Photon Emission from Atomic Structure

The entire key to understanding this is the recognition that each atom has a unique set of atomic energy levels . The basic ideas of atomic energy levels and transitions will become clear when you run the simulation. For now, the details are summarized in the diagrams below. We begin with the simple concept of Energy levels in an atom as shown below

The key point is that atomic energy levels are discrete (or quantised). These levels are occuppied by electrons. Therefore, in any given atom, an electron can only be in a certain, discrete energy state. Each element has a unique set of energy levels. The concept of discrete is important. A photon is either all or nothing. You can not take a photon, steal part of its energy, and still have that photon left. So, in general, there is a one to one correspondence between photon generation and the movement of electrons to different energy levels. Photon Emission:

If the electron in an upper energy level (say E₃) falls to a lower energy level (say E₂) one photon is emitted whose energy is exactly equal to the energy difference between levels E₃ and E₂.

For photons, their energy is inversely proportional to their wavelength. Short wavelength photons (e.g. UV photons) have more energy than long wavelength photons (e.g. IR photons). In the optical, blue photons have shorter wavelength (around 4000 angstroms) than red photons (around 7000 angstroms) and therefore have more energy.

All photons travel at the speed of light, independent of their energy or wavelength.

What the observer therefore sees is photon emission at a unique wavelength and that is what the spectra of elements are.

Hydrogen Atom

Its corresponding emission spectrum

So if we consider the following simplified 3 level atom we can see the mixture of emission and abosrption, each of which is uniquely associated with either the generation of a photon or the removal (absorption) of a photon of a specific energy/wavelength.

Now let's pretend that we can "stimulate" this atom in some way so that the word "absorption" above is replaced with the word "pumping". In other words, we have a mechanism to "pump" the electon from the ground state to the first excited state. When it falls back, it generates a photon of a particular wavelength. Now imagine an entire collection of atoms that can be "pumped" this way the output of this collection of atoms will be light generated at a single frequency or wavelength (this is called monochromatic light).

In principle, this is the basic mechanism behind a LASER.

Familiarize yourself with these concepts via manipulation/examination of this virtual atom . Just click on any of the energy levels to "pump" the electron to that level. As it falls back to the ground state it releases one individual photon of specific energy as the electron is de-excited back to the ground state (its normal state). Note that say, if you pump the electron to energy level = 8, it may or may not fall back to the ground state directly. Sometimes it may "cascade" down through intermediate levels. This occurs because atoms are complicated and now fully predictable in their behavior - just like dogs on the Internet.

The following small YouTube Authorative Video may help: