Slowing down c

No, that’s not a type-o, you CAN slow down the speed of light (which is denoted by the letter “C”, which is translated to English from Latin meaning “constant”). Wow, you learned something from the intro message.. this post has GOT to be chalk full of education!

So, let’s start off with what c is. C is a single letter character in physics, standing for  celeritas (Latin for Swiftness), which as you can probably deduce on your own, means constant. Using this in physics is normally designated for the speed of light in a vacuum, which is about 300,000km per second.

Now, with that understood, know that you CAN’T slow down light…. but you can slow it down too.. because physics doesn’t care about your concepts of right and wrong. Don’t worry, I’m not going to go into the quantum physics and polaritons and other things; I’m going to keep things simple and basic: Physics 101 (This means that if you’re a physics student or otherwise higher educated on the subject, you probably know more about this area than others).

We all know that the shortest possible distance between two objects is a straight line, and with that in mind, the common misconception of this physics problem is that light bounces off of particles and atoms inside the glass, so that by the time it leaves the glass, it’s had to travel farther than an identical beam of light that went straight through air or a vacuum. More travel time does not mean that it has slowed down however, it just means it had to do more “stuff” inside the glass to leave it, so while it does leave the glass at a different measured time, it’s still travelling at or near c.

So what is happening?

Easy! Wave Particle duality! I can’t believe you didn’t know this?! No, I don’t expect any non-university physics educated person to know about that, I’m just being a smart-ass. You should know that light is a particle (a photon) and a wave, and a wave propagates through mediums at different speeds. THAT is what’s happening.

But wait, there’s more.. call now and receive homeostasis!

Now, in physics, Homeostasis has no business being mentioned, but it’s a similar system that does happen everywhere. When you introduce two disruptions such as two waves which are different frequencies, they want to be as organized as possible, and one wave either increases or decreases its wavelength to match the other wave. Unless they have a source of energy that’s greater than colliding wave, always the more powerful wave will slow down to meet the constant, lower frequency wave. This is called destructive interference, and is usually why your radio in your car goes out when you pass by large antennas and towers.

What’s happening in the glass is the light, a wave, is being introduced to the particles of the glass which have their own waves propagating from it. The light then becomes disturbed, and eventually matches the propagating glass waves. As it goes through the glass, it keeps encountering different wave frequencies (since glass is not a perfectly harmonized substance), so the light has to keep changing. These changes are so minor that to the human eye, and even to the technological eye such as spectrometers, even though the wavelength is changing, the colour of that light doesn’t change a noticeable amount.

Wait what? What does colour have to do with this?!

I’ll end this overly confusing article about the wavelengths of light and what that means to us. Waves are measured in wavelengths, and depending on their frequency, determines if they fall into the visible spectrum or not to humans. Depending where it falls in that range determines the colour we determine, from red to violet (see where the terms infra-red and ultra-violet come from now?!).

I expect many questions and corrections on this article, so please feel free to comment below with cited sources and I’ll be happy to discuss or even correct the above with credit to you 🙂

Jeff Wilton

Jeff is the founder and owner of Everyday Science Stuff. ESS is a one man operation, with the core belief that all education should be served without crippling debt tuition, without revenue generating ads and without any restrictions of any kind such as paywalls, forced login and account creations, geographical restrictions, and so on.

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2 Responses

  1. Anderson says:

    Excellent article(my humble opinnion as a person non educated on the matter). I always thought it had to do with the whole “bouncing particles” thing, this of course not only clarified the misconception I had on the matter, but also made me ask myself a few more questions about it. Anyways great site, too bad it seems a bit quiet on the comments side, thanks.

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