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NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions

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Question : 10
Total: 38
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.
Solution:  
Equivalent focal length of the combination
1
feq
 =
1
f1
+
1
f2

or
1
feq
 =
1
30
+
1
−20
 =
2−3
60
or feq = - 60 cm
Hence, system will behave as a diverging lens of focal length 60 cm
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