Today’s post is a light-hearted introduction to interferometry and why we need it to understand light waves.
There are some waves you can detect with your body. Earthquakes will shake the ground, which you will feel if you are standing near the center. Sound waves oscillate through the air and oscillate your eardrums, oscillate even your skin if they are strong enough. You might not see ocean waves if you are standing on the shore, but you can still see them rippling through the water.
These examples show that a wave doesn’t exist without a medium. This becomes a difficult statement to accept when you apply this idea to electromagnetic waves; many experiments have sought to prove that photons also travel through a medium, and while some of these experiments have made incredible discoveries (for instance Michelson and Morley’s famous experiment to prove the existence of a “luminiferous aether” led to the development of special relativity), there is still no convincing evidence.
The problem then becomes this: without a medium, how do we observe basic properties of electromagnetic waves such as phase, frequency or group velocity? (For those without a background in physics, these are all properties that have to do with the position of the peaks of waves, and how they relate to neighbouring peaks.) When you look at a light wave, you see a constant brightness, because across all the peaks and troughs the amplitude will be the same to your measuring device (in this case, your eye.)
It’s important to note that even though you can see light, you’re not seeing the wave itself, only the interaction of the photon with the mechanisms in your eye. It’s like how you can see pictures in an Etch-a-Sketch only when the magnetic grains interact with the screen, but you can’t see them moving in behind when you gently rotate it.
This is where another characteristic of waves in a medium comes to light, and for this I invite you to think back to a time you were on a trampoline. In one moment, your friend jumped at the perfect moment so that you were double-bounced and flew twice as high in the air as you could on your own. But a few moments later your timing was slightly off, and when you went to jump you found that you were stopped short by your friend’s bounce.
These are both events that happen when two waves add together, be they trampoline waves, ocean waves, sound waves, and electromagnetic waves. In the first case you both created peaks at the same place on the trampoline, so when they added together the final peak was twice as big and bounced you twice as high. In the second case you created a peak in a place where your partner made a trough, and when they added together and cancelled out to be flat.
This adding of waves is called interference, and we can see the result even in light waves because it changes the amplitude of the wave. This means that under the right conditions (a topic for another post), you can see an alternating pattern of light and dark lines (or fringes, or circles, or dots, depending on your experiment). You can then pick apart this pattern with careful measurements to learn about the waves’ phases and how they change over time.
Interferometry is a genre of measurement that takes advantage of these interference patterns. Phase measurement and phase retrieval are interesting topics on their own, but interferometers can also be used to make extremely precise measurements of small distances (for example the thickness of an ultrathin graphene layer), can profile very shallow 2D surfaces (like the surface of a cell in your skin), or describe the refractive index of a medium (like what kind of colours changes you could expect from a camera filter.)
The last example is a special subset of interferometry called white-light interferometry. There is no such thing as a purely white photon; when you see white light coming from a computer screen, the plastic lid of a coffee cup, or even the sun, what you’re really seeing is thousands of photons of different colours jumbled up together (the opposite of black, which is the absence of photons.) While classic interferometry uses a light beam of one colour to get a very precise and easy-to-analyse pattern of light and dark fringes, white light interferometry produces a smear of colour that must be picked apart very carefully, however it allows us to see what happens to a wide variety of wavelengths.
I like this topic because it’s another example of how measurement is the comparison of qualities. As we are part of our system, our universe, we cannot make absolute measurements, which is how physics to me is profound and beautiful.