Rhett Allain has a list of 5 Things Every Human Should Know About Light, to tie in with the International Year of Light, and it’s a good list with lots of .gifs. Of course, there are some gaps, so let me offer some additional things that everyone ought to know about light:

Light Is a Particle

Rhett and I have a long-running argument about the use of photons in introductory physics; he’s against them for reasons that make no sense to me. To my mind, it is unquestionably true that light has particle-like properties (and here’s a follow-up with some math), and that’s a thing that everybody ought to know.

Quantum Physics Starts with Light

So, Rhett says light is a wave, and I say it’s a particle. which is it? Well, both, really, and that discovery drove the development of quantum physics, as explained in this video:

(Or you can read the approximate text here)

The particle nature of light was introduced by Planck, and refined by Einstein, and the wave nature of matter starts with Bohr’s quantum model of hydrogen, which was needed to explain the light absorbed and emitted by atoms. So, quantum physics is all about light.

And quantum physics is essential to, well, everything. Understanding the quantum physics of light and matter lets us build lasers and semiconductors, which are essential for modern telecommunications and computing. You wouldn’t be able to read this if not for light leading to quantum physics.

(More in this vein: Four Things Everybody should Know About Quantum Physics and Seven Essential Elements of Quantum Physics. And if you prefer video, here’s a TED-Ed animation about Schrödinger’s cat and computers.)

Everything We Know About the Universe Comes From Light

This isn’t just a statement about our heavy reliance on our eyes for getting around the world. Pretty much everything we know about the origin and fate of the universe comes from studying light. We can even use light to detect the existence of stuff that doesn’t emit any light:

See also this Tor.com article I wrote about spectroscopy, jumping off from Gandalf’s assertion that splitting white light to study it departs from the path of wisdom. Far from it– splitting light is behind some of the most amazing discoveries in the history of science.

So, in summary, light is awesome. The more cool things you can know about it, the better.

(“Featured image” from Russel Dickerson’s site, because it amused me.)


  1. #1 Sili
    February 14, 2015

    Aren’t you in disagreement with Sean Carroll as well (however idiosyncratic his views may be)? I think I recall him giving a talk where he came down on the side of waves.

  2. #2 Ultraviolet Thunder
    February 14, 2015

    Light works hard. It’s almost certain that a lot of your car was cut and welded with continuous wave lasers. The glass screen on your smartphone was probably cut with green picosecond laser pulses.
    From a practical standpoint waves and particles are the same to me as long as they come out of the diffraction optic stacked neatly in a 700fs heap.

  3. #3 Chad Orzel
    February 14, 2015

    Well, the “right” answer is that light is a quantum field, with photons being quantized excitations of that field. The particle vs. wave thing comes down to whether you want to choose to emphasize the wave-like nature of the field, or focus on the fact that it’s quantized, which is a very particle-like characteristic.

    I don’t recall Sean’s exact views on this. The “it’s really a quantum field” thing is the core of all anti-photon arguments, though, so if he said something favoring waves over particles, that would probably be it.

  4. #4 CCPhysicist
    February 15, 2015

    I think the “light is a particle” detail is useful because it is the most trivial way of dispensing with the need to have a medium to carry the “wave”. You don’t need a medium to carry a particle.

    Rhett’s argument about how the fields can mutually support each other in the absense of a medium begs the question of what the fields are doing there in the first place, that is, how you can have action at a distance. It also doesn’t face the question of how electrons have the same wave-like nature as light despite having mass

  5. #5 magnocrat
    February 22, 2015

    The term light to most layman means what is visable. It was centuries before the full electromagnetic spectrum was revealed. Now we see with greatly extended ‘eyes’. Yet for all that extrodinary advancement we still kill each other. Perhaps the advancement is only technological eh?

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