Physics

Physics · modern-physics

Problem

A.3 Particle can undergo instantaneous transition from one state to another only by emitting or absorbing a photon of the corresponding energy difference. Find the wavelength

\lambda_{21}

of the photon emitted during the transition of the particle from the first excited state

\left(E_{2}\right)

to the ground state

\left(E_{1}\right)

. Given constants: Particles and Waves Wave-particle duality, which states that each particle can be described as a wave and vice versa, is one of the central concepts of quantum mechanics. In this problem, we will rely on this notion and just a few other basic assumptions to explore a selection of quantum phenomena covering the two distinct types of particles of the microworld-fermions and bosons. Part A. Quantum particle in a box Consider a particle of mass

m

moving in a one-dimensional potential well, where its potential energy

V(x)

is given by

V(x)= \begin{cases}0, & 0 \leq x \leq L \\ \infty, & x<0 \text { or } x>L\end{cases} \tag{1}

While classical particle can move in such a potential having any kinetic energy, for quantum particle only some specific positive discrete energy levels are allowed. In any such allowed state, the particle can be described as a standing de Broglie wave with nodes at the walls. Context question: A.1 Determine the minimal possible energy

E_{\min }

of the quantum particle in the well. Express your answer in terms of

m, L

, and the Planck's constant

h

. Context answer:

E_{\min }=\frac{h^{2}}{8 m L^{2}}

Extra Supplementary Reading Materials: The particle's state with minimal possible energy is called the ground state, and all the rest allowed states are called excited states. Let us sort all the possible energy values in the increasing order and denote them as

E_{n}

, starting from

E_{1}

for the ground state. Context question: A.2 Find the general expression for the energy

E_{n}

(here

n=1,2,3, \ldots

). Context answer:

E_{n}=\frac{h^{2} n^{2}}{8 m L^{2}}

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