Ray Optics

JEE Main level, single correct, medium

136 of 295

Question 136 of 295

Two thin convex lenses of focal length 30 cm and 10 cm are placed coaxially, 10 cm apart. The power of this combination is :

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Answer: (D) 10 D

f1=30 cm,f2=10 cm\mathrm{f}_1=30 \mathrm{~cm}, \mathrm{f}_2=10 \mathrm{~cm}
1feq=1f1+1f2−df1f2, d=\frac{1}{\mathrm{f}_{\mathrm{eq}}}=\frac{1}{\mathrm{f}_1}+\frac{1}{\mathrm{f}_2}-\frac{\mathrm{d}}{\mathrm{f}_1 \mathrm{f}_2}, \mathrm{~d}= distance between lens
1feq=10.3+10.1−0.1(0.3)(0.1)\frac{1}{\mathrm{f}_{\mathrm{eq}}}=\frac{1}{0.3}+\frac{1}{0.1}-\frac{0.1}{(0.3)(0.1)}
1feq=10.1\frac{1}{\mathrm{f}_{\mathrm{eq}}}=\frac{1}{0.1}
Power =1feq=10D=\frac{1}{\mathrm{f}_{\mathrm{eq}}}=10 \mathrm{D}

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