A biconvex lens has a focal length 2 3


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a biconvex lens has a focal length 2/3 times the radius of curvature of other faces. calculate the refractive index of lens material

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Final Answer : 7/4

Assuming surrounding medium is air or vacuum.
Steps:
1) Radius of Curvature , R(1) = R (say)
Radius of Curvature, R(2) = -R
n - > refractive index of lens.
f = 2R/3 (given)

2) By Lens Makers Formula,
 \frac{1}{f}  = ( \frac{n}{1}  - 1)( \frac{1}{r(1)}  -  \frac{1}{r(2)} ) \\  =  >  \frac{1}{ \frac{2r}{3} }  = ( n - 1)( \frac{1}{r}  -  \frac{1}{( - r)} ) \\  =  >  \frac{3}{2r}  = (n -1 ) \frac{2}{r}  \\  =  > n - 1 =  \frac{3}{4}  \\  =  > n =  \frac{7}{4}

Hence, Refractive Index of Lens is 7/4.


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