Modern Physics PYQ — Page 8

Pure Si at $500 \mathrm{~K}$ has equal number of electron $\left(n_e\right)$ and hole $\left(n_h\right)$ concentrations of $1.5 \times 10^{16} \mathrm{~m}^{-3}$. Doping by indium increases $n_h$ to $4.5 \times 10^{22} \mathrm{~m}^{-3}$. The doped semiconductor is of

Photoelectric emission occurs only when the incident light has more than a certain minimum

The threshold frequency for a photo-sensitive metal is $3.3 \times 10^{14} \mathrm{~Hz}$. If light of frequency $8.2 \times 10^{14} \mathrm{~Hz}$ is incident on this metal, the cut-off voltage for the photo-electric emission is nearly

In the following figure, the diodes which are forward biased, are A.  B. C. D.

The half-life of a radioactive isotope $X$ is $50 \mathrm{yr}$. It decays to another element $Y$ which is stable. The two elements $X$ and $Y$ were found to be in the ratio of $1: 15$ in a sample of a given rock. The age of the rock was estimated to be

Out of the following which one is not a possible energy for a photon to be emitted by hydrogen atom according to Bohr's atomic model?

Electrons used in an electron microscope are accelerated by a voltage of $25 \mathrm{kV}$. If the voltage is increased to $100 \mathrm{kV}$ then the de-Broglie wavelength associated with the electrons would

Light of two different frequencies whose photons have energies $1 \mathrm{eV}$ and $2.5 \mathrm{eV}$ respectively illuminate a metallic surface whose work function is $0.5 \mathrm{eV}$ successively. Ratio of maximum speeds of emitted electrons will be

A zener diode, having breakdown voltage equal to $15 \mathrm{~V}$, is used in a voltage regulator circuit shown in figure. The current through the diode is 

The wavelength of the first line of Lyman series for hydrogen atom is equal to that of the second line of Balmer series for a hydrogen like ion. The atomic number $Z$ of hydrogen like ion is

A radioactive nucleus of mass $M$ emits a photon of frequency $v$ and the nucleus recoils. The recoil energy will be

A nucleus ${ }_n^m X$ emits one $\alpha$-particle and two $\beta^{-}$particles. The resulting nucleus is

Fusion reaction takes place at high temperature because

Two radioactive nuclei $P$ and $Q$, in a given sample decay into a stable nucleous $R$. At time $t=0$, number of $P$ species are $4 N_0$ and that of $Q$ are $N_0$. Half-life of $P$ (for conversion to $R$ ) is 1 min where as that of $Q$ is $2 \mathrm{~min}$. Initially there are no nuclei of $R$ present in the sample. When number of nuclei of $P$ and $Q$ are equal, the number of nuclei of $R$ present in the sample would be

The power obtained in a reactor using $\mathrm{U}^{235}$ disintegration is $1000 \mathrm{~kW}$. The mass decay of $\mathrm{U}^{235}$ per hour is

The potential difference that must be applied to stop the fastest photoelectrons emitted by a nickel surface, having work function $5.01 \mathrm{eV}$, when ultraviolet light of $200 \mathrm{~nm}$ falls on it, must be

The mass of a ${ }_3 \mathrm{Li}^7$ nucleus is $0.042 \mathrm{u}$ less than the sum of the masses of all its nucleons. The binding energy per nucleon of ${ }_3 \mathrm{Li}^7$ nucleus is nearly

The device that can act as a complete electronic circuit is

The energy of a hydrogen atom in the ground state is $-13.6 \mathrm{eV}$. The energy of a $\mathrm{He}^{+}$ion in the first excited state will be

The electron in the hydrogen atom jumps from excited state $(n=3)$ to its ground state $(n=1)$ and the photons thus emitted irradiate a photosensitive material. If the work function of the material is $5.1 \mathrm{eV}$, the stopping potential is estimated to be (the energy of the electron in $n$th state $\mathrm{E}_{\mathrm{n}}=-\frac{13.6}{\mathrm{n}^2} \mathrm{eV}$ )

A source $\mathrm{S}_1$ is producing, $10^{15}$ photons/s of wavelength $5000 Å$. Another source $S_2$ is producing $1.02 \times 10^{15}$ photons per second of wavelength $5100 Å$. Then, (power of $\left.S_2\right) /\left(\right.$ power of $S_1$ ) is equal to

The binding energy per nucleon in deuterium and helium nuclei are $1.1 \mathrm{MeV}$ and $7.0 \mathrm{MeV}$, respectively. When two deuterium nuclei fuse to form a helium nucleus the energy released in the fusion is

Which one of the following statement is false?

When monochromatic radiation of intensity $I$ falls on a metal surface, the number of photoelectron and their maximum kinetic energy are $N$ and $T$ respectively. If the intensity of radiation is 2I, the number of emitted electrons and their maximum kinetic energy are respectively