A thin lens made of material of refractive index $n_l$ forms the image at the position $I$ of a point object held at $O$ on the central axis of the lens, as is shown in the ray diagram below. Consider that the refractive index of the medium is $n_m$ and $n_l>n_m$ for the figure given. Where does the image form when $n_l

A conducting sphere of radius $a$ initially contains a uniform volume charge density $\rho$. What is the electric flux $\Phi$ through a spherical surface of radius $2 a$ centered at a point on the surface of the charged sphere (see figure) at a later time $t$ ? ( $\epsilon_0$ is the permittivity of free space.)

The figures in the options show the photocurrent $J$ of a photoelectric material versus the frequency $f$ of an incident light beam. The light beam can have three different intensities $I_1, I_2, I_3$, with $I_3>I_2>$ $I_1$. Which of the options is correct?