Question

A convex lens of focal length is 2/3 times R . find refractive index of the material

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Answer 1

Answer: Assuming surrounding medium is air or vacuum. n - > refractive index of lens. Hence, Refractive Index of Lens is 7/4.

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Answer 2

$\large\bold{\underline{GIVEN :}}$

•A convex lens of focal length is 2/3 times R

•We need to find the refractive index of the material

$\large\bold{\underline{SOLUTION :}}$

From Lens makers formula we know that,

1/f = (n – 1)(1/R1 – 1/R2) ———(i) { were n is refractive index}

We know that,

For biconvex lens

R1 = + R and R2 = – R

${\red{\boxed{\tt{f= \:2/3 \:R}}}}$ (Given)

Substituting the value of from the above equation in equation (i) we get,

1/ 2/3 R= (n-1) ( 1/ R – 1/ -R)

3/2R = (n -1) (2/R)

(n -1) = 3/4

n = 1 + 3/4

$\red{\boxed{\sf{n = 7/4}}}$

Hence n = refractive index of the material = 7/4 .