If a concave lens of focal length 12cm is used to produce a real image four times the size of the object, how far from the lens must the object be placed?
10 cm
15 cm
20 cm
25 cm
Explanation
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Discussions (12)
Remember the focal point and image distance of a CONCAVE LENS is NEGATIVE
but looking at rhe question, a REAL image is formed and whenever a real image is formed, image distance V is positive so
m=v/u
4=V/u
4u =V
-1/f = 1/u + 1/v
-1/12 = 1/u+1/4u
-1/12 = 4+1/ 4u
-1/12 = 5/4u
12*5 = -4u
-u = 15
we will neglect the minus because the object distance is always positive for both concave and convex mirror
so u=15
option b
I think there is a mistake in the question it's with suppose to be convex lens or concave mirror based on the answers
pls oohhh what is the difference between mirror and lens formula
because am seeing this for lens formula 1/f=1/v-1/u
while this for mirror
1/f=1/v+1/u
and when ever i see lens i get confused and use the lens formula
because the question here is about for lens
so pls direct me
I thought image formed by a convex mirror(v) and the focal length are always negative..
