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Max Planck, the Theory of Solar Panels

Max Planck - a great name for a great physicist. Today's blog entry is going to have a historical twist as we go back in time to think about where solar panels come from.

Light is a type of wave, and an unusual one. Waves are typically defined as either longitudinal or transverse, so I'll start by explaining these. Transverse waves oscillate (wobble up and down) in a direction perpendicular to the wave's direction of travel. That's quite a bit of jargon, so I'll use a diagram:


Let's think of a real-life example. If two people are standing facing each other and holding a Slinky, then if one person moves his arm up and down, it will create a transverse wave in the slinky. If the person now stops flapping his arms about and instead decides to push the end of the slinky towards the other person, and then to pull it away from them repeatedly, a longitudinal wave will be created.

So how is light unusual, and how does this relate to Max Planck? Light exhibits properties of both longitudinal and transverse waves. It is longitudinal in the sense that it is a ray of photons, which are packets of information, and these come in clusters (like the tightly packed areas of a slinky compared to the stretched out areas). It exhibits transverse behaviour because it changes direction when it enters a medium of a different density at any non-perpendicular angle (I'll go into that another time, maybe). Max Planck defined a formula used to calculate that the energy within photons is directly proportional to the light frequency, basically describing what light itself is. If photon energy is greater than the forces that hold the electrons in a metal, then electrons may be discharged, which is the definition of a current - electrical power.

Therefore, with the appropriate materials (those metals with weak forces holding the electrons in place), solar panels can be created!