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