Question

15. Two lenses of power +2.5D and -3.75 D are combined to form a compound lens, it's focal in cm will be a) 40 b) -40 c) 80​ d) -80

Answer 1

effective power(P) = p1 + p2

here p1 = 2.5 D

P2 = -3.75 D

therefore,

effective power (P) = 2.5+(-3.75)

= 2.5 - 3.75

= -1.25

$p = \frac{100}{f}$

$- 1.25 = \frac{100}{f}$

therefore,

$f = - 80 \: cm$

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