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NCERT Class XII Chapter
Ray Optics and Optical Instruments
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Question : 10 of 38
Marks: +1, -0
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
1feq\frac{1}{f_{eq}} = 1f1+1f2\frac{1}{f_1} + \frac{1}{f_2}
or 1feq\frac{1}{f_{eq}} = 130+1−20\frac{1}{30} + \frac{1}{-20} = 2−360\frac{2-3}{60}
or feqf_{eq} = - 60 cm
Hence, system will behave as a diverging lens of focal length 60 cm
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