7.1 Introduction to Light and Optics

If you want to really understand just how weird light is, you have to imagine the perspective of someone from a long, long time ago.

Why not choose, for example, a farmer in Southeast Asia a few thousand years ago. Put yourself into the farmer’s shoes – unless this is a barefoot farmer, in which case I’m speaking only metaphorically. You’re relaxing after a long day of work. Feel the rough, scratching fabric of the shirt on your back. Feel the soft evening breeze blowing through your hair as you watch the sun set over the countryside of what is now Vietnam.

As the daylight slowly fades, what do you see? Well, if you’re a particularly perceptive farmer, the answer is: quite a lot.

For example, you see that every solid object casts a shadow, and that these shadows stretch out longer as the sun goes down. If you’re lucky and the clouds are piled up just right, you might even be able to see them casting shadows as well.

Then, as the sun gets lower and lower, you see the colour of the entire sky slowly changing. Once the last edge of the sun sinks below the horizon, you start to be able to see less and less, and all of the brilliant colours of the daytime seem to have disappeared, replaced instead by a dim world of blacks, whites, and greys.

If the moon is full on this particular evening, though, you can still see most of the things around you – a tree, perhaps, looking somewhat ghostly, and your fields, and the walls of your house, and some bored-looking cattle. Surrounding that moon, meanwhile, you see an uncountable number of stars.

And if your mind got to wandering, you might start to consider how it is that you see anything at all. And what, you might ask yourself, is all this light shining around everywhere, anyway?

These are the kinds of musings and questions that bothered philosophers and scientists for most of human history. One popular answer to the riddle of light and human vision had it that we could see because we all shot beams of light out of our eyeballs.1 However, this theory, which I’ve named the “Everyone Is Cyclops From The X-Men” Theory, is pretty easy to argue against – all you have to do is point out that we can’t see things at night, and it becomes clear that the eye-beams probably don’t exist.

It wasn’t until the 1600s (which, if you’ve been reading along, is about the time when pretty much all of classical physics was discovered) that people like René Descartes and Isaac Newton formulated a good, modern description of light as something that emanated from luminous objects. Newton in particular took this theory further, stating that light is made up of tiny particles rather than waves, and used these kinds of ideas to come up with explanations for reflection and refraction. He even explained the phenomenon of white light splitting into a rainbow after passing through a prism, as it does on the cover of Pink Floyd’s seminal 1973 album, Dark Side Of The Moon.2

Light became even more important when, in 1905, Einstein published papers on both the Photoelectric Effect and Special Relativity.3 The former showed that light often did exhibit wave-like characteristics, despite behaving sometimes like a particle. The latter paper, though, really blew everyone’s minds. It demonstrated that the speed at which light travels also happens to be the fastest possible speed anything can ever travel.

But once again, I’ll have to hold off on talking too much about Special Relativity – after all, I’ve got a whole chapter dedicated to Einstein’s work later on.

So before we get too far off track, let’s talk about light: what is it, how does it work, and how do we see things?


Next: 7.2 – Light

Previous: 6.5 – The Doppler Effect

  1. I’m fairly certain that this is not a joke, and that people actually thought this.
  2. The name of the album is both unrelated to the cover art and also terribly unscientific, by the way, although the term seems to have become engrained into the public consciousness, as evidenced by the Transformers movie of the same name. In fact, there is no dark side of the moon; there’s a far side of the moon, which never faces us but which faces the sun for half of every moon-day (which lasts about 29.5 Earth-days).
  3. And yes, pretty much everything in physics can be traced back to Newton and/or Einstein.