Question

Two thin lenses of focal length +10cm and -5cm are kept in contact.what is the focal length

Answer 1

Let 10cm be f1 and - 5cm be f2
Using the formula,
1/f=1/f1+1/f2
1/f= 1/10-1/5
1/f= 1-2/10
1/f = - 1/10
On taking reciprocal
Final focal length is equal to - 10cm
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