First slide

Ray optics

Question

Image of an object at infinity is formed by a convex lens of focal length 30 cm such that the size of the image is 2 cm. If a concave lens of focal length 20 cm is placed in between the convex lens and the image, at a distance 26 cm from the convex lens, size of the new image is

Moderate

A

2.5cm
B

2.0cm
C

1.025cm
D

1.05cm

Solution

u = 4 cm
First image will act as an object for second lens u = - 4 cm for concave lens
$\frac{1}{u} + \frac{1}{v} = \frac{1}{f}; - \frac{1}{4} + \frac{1}{v} = - \frac{1}{f}, V = 5 cm m = \frac{I}{O} = \frac{v}{u}; I = 0 (\frac{v}{u}) = (2) (\frac{5}{4}) = 2.5 cm$

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