Question

A student has to project a three times magnified image of a candle flame on a wall name the type of the lens required for the purpose .if the candle flame is at a distance of 6 meters from the wall find the focal length of the lens

Answer 1

The student should use convex lens
Distance btwn wall and flame = 6m
Considering sign conventions
Image distance = 6+ object distance
v = 6 + u
m = v/u
-3 = 6+u /u ( m= -3 as three timez magnified image is obtained)
-3u = 6+u
4u = -6
Therefore, u = - 1.5m
And v= 6 - (-1.5)
= 7.5m

Now,
I/v - 1/u = 1/f

1/7.5 - 1/-1.5 = 1/f
1+5/7.5 = 1/f
6/7.5 =1/f

Therefore, f = 7.5/6
= 1.25m
Hence, 1.25m is the required answer