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The Daily Tar Heel

As a student of physics, I’ve noticed that a lot of times, it’s helpful to think of certain difficult concepts in terms of simple things with which I’m already somewhat familiar.

For example, I sometimes think of electrical circuits as water flowing through a series of little pipes. In this analogy, pumps represent batteries, water flowing past a certain point represents electrical current and some sort of obstacle that impedes the flow is a resistor.

There are some things in life, however, that have no simple, intuitive explanation. These are the hardest to fully understand but also the most interesting. Light is one of those things.

Basically, you can think of light as simply a wave that travels through space, just like an ocean wave or “the wave” at Kenan Stadium on game days.

But unlike the crowd wave, which cannot exist without a few thousand antsy Tar Heels to push it along, light doesn’t need a medium through which to travel— it is actually self-propagating. A changing electric field produces a changing magnetic field, which then produces a changing electric field — a cycle which will repeat forever, as long as nothing gets in the way.

Well that’s not terribly difficult to understand. But wait!

In 1905, Albert Einstein discovered the photoelectric effect, by which light can exchange energy with electrons in a metal. But this energy only occurs in discrete chunks, called photons.

In the photon model of light, a beam consists of a bunch of little particles flying around, like a stream of infinitesimally small baseballs. Only, these baseballs have no mass whatsoever, and fly around at the speed of light.

So, which is it? In fact, both of these seemingly contradictory ways of describing light are entirely accurate. Light is both a particle and a wave. Your choice to make use of one or the other only depends on the type of problem you’re working on at the time.

Another bizarre property is that light carries momentum, despite having no mass. In fact, this was shown more than 100 years ago.

Typically, we think of momentum as a measure of the “quantity of motion” of an object: Heavy, fast things have a lot and light, and slow things have a little. So, intuitively, a massless object should never be able to have any momentum. Oddly enough, if you stood in front of an intense enough light source, you’d feel it pushing you back. In fact, one proposed method of spacecraft propulsion as an alternative to rocket fuel is to use what are called solar sails; essentially giant mirrors that are pushed by sunlight.

It’s funny how we make use of the properties of light all the time, yet its true nature is so difficult to pin down. And despite being a weird and unique natural phenomenon, the behavior of light is very well understood by scientists.

There are many stranger things out there that have little hope of ever being explained as something simple or intuitive. Maybe the best way to go about studying certain things is to keep in mind the old sports cliche: “It is what it is.”

Nick Mykins is a senior physics major from Raleigh, N.C. Contact him at

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