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JIPMER 2006
Exam
JIPMER
Year
2006
Subject
Physics
Chapter
Ray Optics

JIPMER 2006 · Physics · Ray Optics

If a convex lens of focal length 80 cm and a concave lens of focal...

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

Options

  1. A+0.65D+0.65 D
  2. B−0.65D-0.65 D
  3. C+0.75D+0.75 D
  4. D−0.75D-0.75 D

Correct answer

D. −0.75D-0.75 D

Step-by-step solution

f1=+80 cm,f2=−50 cmf_1=+80 \mathrm{~cm}, f_2=-50 \mathrm{~cm}. Resultant focal length F=f1f2f1+f2=80×−5080−50=−400030=−40003 cm.\begin{aligned} F & =\frac{f_1 f_2}{f_1+f_2} \\ & =\frac{80 \times-50}{80-50}=\frac{-4000}{30} \\ & =-\frac{4000}{3} \mathrm{~cm} .\end{aligned} Resultant power P=100F=100−400/3=−34=−0.75D.\begin{aligned} P & =\frac{100}{F}=\frac{100}{-400 / 3} \\ & =-\frac{3}{4} \\ & =-0.75 \mathrm{D} .\end{aligned}
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