Question

What is the refraction index of the material of a plano-convex lens, if the radius of curvature of the convex surface is $10 \,cm$ and focal length of the lens is $30 \,cm$ ?

• A. $\frac{1}{3}$
• B. $1$
• C. $\frac{4}{3}$
• D. $\frac{2}{3}$

Answer 1

Answer: (C)

$ R_{1} =\infty $
$ R_{2} =-10 \,cm $
$ f =30 \,cm $
$ \mu =? $
$ \frac{1}{f} =(\mu-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right) $
$ \frac{1}{30} =(\mu-1)\left(\frac{1}{\infty}-\frac{1}{-10}\right)=\frac{\mu-1}{10} $
$ \mu =1+\frac{1}{3}=\frac{4}{3}$