The name of the game.

On November 11, 2008, I was criticized about using the term blue and red photons in a blog on solar cells.

user kardinal wrote:

I do not like if someone writes about ”red/blue photons”.
If you are a physicist you are definitively not allowed to say so…

and later …

A photon doesn’t have any color, it is a particle like electrons,
neutrons, protons, positrons. Only few people would call an electron
as being blue or red…

No physicist would do such a defection.

As a physicist myself I felt somewhat insulted and decided if I ever would start a blog of my own, than this would be the title. And while for years fighting and refusing as a Web 1.0-user to jump onto the Web 2.0 hipe I now have a Facebook account and finally even decided to start a blog.

Are there green photons?

Physics has two main models which can describe the behavior of light: as an electromagnetic wave and as a particle. Both models are mutually exclusive, which means that there are phenomena which can be explained by only one of the models, but not by the other. In the wave model of light, light is characterized by its wavelength. It is possible to measure this wavelength and it was found that the visible part of the electromagnetic spectrum covers the wavelength range between about 400 nm and 750 nm. We see light with a wavelength around 450 nm as blue, around 550 nm as green, 580 nm as yellow and above 610 nm as red – the colors of the rainbow.

This was the common knowledge and the state of science until 1900-1905 when Max Planck and Albert Einstein started to play around with the idea of smallest particles of light, photons. The photon is an elementary particle from the family of bosons. The quantum theory of light became necessary in order to explain the photoelectric effect: it doesn’t matter how much energy in the form of red light you shine on a metal surface, no electric charges can be made to leave the surface. Use blue light, however, and you will see an effect. This is a contradiction to classical physics where it always is the amount of incoming energy which causes an effect.

The solution was the assumption that light energy comes in small packets. Only if the energy per packet is high enough to make the metal electrons leave the surface, they will leave the surface. And of course there is a mathematical link between these theories: the shorter the wavelength of light, the higher the energy of the associated photons, and in order to honor Max Planck the numerical factor between both entities is known as the Planck constant, h.

Coming back to the title of my blog, the photons which in the particle theory correspond to green light with a wavelength of 550 nm, have an energy of 2.25 eV each, or 3.6·10^-19 J if you prefer SI-units. These photons have the ability to excite a certain group of cone cells in the retina of the human eye, which is then interpreted by the human brain as green light. Of course I never implied that certain photons would be painted in a certain color – this would be ridiculous. But we see photons with an energy of around 2.2 eV as green and therefore it is no defection to physics to distinguish between different photon energies in terms of the color of light which they described.

For we may not forget that both the wave model and the particle model of light are just bare images which our neolithic brains invented in order to allow us to grasp the world outside our cranium. All these models — as detailed they might be — are just different levels of lying to children¹.

So what is this blog about?

Since I started my first homepage at Geocities in 1996 I had high ambitions on its contents. The focus of this blog will most probably end up to be on electronics projects, on books I read and have read, on photography and research, all spiced with a certain amount of home-grown chile peppers.

Uppsala, 2010-02-23

Uwe

[1] see e.g. The Science of Discworld by Terry Pratchett, Ian Stewart and Jack Cohen.