f=1/R`

Now recall the ray diagrams , if object is at C the image will also be at C.

Now, since there is a mirror so to form image of object on object itself the object distance should be `10cm`.
Answer `=10cm`