Class 12 Physics
CBSE and UP Board
Ray Optics and Optical Instruments
Chapter-9 Exercise 9.10
NCERT Solutions for Class 12 Physics Chapter 9 Question-10
What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.
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Focal length of the convex lens, f1 = 30 cm and
focal length of the concave lens, f2 = -20 cm
Focal length of the system of lenses = f
The equivalent focal length of a system of two lenses in contact is given as:
1/f = 1/f1 + 1/f2
=> 1/f = 1/30 -1/20 = (2-3)/60 = -1/60
Therefore ,
f = -60 cm
Hence, the focal length of the combination of lenses is 60 cm. The negative sign indicates that the system of lenses acts as a diverging lens.