A) 70 mm
B) 35 mm
C) 40 mm
D) 20 mm
Correct Answer: A
Solution :
[a]As in case of projector, \[m=\frac{I}{O}=\frac{v}{u}\] |
So, \[-\frac{20\times 100\,cm}{3.5\,cm}=\frac{40\times 100}{u}\] |
i.e., \[u=-7\text{ }cm\] |
i.e., film is at a distance of 7 cm in front of projection lens. |
And from lens formula \[\frac{1}{v}-\frac{1}{u}=\frac{1}{f},\] here we have \[\frac{1}{4000}-\frac{1}{-7}=\frac{1}{f}\] or \[\cong 7\,cm=70\,mm\] |
[as (1/4000) << (1/7)] |
i.e., focal length of projection lens is 70 nun. |
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