First slide

Ray optics

Question

A screen is placed at 80 cm from an object. The image of the object on the screen is formed by a convex lens at two different location separated by 20 cm. Focal length of lens in cm is

Moderate

A

$\frac{75}{4}$
B

$\frac{75}{2}$
C

$\frac{35}{4}$
D

75

Solution

focal length in lens displacement method is $f = \frac{L^{2} - D^{2}}{4 L} D = 20 c m a n d L = 80 c m$

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