A) 0.5m
B) 1.0m
C) 1.5 m
D) 2.0 m
Correct Answer: B
Solution :
Let a large convex lens is placed between two walls at a distance x from wall on which an electric bulb is fixed. Using lens formula, \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\] Putting\[u=x\] and \[v=4-x\] \[\therefore \] \[\frac{1}{f}=\frac{1}{4-x}-\frac{1}{-x}\] Or \[\frac{1}{f}=\frac{x+4-x}{(4-x)(x)}\] Or \[\frac{1}{f}=\frac{4}{(4-x)(x)}\] Or \[f=\frac{(4-x)(x)}{4}\] ?.. (i) Now magnification, \[m=\frac{v}{u}=\frac{4-x}{x}\] or \[1=\frac{4-x}{x}\] or \[x=4-x\] or \[2x=4\] or \[x=2m\] Hence, from equation (i), \[f=\frac{(4-2)(2)}{4}=\frac{2\times 2}{4}=1m\]You need to login to perform this action.
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