Thu January 24, 2013 By: Puneeta Budhwar

real image of an object is formed on the other side of convex lens on a screen 30 cm from the lens. when the lens is shifted 5cm towards the screen,the screen needs to be shifted by 5cm towards the lens to given another image of the same object.what is the focal length of lens?

Expert Reply
Fri February 01, 2013
this problem can be solved using the lens formula:
1/v - 1/u = 1/f
now first case
v = 30cm
1/u -1/30 =1/f            -1
second case
v' = 30-5-5 = 20cm
u' = u+5
 1/(u+5) -1/20 =1/f    -2
from equation 1 and 2 we get that,
1/(u+5) -1/20 = 1/u -1/30
it can be solved to find u and then f
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