Assertion- The red light is just able to emit photoelectrons from a metal surface, then violet light cannot emit the photoelectrons from that metal.

Reason- The energy of violet light photons is more than red light photons.

Assertion- If the wavelength of incident photons is halved, then the maximum kinetic energy of photoelectrons will become two times.

Reason- Energy of the photon$\propto \frac{1}{\lambda }$.

Assertion- Second orbit circumference of a hydrogen atom is two times the de-Broglie wavelength of electrons in that orbit.

Reason- de-Broglie wavelength of an electron in the ground state is minimum.

Assertion- A particle at rest breaks into two particles of different masses. They fly off in different directions. Their de-Broglie wavelengths will be different.

Reason- Their speed will be different.

Assertion- In one half-life of a radioactive substance, more number of nuclei are decayed than in one average life.

Reason- Average life$=\frac{\text{Half-life}}{\ln 2}$.

Assertion- Two different radioactive substances have initially the same number of nuclei. Their decay constants are ${\lambda }_1$ and ${\lambda }_2\left(<{\lambda }_1\right)$. Then, initially, the first radioactive substance decays at a faster rate.

Reason- The half-life of a first radioactive substance is less.

Assertion- In any nuclear process energy is released, if the total binding energy of daughter nuclei is more than the total binding energy of parent nuclei.

Reason- If energy is released then the total mass of daughter nuclei is less than the total mass of parent nuclei. 

Assertion- The density of a nucleus is independent of the mass number.

Reason- The radius of a nucleus$\propto A^3$. Where, $A$ is the mass number.

Assertion- The energy $E_1$ is required to remove the first electron from the helium atom and energy $E_2$ is to required to remove the second electron. Then, $E_1<E_2$.

Reason- Ionisation energy of a single electron of ${\mathrm{He}}^{+}$ is $54.4 \mathrm{eV}$.