First slide

Ray optics

Question

Two converging lenses are placed 20.0 cm apart, as shown in figure. If the first lens has a focal length of 10.0 cm and the second has a focal length of 20.0 cm, locate the final image formed of an object 30.0 cm in front of the first lens.

Easy

A

6.67 cm left of the second lens
B

6.67 cm right of the second lens
C

15.0 cm left of the second lens
D

15.0 cm right of the second lens

Solution

Image due first lens
$\frac{1}{30} + \frac{1}{v} = \frac{1}{10}$
$\frac{1}{v} = \frac{1}{10} - \frac{1}{30} = \frac{2}{30} = \frac{1}{15}$
V = 15 cm
It’s distance from the 2nd lens is 5cm
For second law
$\frac{1}{5} + \frac{1}{v_{f}} = \frac{1}{20}$
$\frac{1}{v_{f}} = \frac{1}{20} - \frac{1}{5} = \frac{1 - 4}{20}$
$v_{f} = \frac{- 20}{3} = - 6.67 c m$

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