Equation of eigen value of the momentum of a particle in one dimension box: The eigen value of the momentum $P_{n}$ of a particle in one dimension box moving along the x-axis is given by $P^{2}_{n} = 2 m E_{n}$ $P^{2}_{n} = 2 m \frac{n^{2} \pi^{2} \hbar^{2}}{2 m L^{2}} \qquad \left( \because E_{n}= \frac{n^{2} \pi^{2} \hbar^{2}}{2 m L^{2}} \right)$ $P^{2}_{n} = \frac{n^{2} \pi^{2} \hbar^{2}}{L^{2}}$ $P_{n} = \pm \frac{n \pi \hbar}{L}$ $P_{n} = \pm \frac{n h}{2L} \qquad \left( \hbar = \frac{h}{2 \pi} \right)$ The $\pm$ sign indicates that the particle is moving back and forth in the infinite potential box. The above equation shows that eigen value of the momentum of the particle is discrete and the difference between the momentum corresponding to two consecutive energy levels is always constant and equal to $\frac{h}{2L}$

(The Advance Learning Institute of Physics and Technology)