1.3 Shedding some Light
Light shows wave-particle duality.
At least as far back as ancient Greece, people have enjoyed arguing about the nature of light. In the 17th century the big disagreement was about whether light was made up of particles or waves (“plus ça change, plus c'est la même chose,” mutter a bunch of ghostly frenchmen who we’ll meet later). Isaac Newton (“probably the greatest mind in the history of physics; call me Sir Isaac”) was a fan of the particles (which he called corpuscles) because light travels in straight lines like particles obeying his laws in the absence of a force. Except sometimes it doesn’t: light can be diffracted, bending around things to create effects like the fringes at the edge of a shadow. This really needs a wavy explanation and a detailed wave theory of light which seemed to be capable of explaining pretty much everything light did was developed by Christian Huygens and others. Later Thomas Young and his famous slits (see below) provided convincing evidence of interference effects for light – a quintessential wave property – and then James Clerk Maxwell developed his spectacularly successful theory of electromagnetic waves which seemed finally to nail it: light is a wave. Except ……..
Here’s a tricky one to explain: Nefertiti has been given an unusual birthday present; a jar containing a mixture of chlorine and hydrogen. She wants to explode it and she reasons that she could use light energy to break open the relatively weak bonds that hold the Cl2 molecules together to form super-reactive chlorine atoms and initiate the reaction. Seems like a good plan. “Allow me”, you say, and wheel in your powerful, state of the art helium-neon laser. You direct your intense beam of pure red light on the jar and ….. nothing happens. “Don’t worry”, you say, a little flustered, and you crank the power up to 11. Still nothing happens. Cormorant flies in. “Having trouble?” he enquires. “Try this”. He produces the very safe, low power UV disco light that he uses to help create a nice ambiance in his nest. You smirk. The mixture explodes.
What does this mean (apart from the fact that you’ve been humiliated by a seabird with questionable aesthetic values)? Your laser beam was delivering much more power than his blacklight: in wave theory terms, your waves had an amplitude that dwarfed his - but your waves didn’t work. “What it means”, says Nefertiti, “is that light is composed of particles. Although Cormorant’s uv-light had much less power in total, that was because it was pumping out fewer particles in a given time. However each individual particle had more energy than yours. If a chlorine molecule can only absorb one light particle at a time, it makes sense that a single uv-particle might be able to deliver enough energy to break the bond open, while red light particles – no matter how many there are – cannot”.
“Impressive reasoning” opines the one ghost we instantly recognise, with his pale face, long hair, awkward gait and a cigar in the mouth (his words). It is Albert Einstein. “I used essentially the same reasoning in 1905, to explain a similar situation where a minimum particle energy is needed to make something happen: in my case ejection of an electron from an illuminated metal. In fact I received the first of my Nobel prizes for this.” Uncle Albert referred to these particles as quanta of light; today we call them photons.
We are left, then, with the idea that light is neither particle nor wave, since neither of these models fully explains its properties. Light is what it is. But depending on the situation, its behaviour can sometimes be best understood by using a wave model and other times using a particle model. This is called wave-particle duality. It’s an extraordinary thing but how does it helps us with our quest to understand quantised energy levels? Let’s continue with the story.