Assertion (A): We cannot form a $p-n$ junction diode by taking a slab of a $p$-type semiconductor and physically joining it to another slab of a $n$-type semiconductor.

Reason (R): In a $p$-type semiconductor $\eta_e \gg \eta_h$ while in a $n$-type semiconductor $\eta_h \gg \eta_{e^*}$

Assertion (A) : The potential energy of an electon revolving in any stationary orbit in a hydrogen atom is positive.

Reason (R) : The total energy of a charged particle is always positive.

A battery of emf $E$ and internal resistance $r$ is connected to a rheostat. When a current of 2 A is drawn from the battery, the potential difference across the rheostat is $\mathbf{5 ~ V}$. The potential difference becomes 4 V when a current of 4 A is drawn from the battery. Calculate the value of $E$ and $r$.

(a) In a diffraction experiment, the slit is illuminated by light of wavelength 600 nm . The first minimum of the pattern falls at $\theta=30^{\circ}$. Calculate the width of the slit.

OR

(b) In a Young's double-slit experiment, two light waves, each of intensity $I_0$, interfere at a point, having a path difference $\frac{\lambda}{8}$ on the screen. Find the intensity at this point.