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  4. If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm combined together, what will be their result power ?

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Q. If a convex lens of focal length $80 \,cm$ and a concave lens of focal length $50\, cm$ combined together, what will be their result power ?

JIPMERJIPMER 2006

Solution:

$f_{1}=+80\, cm,\, f_{2}=-50\, cm$
Resultant focal length
$F =\frac{f_{1} f_{2}}{f_{1}+f_{2}}$
$=\frac{80 \times-50}{80-50}=\frac{-4000}{30}$
$=-\frac{4000}{3} cm$
Resultant power $P=\frac{100}{F}=\frac{100}{-400 / 3}$
$=-\frac{3}{4}$
$=-0.75\, D$