A lantern gives an image of 3m square of a slide 7.62cm square on a...

WAEC 1998

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

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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 = \(\frac{25440}{1025.44} = 24.8\)

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