Modern Physics PYQ — Page 7

An electron in hydrogen atom makes a transition $n_1 \rightarrow n_2$ where $n_1$ are principle quantum numbers of the two states. Assuming Bohr's model to be valid, the time period of the electron in the initial state is eight times that in the final state. The possible values of $n_1$ and $n_2$ are:

A source of light is placed at a distance of $50 \mathrm{~cm}$ from a photo cells and the stopping potential is found to be $\mathrm{V}_{0^*}$. If the distance between the light source and photo cells is made $25 \mathrm{~cm}$, the new stopping potential will be:

A certain mass of hydrogen is changed to helium by the process of fusion. The mass defect in fusion reaction is $0.02866 \mathrm{u}$. The energy liberated per $u$ is (given $1 \mathrm{u}=931 \mathrm{MeV}$ )

$\alpha$-particle, $\beta$-particle and $\gamma$-rays are all having same energy. Their penetrating power in a given medium in incresing order will be:

Two radiations of photon energies $1 \mathrm{eV}$ and $2.5 \mathrm{eV}$, successively illuminate a photosensitive metallic surface of work function $0.5 \mathrm{eV}$. The ratio of the maximum speeds of the emitted electrons is

Two ideal diodes are connected to a battery as shown in the circuit. The current supplied by the battery is 

To get an output $Y=1$ in given circuit which of the following input will be correct 

The transition from the state $n=3$ to $n=1$ in a hydrogen like atom results in ultraviolet radiation. Infrared radiation will be obtained in the transition from

The half life of a radioactive nucleus is 50 days. The time interval $\left(t_2-t_1\right)$ between the time $t_2$ when $\frac{2}{3}$ of it has decayed and the time $t_1$ when $\frac{1}{3}$ of it had decayed is

The figure shows a logic circuit with two inputs $A$ and $B$ and the output $C$. The voltage wave forms across $A, B$ and $C$ are as given. The logic circuit gate is 

Monochromatic radiation emitted when electron on hydrogen atom jumps from first excited to the ground state irradiates a photosensitive material. The stopping potential is measured to be $3.57 \mathrm{~V}$. The threshold frequency of the material is

If the nuclear radius of ${ }^{27} \mathrm{Al}$ is $3.6 \mathrm{Fermi}$, the approximate nuclear radius of ${ }^{64} \mathrm{Cu}$ in Fermi is

If the momentum of an electron is changed by $p$, then the de-Broglie wavelength associated with it changes by $0.5 \%$. The initial momentum of electron will be

Electron in hydrogen atom first jumps from third excited state to second excited state and then from second excited to the first excited state. The ratio of the wavelengths $\lambda_1: \lambda_2$ emitted in the two cases is

$C$ and Si both have same lattice structure, having 4 bonding electrons in each. However, $C$ is insulator whereas $\mathrm{Si}$ is intrinsic semiconductor. This is because

An electron of a stationary hydrogen atom passes from the fifth energy level to the ground level. The velocity that the atom acquired as a result of photon emission will be ( $m$ is the mass of the electron, $R$ Rydberg constant and $h$ Planck's constant)

An $\alpha$-particle moves in a circular path of radius $0.83 \mathrm{~cm}$ in the presence of a magnetic field of $0.25 \mathrm{~Wb} / \mathrm{m}^2$. The de-Broglie wavelength associated with the particle will be

A $200 \mathrm{~W}$ sodium street lamp emits yellow light of wavelength $0.6 \mu \mathrm{m}$. Assuming it to be $25 \%$ efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is

A mixture consists of two radioactive materials $A_1$ and $A_2$ with half lives of $20 \mathrm{~s}$ and $10 \mathrm{~s}$ respectively. Initially the mixture has $40 \mathrm{~g}$ of $A_1$ and $160 \mathrm{~g}$ of $A_2$. The amount of the two in the mixture will become equal after

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 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

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

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 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