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
- 40.3cm
- 26.1cm
-
24.8cm
- 0.7cm
- 0.3cm
The correct answer is: 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 = \(\frac{25440}{1025.44} = 24.8\)