The image of a small electric bulb fixed on the wall of a room is to be obtained on opposite wall 3m away by means of large convex lens.What is the maximum possible focal length for this purpose ??
ans:75cm
but m not able to get it .........
plz TiiTians ............. neone plz explain
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2 Answers
13lvl + lul = 3m
u = -l u l
v = l 3-u l
1/f = 1/(3-u) + 1/u
=> f =(3u-u2)/3
=> f' = 3 -2u
Fr "f" 2b maximum, f'=0 & f"<0.
=> u =1.5m, v=1.5m
=> f= 0.75m
