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?

  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.

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6.5 The Doppler Effect

Critical Questions:

  • Why does the sound of an ambulance or car engine seem to change as the vehicle passes by?
  • Why do we hear a loud bang when a supersonic jet flies by?
  • What does all this have to do with the Big Bang?

I’d like you to do me a favour. This is especially important if you’re in a public place, surrounded by lots of strangers.

I’d like you to imitate, out loud and at a significant volume, the sound of a racecar going by.

You can imagine Ricky Bobby driving if it helps.
You can imagine Ricky Bobby driving if it helps.

The noise you made probably sounded something like, “weeeeeeeeEEEEEOOOOOOOooooo”. It’s a fairly common sound effect, one most people can perform even if they’ve never attended a car race. But why does it sound like that? Why does the car’s engine go from a higher pitch to a lower one as it passes us by?


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6.4 Resonance

Critical Questions:

  • What is resonance?
  • Can you shatter a wine glass just by singing at it?

In the same way that it must be illegal to talk about buoyancy without telling the story of Archimedes and the crown, everyone who teaches resonance brings up the Tacoma Narrows Bridge at one point or another.

This is probably because there is some cool old black-and-white footage of the bridge swaying and rippling impossibly in a stiff breeze, as follows:

It’s clear from the video that this was a windy day, but not tornado-windy. So what made the bridge move like that?


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6.3 Sound

Critical Questions:

  • What is a sound wave?
  • How do we hear sounds?
  • How do multiple sound sources mix together to form one sound signal?
  • Why does a flute sound different than a piano?

The well-known Zen koan asks: if a tree falls in the forest, does it make a sound? If you’re ok with the idea of not reaching enlightenment just now, physics can give a rather bland answer to the question.

Thinkin' real hard about physics right now
Thinkin’ real hard about physics right now [source]
Sound is nothing more than vibrating air. When you speak, for example, your vocal cords are moving rapidly back and forth, striking air molecules with every forward motion. These first few molecules rush forwards until they run into other molecules, at which point they collide and move back, repeating the cycle. This vibration spreads and moves through the air as a wave, causing molecules in all directions to begin vibrating. At some point, the air near someone else’s ear might vibrate as well, and the wave will travel the short distance down the ear canal until it reaches the eardrum (or ‘tympanic membrane’, which sounds much more official).


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6.2 Waves

Critical Questions:

  • What is a wave?
  • How does a wave travel?

There’s a story I heard somewhere – I have no idea whether it’s true or not, but I like it anyway. The story is that a group of people somewhere in the world developed a language that had an interesting way of referring to water waves. Instead of pointing at the ocean and saying, “Hey look, there’s a wave,” they would point at the ocean and say, “There is waving.”

If this is a true story, two things are possible: the first is that these people disliked the expression “Hey look,” and the other is that the speakers of this language understood wave behaviour much better than most.

if you ask me, he's kind of showboating [source]
if you ask me, he’s kind of showboating [source]

The reason “waving” is more appropriate than “a wave” is that waves are best understood not as objects but as actions. A water wave, for example, isn’t a thing in many senses of the word – it is definitely not solid, it may have a beginning but often lacks a clearly definable end, and it can’t be picked up and separated from the water it’s made of.


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6.1 Introduction to Waves and Sound

There is a secret about waves. It is the kind of secret that, if you fully understand it and its implications, may well blow your mind and leave you scribbling paranoid manifestos on attic walls.

Or worse, you might only think you understand this secret, and go off and make some horrible and misleading movie like What The Bleep Do We Know1 based on your flawed understanding.

What the bleep do we know
Augh, I wish I could un-see this movie

It is the kind of secret that has a lot in common with some of the best conspiracy theories: it’s far-fetched and far-reaching, and for many years it attracted only a small handful of dedicated believers trying in vain to convince everyone else that it was true.


  1. Just don’t even bother googling it, seriously.

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To Stir, Or Not To Stir

In my physics undergrad, we had one prof who seemed a bit strange. Actually, being only a bit strange seems something of an accomplishment for a physics professor, as most of them ran the gamut from odd to unintelligibly bizarre. But the one I’m talking about here was a younger guy who taught us about thermodynamics.

He was odd in a number of ways, but one sticks out in my memory: one day, in class, he said he couldn’t understand why anyone would ever stir a hot cup of tea, because stirring adds energy, which would only make the liquid hotter. He also said that people switched from wooden spoons to metal ones for the sole reason that they conducted heat out of hot liquids, allowing them to cool down more quickly.

That is patently absurd. We use metal spoons because they make a pleasant 'tinking' sound when you stir.

Both of these comments were strange because they seemed to betray a pretty deep misunderstanding of the physical world – something that would cause problems for, say, a physics professor.

Luckily for us, Randall Munroe of xkcd fame has tackled the issue of stirring hot drinks in his latest instalment of What If: Stirring Tea. The question asked:

‘I was absentmindedly stirring a cup of hot tea, when I got to thinking, “aren’t I actually adding kinetic energy into this cup?” I know that stirring does help to cool down the tea, but what if I were to stir it faster? Would I be able to boil a cup of water by stirring?’

Read the whole thing here!

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Wired: The Experiments Most Likely to Shake Up the Future of Physics

LUX Detector
LUX: A dark matter detector and art installation

If you’ve been following Pop Physics (especially if you’ve stuck through the overlong and unannounced hiatus that started with the new school year) and are still in school, there’s an excellent chance that you’ll end up working for one of these projects someday!
The Experiments Most Likely to Shake Up the Future of Physics

Here’s one of my favourites:

“NOvA will attempt to figure out this mass hierarchy by shooting a beam of neutrinos from Fermilab near Chicago 810 kilometers away to a detector in Ash River, Minnesota. A similar experiment in Japan called T2K is also sending neutrinos across 295 kilometers. As they pass through the Earth, neutrinos oscillate between their three different types. By comparing how the neutrinos look when they are first shot out versus how they appear at the distant detector, NOvA and T2K will be able to determine their properties with high precision.”


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