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  4. A double convex lens has two surfaces of equal radii 15 cm and refractive index μ=1.5, its focal length is equal to

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Q. A double convex lens has two surfaces of equal radii $15\, cm$ and refractive index $\mu=1.5$, its focal length is equal to

Ray Optics and Optical Instruments

Solution:

$\frac{1}{f}=(\mu-1)\left(\frac{1}{R}-\frac{1}{-R}\right)$
$\frac{1}{f}=0.5\left(\frac{2}{15}\right)$
$\therefore f=15 \,cm$