A lantern gives an image of 3m square of a slid... - WAEC Physics 1998 Question
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
correct option: c
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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