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NEET 2008
Exam
NEET
Year
2008
Subject
Physics
Chapter
Ray Optics

NEET 2008 · Physics · Ray Optics

Two thin lenses of focal lengths f_1 and f_2 are in contact and...

Two thin lenses of focal lengths f1f_1 and f2f_2 are in contact and coaxial. The power of the combination is

Options

  1. Af1f2\sqrt{\frac{f_1}{f_2}}
  2. Bf2f1\sqrt{\frac{f_2}{f_1}}
  3. Cf1+f2f1f2\frac{f_1+f_2}{f_1 f_2}
  4. Df1+f2f1f2\frac{f_1+f_2}{f_1 f_2}

Correct answer

D. f1+f2f1f2\frac{f_1+f_2}{f_1 f_2}

Step-by-step solution

If two thin lenses of focal lengths f1,f2f_1, f_2 are placed in contact coaxially, then equivalent focal length of combination is 1F=1f1+1f2−0f1f2=1f1+1f2\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{0}{f_1 f_2}=\frac{1}{f_1}+\frac{1}{f_2}Power for the combination is P=1F=1f1+1f2=f1+f2f1f2\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{f_1+f_2}{f_1 f_2}
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