"The phenomenon that required quantum mechanics is known as black body radiation," Risu explained. "It has to do with something very ordinary and, on the face of it, simple: Heat. If a piece of metal is brought to high temperatures, it glows red hot. The explanation for these observations in classical physics is known as the Rayleigh-Jeans law, but while it works well for heat radiating at long wavelengths, something weird happens at very short wavelengths - the law predicts that the hot piece of metal should emit an infinite amount of energy. Since that obviously doesn't happen, a new explanation had to be found.

"In 1900, the German physicist Max Planck proposed a model that worked. This name is important to remember, by the way," Risu added, "Max Planck is generally considered one of the founding figures of quantum mechanics. In his model, energy could only be transported in little packets he called quanta. Singular quantum, plural quanta. Unlike in classical physics, this energy couldn't be arbitrarily transmitted, it had to be moved, quantum after quantum. Albert Einstein, who was rather sceptical about quantum mechanics for several years, used this explanation to show that a beam of light could be a stream of quanta - they're now called photons."

Risu paused. "At this point, we need to talk about light. Someone mentioned it earlier, light can be considered to be both a stream of particles or a wave. I said at the time that this is called wave-particle duality. It's one of those things that quantum mechanics has come up with that challenge our day-to-day understanding of the Universe. This applies to all matter, incidentally, not just light.
Until quantum mechanics came along, most people thought the debate about whether light was made up of particles or waves had been settled in favour of waves. It explained how light could move at the cosmic speed limit - since particles are usually considered to have mass, and therefore cannot speed up to the speed of light.

"An absolutely classic proof of the dual nature of light is the so-called double-slit experiment. Does anyone happen to have heard of this? Could you explain the experimental set-up?"