Light is everywhere. By day it shines down on us from the sun, by night it comes from our lamps. But how well do we really understand it?
Light is a wave
Waves, like ripples on water, or sound moving through the air. This means, among other things, that if light moves through a small slit, it will ripple out in all directions. See the diagram below.
One way to reproduce this is to create slits like this, then point a laser pointer at them. If the slit is small enough, you’ll see a pattern of light on the other side — word of warning, don’t look directly into a laser pointer, it will destroy your retinas.
With a setup like above on a table, if you were to project the result on a wall, you’d get a horizontal bar of light on the wall, like this:
When you create two slits, and use two laser pointers, you get an interference pattern like the one below.
Now it gets interesting. What will the projection on the wall look like? A horizontal bar like above?
Because light is a wave, it consists of a sine, with peaks and valleys. We can’t see those peaks and valleys in this case, but you can for waves in water, for example. The principle is the same. When two ripples like above form an interference pattern, the peaks and valleys are combined. Where two peaks intersect, they become higher. Where a peak and a valley intersect, the two cancel each other out.
This happens for light as well, and you get a pattern like this, because the canceled out waves are dark and the higher peaks are bright:
So, in short: light is a wave.
Light is a particle
About a century ago, scientists had concluded it was waves all the way. Then, unfortunately, they ran into a problem.
When you shine light onto certain metals, they start to shed electrons, the so-called photoelectric effect. The wave-as-light theory predicts that this would happen when enough energy was transferred to the metal, and the resulting electrons would be thrown out.
Unfortunately, actually testing this showed something different. Electrons were ejected in proportion to the frequency of the light, in other words, the distance between the peaks and valleys in the wave. That frequency determines the color, meaning the ejected electron rate is related to the color not the brightness.
As it turns out, the only way to solve this, is by viewing light as particles. Photons are packets of energy that make up light. The frequency of the light determines the energy of the packets, which is why the frequency determines the electron ejection on metals.
Since Einstein (yes, him again) came up with this, we’ve seen a lot more things that can only be explained by light as energy packets. Quantum interactions with other particles, for example, can only be explained by introducing photons.
So light is a particle and a wave. That can’t be right, can it? Well, apparently it can be. Both models are required to explain certain phenomenons, but they also seem to contradict each other.
Imagine the slit experiment we did above. How does that translate to particles? What if we send only a single particle through the slits? One particle can’t form an interference pattern, right?
Guess what, that’s not what happens. A single photon still behaves as if there is an interference pattern. Welcome to the world of quantum mechanics.
Sometimes it appears that science is ‘fixed’ or ‘done’, but people thought the same thing by the end of the nineteenth century. Einstein shattered that idea, and we still haven’t figured out what light really is. Science still changes, and insights differ over time. There are still mysteries to solve, like the particle-wave duality.
And thankfully, that also means we can write scifi stories to fill those gaps.