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1 Answer
Chirag Mehta
Mar 2, 2018

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
meritnation

#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#

mini physics
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#

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