Re: How big is a photon and how does that compare to it's wavelength?

Light indeed behaves like both a wave and a particle. However, a photon does not have a "size" in the same sense as, say, a basketball. A wave can diffract and refract in media, whereas a basketball cannot. But to answer your direct question, the "size" of a photon is its wavelength.

There are a few equations you should have in the back of your mind at all times. One is E = h v, where E is engery, h is Planck's constant, and v (pronounced noo) is the frequency. Also, lambda = c / v, where lambda is the wavelength, c the speed of light, and v the frequency. From these two equations, you can learn a lot about light.

The photon does not have a shape, per se. It is only a quantum of energy with a certain frequency associated with it. And given the medium, the photon exhibits a particular wavelength. The wavelength is not a physical parameter necessarily, but it can act like one. Particularly, a diffraction grating disperses light at different angles based on the physical spacing between the ridges of the glass. However, this is more a quantum mechanical effect than a physical photon-matter size relation. Basically, the wavelength of the photon determines how it can fit within the boundary conditions set up by the glass ridges.

Matter also behaves like a wave. You have probably studied the DeBroglie wavelength. Matter does have a wavelength associated with it, but the wavelength does not imply a change in the physical parameters of the matter itself. Let me explain it this way.

First, light can penetrate metal without too much difficulty. Quantum mechanics allows us to model a thin layer of metal as a boundary value problem. Light falling on a surface has a probability of tunneling into the metal or reflecting off it it. The probability is dependent on the change in potential energy on either side of the boundary. If you can fit one half wavelength of light within the layer of metal, the probability that the light will tunnel into and back out is maximized. This is the basis of many types of optical filters, and why oil makes those colored patterns on the surface of water.

However, let us think about the macroscopic world instead of the microscopic. Imagine a cliff and a car driving toward it. The car has some wavelength based on its speed. Classical mechanics tells us that if the car continues to the cliff edge, it will simply drive right off and fall into the ravine. However, quantum mechanics tells us that based on the car's wavelength, the car will react differently to the gravitational potential energy change represented by the cliff. There is actually a non- zero probability that the car will "reflect" off of the boundary of the cliff.

This problem illustrates the breakdown of Newtonian physics when applied to the microscopic world. You just can't think of getting out a miniature yardstick and holding it up to a proton and measuring its' dimensions. The quantum world is constantly jiggling around. While the proton has some physical size, it is constantly bouncing around, and its position is really only known as a probability density bound by the Heisenberg Uncertainty principle. In the Newtonian view, an object at rest is at rest, and there is no other way to look at it.

As for the circle of an orbiting electron and the photon that is created, you are on the right track. The orbit of the electron determines its kinetic energy. If an electron is kicked into a higher-energy orbit, it will want to fall back to its original energy. The difference in orbital kinetic energy gives you the photon's energy, and thus the wavelength.

Check out some information on the Heisenberg Uncertainty principle and quantum mechanics.

Hope this helps! -Fred