(a) Name the parts of the electromagnetic spectrum which are (i) also known as heat waves' and (ii) absorbed by ozone layer in the atmosphere.

(b) Write briefly one method each, of the production and detection of these radiations.

(a) Explain the characteristics of a $p-n$ junction diode that makes it suitable for its use as a rectifier.

(b) With the help of a circuit diagram, explain the working of a full wave rectifier.

Explain the following, giving reasons:

(a) A doped semiconductor is electrically neutral.

(b) In a $p$ - $n$ junction under equilibrium, there is no net current.

(c) In a diode, the reverse current is practically not dependent on the applied voltage.

Dielectrics play an important role in design of capacitors. The molecules of a dielectric may be polar or non-polar. When a dielectric slab is placed in an external electric field, opposite charges appear on the two surfaces of the slab perpendicular to electric field. Due to this an electric field is established inside the dielectric.

The capacitance of a capacitor is determined by the dielectric constant of the material that fills the space between the plates. Consequently, the energy storage capacity of a capacitor is also affected. Like resistors, capacitors can also be arranged in series and/or parallel.

(i) Which of the following is a polar molecule?

(A) $\mathrm{O}_2$

(B) $\mathrm{H}_2$

(C) $\mathrm{N}_2$

(D) HCl

(ii) Which of the following statements about dielectrics is correct?

(A) A polar dielectric has a net dipole moment in absence of an external electric field which gets modified due to the induced dipoles.

(B) The net dipole moments of induced dipoles is along the direction of the applied electric field.

(C) Dielectrics contain free charges.

(D) The electric field produced due to induced surface charges inside a dielectric is along the external electric field.

(iii) When a dielectric slab is inserted between the plates of an isolated charged capacitor, the energy stored in it

(A) increases and the electric field inside it also increases.

(B) decreases and the electric field also decreases.

(C) decreases and the electric field increases.

(D) increases and the electric field decreases.

(iv) (a) An air-filled capacitor with plate area A and plate separation $d$ has capacitance $\mathrm{C}_0$. A slab of dielectric constant K, area A and thickness $\left(\frac{d}{5}\right)$ is inserted between the plates. The capacitance of the capacitor will become

(A) $\left[\frac{4 \mathrm{~K}}{5 \mathrm{~K}+1}\right] \mathrm{C}_0$

(B) $\left[\frac{K+5}{4}\right] C_0$

(C) $\left[\frac{5 \mathrm{~K}}{4 \mathrm{~K}+1}\right] \mathrm{C}_0$

(D) $\left[\frac{K+4}{5 K}\right] C_0$

OR

(b) Two capacitors of capacitances $2 \mathrm{C}_0$ and $6 \mathrm{C}_0$ are first connected in series and then in parallel across the same battery. The ratio of energies stored in series combination to that in parallel is

(A) $\frac{1}{4}$

(B) $\frac{1}{6}$

(C) $\frac{2}{15}$

(D) $\frac{3}{16}$