• # question_answer A 35 mm film is to be projected on a 20 m wide screen situated at a distance of 40 m from the film-projector. Calculate the focal length of projection lens. A) 70 mm B) 35 mm C) 40 mm D) 20 mm

[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.