Question

A convex lens held some distance above
a 6 cm long pencil produces its image
of SOME size. On shifting the lens by a
distance equal to its focal length, it again
produces the image of the SAME size as
earlier. Determine the image size.​

Answer 1

The Size of the image will be 2 cm

Explanation:

According to the given condition in the question,  

f/f+u   = - f/f+u+f

By solving this for f we get,

f = (-2/3 )u             …….. (i)

we know that,

1/f = 1/v – 1/u      ....….. (ii)

Put equation (i) in (ii) and solve for v

v = u/3

Now,  

Magnification = v/u = (u/3)/u  

M = 1/3

Now,

1/3 = v/6

v = 2 cm