Photons - How We See

A "photon" is the tiniest piece of light. We are bathed in a seething ocean of photons, and not just those we see as light. Radio, television, and microwaves are photons. And photons bind electrons, atoms, and molecules.

http://www.msim.org.uk/images/Red-apple-fallingWEB.jpg

Before saying more about photons, it is prudent to say what physics tries to do. Physics tries only to describe how objects behave, what they do in response forces. Physics does not try to say why things behave that way, nor what mechanism makes that behavior happen. Mechanism would be nice, but no one knows what "really" happens.

Newton did not discover gravity; others had noticed that apples fall to the ground. What Newton did was to describe how gravity makes object behave. His great leap was to see that planets orbit the sun following the same behavior. His gravity was not earthbound, but reached across all of space. Einstein went further by showing how a description of space-time itself would produce gravity as a by-product. His description is more accurate than Newton's, if you are moving fast enough.

Wave or Particle?

In the latter half of the nineteenth century physicists saw objects as collections of particles (atoms) and forces as waves. We can see both in a shoreline.

But what is light? a particle or a wave?

http://66.199.135.31/s/mzig/products/PT-SSL-77-0.400x400.jpg

The Physics Classroom describes many of the ways in which light behaves like a wave, including: "Light reflects .... Light refracts .... Light diffracts .... Light undergoes interference in the same manner that any wave would interfere. And light exhibits the Doppler effect ..." Science Trek shows the full gamut of electro-magnetic waves from television signals at hundreds of meters down to gamma rays at billionths of meters. Their next page has a cool demo where you can make electrons fly around a positive charge.

The most crucial argument for the particle nature of light is the photoelectric effect. Light can drive a current of electrons through space, but it can only impart energy in discrete amounts. Einstein got his Nobel Prize for explaining this. The PHYSCHEM website has a good description of the photoelectric effect. They conclude:

The wave model for light fails to account for the photoelectric effect because the energy transmitted by a wave is proportional to the amplitude. This means that if we increase the intensity of the incident radiation, photoelectrons should be emitted regardless of the frequency of the light.... this is not observed. The wave model cannot account for the emission of photoelectrons only after the frequency of the incident light passes a certain threshold value.

Albert Einstein, extending earlier ideas of Max Planck, proposed that the energy of radiation was in discrete packets, or quanta. Each packet of light energy is called a photon. According to Planck, the energy of the photon is proportional to the frequency of the radiation, i.e.:

where h, Planck's constant, has the value of 6.6 x 10^-34 J.s, and c is the speed of light in a vacuum, with a value of 3.0 x 10⁸ m.s^-1. When the light interacts with matter, energy is absorbed only as discrete packets and all the energy of the packet is transferred. Thus, if the energy of a photon is sufficient to excite an electron, it will do so, but if it it insufficient, it will not.

So, wave or particle? Physicists have waffled and arrived at a concoction called the "dual wave-particle" nature of light. Remember, they aren't trying to say what light is, merely how it behaves.

Electrons Meet Photons

The most important thing to know about photons is when they meet electrons. When an electron "absorbs" a photon, the elctron gets a little kick in its energy. If this electron is in an atom, it jumps from its present orbit to the next outer orbit. An electron can also lose energy; then it emits a phton of the same frequency as the one whose energy it needed to jump to that outer orbit. It is these emited photons that we see as light from the sun.

What happens when a photon hits your eye?

In essence, the electromagnetic energy of the photon is converted to mechanical motion. A single "rhodopsin" molecule absorbs energy from the photon and bends in the middle. A VChemLib page describes rhodopsin:

The photosensitive molecule involved in vision is called rhodopsin, (also known as visual purple) which consists of a large protein (having a molecular weight of around 38,000) called opsin, joined to 11-cis-retinal via a protonated Schiff base on one of its lysine side-chains.

http://www.ch.ic.ac.uk/vchemlib/mim/bristol/retinal/cretinal_mol.htm

The retinal molecule is vitamin A. Our bodies cannot make it, but the plants we eat can.
The page goes on to describe the response of rhodopsin to a single photon:

the molecule absorbs the energy and the cis-double-bond ... in the retinal is temporarily converted into a single bond. This means the molecule can now rotate around this bond, which it does by swivelling through 180°. The double bond then reforms and locks the molecule back into position in a trans configuration. Thus the light has isomerised the molecule from cis to trans, and as it did so, it changed the shape of the retinal from curved to straight. Essentially, the energy in a photon has been converted into atomic motion.

Because it has straightened, the molecule no longer fits and starts a little dance with its neighbors. The dance fires neurons. These cascade through other neurons, eventually making your mind believe it has seen (a little piece of) something. Remarkably, the vision systems in ALL sighted animals use this same mechanism.