JEE Main 2026 Model Paper 1

A biconvex lens behaves as a plane sheet of glass when its focal length becomes infinite, i.e. when it causes no refraction of light at its curved surfaces.Using the lens maker's formula, the focal length of a thin lens in a surrounding medium is given by $$\frac{1}{f} = \left(\frac{n_{\text{lens}}}{n_{\text{medium}}} - 1\right)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$$Here, $n_{\text{lens}} = 1.47$ (glass) and $n_{\text{medium}}$ is the refractive index of the liquid.For the lens to act as a plane sheet, the light must pass through without bending, which requires $f \to \infty$, so $\frac{1}{f} = 0$.This happens only when $$\frac{n_{\text{lens}}}{n_{\text{medium}}} - 1 = 0$$which gives $n_{\text{medium}} = n_{\text{lens}} = 1.47$.Thus, the liquid must have a refractive index equal to that of the glass.Hence, the correct option is: Equal to that of glass.