The physics and biological mechanics of vision are astounding – I find it amazing that we can see at all. A Photon of light is very small (measured by the width of its wavelength). For example, the wavelength of green light is about 500 nanometers, or about two thousandths of a millimeter. That begs the question: why don’t photons just go right though us? Lets look at the problem with an explanation of why photons can pass through glass:
|“Particles can pass though objects. For example, right now there are 100 billion solar neutrinos per second passing through every square centimeter of your body. The neutrinos are particles that only have VERY weak interactions with the matter that our bodies are made of so almost all of them pass through without interacting. So, in general, there is no problem with particles passing through matter if they do not interact with the matter – there is lots of space between the nuclei of atoms.
Now light particles, photons, are packets of electromagnetic radiation and the electric fields and magnetic fields can interact with charged particles. Thus photons cannot penetrate through metal because the free electrons that make metals conductors will easily interact with and absorb the photons immediately.
However the electrons in glass are tightly bound to atoms so they are not free to move like the electrons in a metal and therefore they do not absorb the photons. … If the energy of the photon happened to equal the difference between a bound electrons energy level and another unoccupied electron energy level, then the photon would get absorbed by causing the electron to transition between those energy levels. However, there are no such energy levels available in glass for visible light photons. On the other hand, ultraviolet photons do get absorbed because there are available energy levels at the energies of those photons.”
– Frank Heile, Physicist
This helps us better understand some of the properties of our eye lens and our retina. But what does happen when a photon strikes the retina and stimulates a rod or cone?
First, it’s important to know that only 1 to 3 percent of photons actually reach a photoreceptor cell – there’s a lot of light that we don’t see and probably the only reason we are able to detect light hinges on:
- the number of photons passing into the eye (as many as 10^14 photons per second) and
- the massive number of cone and rod cells (120 million rod cells and 6 million cone cells).
When photons “hit” a molecule (on a cone or rod) it raises an electron to a higher level (the molecule absorbs photons in the electrons on the surface, transforming the energy into density vibration). Once stimulated our photoreceptor cells convert light (visible electromagnetic radiation) into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell’s membrane potential. The process happens at the atomic level an our photoreceptors are capable of detecting a single photon of light. Astronauts have reported seeing other colors such as yellow and pale green lights that were determined to be cosmic rays (gamma and x rays less than the diameter of an atom).
But some stimuli may be happening at an ever-smaller scale. Physicists now believe that the human eye is capable of detecting quantum effects. To date I know of no direct experiments with humans that prove that conjecture, but there is evidence from experiments with European Robins that does.
It’s long been thought that birds navigate using Earth’s magnetic fields (which is very weak, too weak to detect), Experiments indicated that the robin’s magnetic detection is in the eye – the robin’s chemical compass can see the state of quantum entanglement of photons (which are more likely at magnetic north), the direct observation, by an eye of a quantum state. See: The Secrets of Quantum Physics: Einstein’s Nightmare, Physicist Jim Al-Khalili 
That Darn Cat
There is a line of thought called Biocentrism that posits our perception is creates the universe. This argument is based on a concept expressed in the Schrodinger’s Cat thought experiment.
|“A cat imagined as being enclosed in a box with a radioactive source and a poison that will be released when the source (unpredictably) emits radiation, the cat being considered (according to quantum mechanics) to be simultaneously both dead and alive until the box is opened and the cat observed.”|
Biocentrism says that the quantum nature of matter sets up a universe where our perception actually creates the universe we experience. But the concept goes further, suggesting that time is an illusion, and that we are living all of our possible decisions/perceptions at the same “moment.”
I’ll revisit this line of thought in the next article, which covers the olfactory system.
None known, but isn’t it interesting?
- Migration via quantum mechanics
- 2012 Nobel Prize: How Do We See Light?
- The Human Eye Can See Individual Particles of Light
- If light is made of particles, how does it pass through glass?
- The shape and size of a photon
- Physicists Explain How Human Eyes Can Detect Quantum Effects
- Photoreceptor cell
- Cosmic ray visual phenomena
- Cosmic rays
- Quantum Mechanical Basis of Vision