First slide

Ray optics

Question

Two point objects are placed on principal axis of a thin converging lens. One is 20 cm from the lens and other is on the other side of lens at a distance of 40 cm from the lens. The images of both objects coincide. The magnitude of focal length of lens is

Moderate

A

80cm
B

40cm
C

(80/3)cm
D

(40/3)cm

Solution

for object O1 image is virtual i.e. on the same side of the object
$- \frac{1}{v} + \frac{1}{20} = \frac{1}{f} ...... (1)$
for object O2 image is real

$\frac{1}{v} + \frac{1}{40} = - \frac{1}{f} ....... (2)$
from equation 1 and 2 we get
f = 80/3.

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