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GUJCET · Physics · Ray Optics and Optical Instruments
A plano convex lens is made of material having refractive index 1.5. The radius of curvature of surface is 60 cm . The focal length of the lens is ________ cm .
- A -60
- B 120
- C 60
- D -120
Answer & Solution
Correct Answer
(B) 120
Step-by-step Solution
Detailed explanation
(B) 120
from lens maker formula,
\(
\begin{aligned}
\frac{1}{f} & =(n-1)\left(\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right) \\
\therefore \frac{1}{f} & =(1.5-1)\left(\frac{1}{\infty}-\frac{1}{-60}\right) \\
\therefore \frac{1}{f} & =0.5 \times \frac{1}{60} \\
\therefore f & =120 \mathrm{~cm}
\end{aligned}
\)
from lens maker formula,
\(
\begin{aligned}
\frac{1}{f} & =(n-1)\left(\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right) \\
\therefore \frac{1}{f} & =(1.5-1)\left(\frac{1}{\infty}-\frac{1}{-60}\right) \\
\therefore \frac{1}{f} & =0.5 \times \frac{1}{60} \\
\therefore f & =120 \mathrm{~cm}
\end{aligned}
\)
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