The refractive index of material of a Plano-convex lens, if the radius of curvature of the plane surface $\left(R_1\right)$ is infinite while the radius of curvature of convex surface $\left(R_2\right)$ is $20 \mathrm{c}\mathrm{m}$ and focal length of the lens is $30 \mathrm{c}\mathrm{m}$, will be:

Where is the object placed in reference to the principal focus of a magnifying glass so as the see its enlarges image?

A student did an experiment with a convex lens. He put an object at different distances $25 \mathrm{cm}, 30 \mathrm{cm}, 40 \mathrm{cm}, 60 \mathrm{cm}$ and $120 \mathrm{cm}$ from the lens. In each case he measured the distance of the image from the lens. His results were $100 \mathrm{cm}, 24 \mathrm{cm}, 60 \mathrm{cm}, 30 \mathrm{cm}$ and $40 \mathrm{cm}$, respectively. Unfortunately his results are written in wrong order.

Which of the object distances gives the biggest image?