Photon Emission from Atomic Structure

The most important physical improvment to the Internet has been through the use of Lasers and Laser Modulation in order to greatly increase bandwidth. To understand Lasers, however, requires understanding the rules of photon emission from atoms.

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.

Familiarize yourself with these concepts via manipulation/examination of this virtual atom .

You can adjust the spacing between the 4 energy levels by grabbing a circule and moving it. While this is not a good characterization of the actual structure of an atom, in terms of energy levels an discrete photon emission its fine. So do this

  • Hit the Jump button for orbit 4 (and ignore what happens)
  • Jump from Orbit 4 to orbit 2 and notice that a photon is emitted of 606 nanometers which corresponds to the green part of the spectrum
  • Now grab orbit 2 that out a small bit
  • Return to orbit 4
  • Jump back to orbit 2 and notice that the emitted photon has increased in wavelength precisely because the energy difference between level 4 and level 2 is now smaller.


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.




The following small YouTube Authorative Video may help: