Question

A diverging lens of focal length 20 cm is placed
in contact with another diverging lens of focal
length 12 cm. The focal length of the
combination will be
(1) - 7.5 cm
(2) +8.3 cm
(3) + 9.2 cm
(4) + 10.4 cm

please explain with solution or i will report it
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Answer 1

Answer:

1. -7.5 cm

Step-by-step explanation:

when two lens are in contact then net focal length is given by

1/ f=1/ f1+1/F2

1/f= -(1/20+1/12)

1/f=-(3+5)/60

1/f=-8/60=-2/15

thus,f=-15/2=-7.5