A lantern gives an image of 3m square of a slide 7.62cm square on a screen. If the screen is 10m from the projection lens of the lantern, calculate the focal length of the lens
-
A.
40.3cm -
B.
26.1cm -
C.
24.8cm -
D.
0.7cm -
E.
0.3cm
Correct Answer: Option C
Explanation
M = (frac{V_H}{U_H} = frac{300}{7.62})
= 39.3
U = (frac{V}{M} = frac{1000}{39.3}) = 25.44
(frac{1}{f} = frac{I}{U} + frac{I}{V} = frac{I}{25.44} = frac{I}{1000} = frac{1025.44}{25440})
f = (frac{25440}{1025.44} = 24.8)