A combination of convex and concave lenses has power 4D. If the convex lens has power 5D the focal length of the concave lens will be

Question

A combination of convex and concave lenses has power 4D. If the convex lens has power 5D the focal length of the concave lens will be (A) 100 cm (B)  -100cm  (C)  -1 cm  (D)  -(100/9) cm

Tardigrade Admin · Accepted Answer

Power of combination P=P1+P2 ⇒ 4+5+P2;P2=4-5=-1 D ∴ -1=P2=(100/f(cm))⇒ f=-100 cm

Q. A combination of convex and concave lenses has power 4D. If the convex lens has power 5D the focal length of the concave lens will be

100 cm

$- 100 c m$

$- 1 c m$

$- \frac{100}{9} c m$

Power of combination $P = P_{1} + P_{2}$ $\Rightarrow$ $4 + 5 + P_{2}; P_{2} = 4 - 5 = - 1 D$ $∴$ $- 1 = P_{2} = \frac{100}{f ( c m )} \Rightarrow f = - 100 c m$

Q. A combination of convex and concave lenses has power 4D. If the convex lens has power 5D the focal length of the concave lens will be

100 cm

−100cm

−1cm

−9100​cm

Power of combination P=P1​+P2​ ⇒ 4+5+P2​;P2​=4−5=−1D ∴ −1=P2​=f(cm)100​⇒f=−100cm

$- 100 c m$

$- 1 c m$

$- \frac{100}{9} c m$

Power of combination $P = P_{1} + P_{2}$ $\Rightarrow$ $4 + 5 + P_{2}; P_{2} = 4 - 5 = - 1 D$ $∴$ $- 1 = P_{2} = \frac{100}{f ( c m )} \Rightarrow f = - 100 c m$