\(\frac{1}{f}=\left(\frac{\mu_{2}}{\mu_{1}}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)\)
\(\mu_{1}\) and \(\mu_{2}\) are the refrative index of medium and material of lens respectively.
If \(\mu_{1}=\mu_{2},\) then the focal length of lens will be infinite. In this case the lens acts as a plane sheet of glass.