How to slow down the speed of light

Pump the brakes there, chief.

Sep 30, 2021

At It’s Not Rocket Science, we’ve talked about light before. Mostly about what it would look and feel like to travel that fast. But we never took the time to stop and think about if we need to be going that fast. Today we’re going to talk about the ways in which we can actually slow light down, and what happens when we manage to do it.

Light moves differently in different mediums. Imagine running down the street, the wind blowing past your face. As hard as that can sometimes be, it would be way easier than trying to run the same speed in water. This is because water is denser, and therefore requires more work to move through. With light, the underlying physics are different but the outcome is largely the same, the denser a medium (gas, liquid, solid) is, the slower light moves through it. This means that light moves fastest in a vacuum, reaching the true speed of light. Everywhere else, however, light moves more slowly. We just added more complexity to the concept of light.

Believe it or not, this is actually why light seems to bend in water, or a transparent crystal. [1]

But Matt, you’re thinking, why does it bend? Wouldn’t it just continue to travel the same direction but slower? No, silly.

Here’s the funny thing about light: it acts like a water skier. Bear with me here. Imagine you’re water skiing. The sun is shining; it’s a good day. You’re shredding the wakes when all of a sudden you catch an edge and bang. Your body rotates faster than you can blink and you faceplant into the water. This sudden change of direction happens with light too. In the image above, the light is traveling straight, and “catches” the edge of the new medium. This causes a change of direction that is relative to the difference in material properties. Once it changes direction, it travels straight, at the speed of light for that medium, until it catches another edge. This concept is called refraction.

We can play with these properties like in the image above so that different wavelengths of light refract differently. This causes the different colors to separate and form a rainbow. This effect is actually how rainbows themselves are created, but the medium refracting the light are raindrops. This is also why when you put your hand underwater and look at it from above, it appears to be in a different place than it actually is. The light from your hand is refracting and appears to come from a different location.

So there you have it! If you want to slow down light just don’t be in a vacuum. Oh wait, you’re already doing that. Nice work! In reality, these changes are very small, but they’re enough to bend light and make some pretty cool effects. Oh and scientists use them for, like, experiments and stuff.

Thanks for reading this week’s edition of It’s Not Rocket Science, see you next week!

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