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Do Xrays, UV, IR, Microwave, and Radio waves all travel at the same speed as Visible Light?

by on December 24, 2013


Do Xrays, UV, IR, Microwave, and Radio waves all travel at the same speed as Visible Light? Do the difference in wavelengths affect the speed of travel?

The TL;DR part is this:

In vacuum they all travel at the same speed.

In a medium the speed can and does depend on wavelength.

Here is the the long technical conversation between two of our lab minions:

When talking waves, Speed = Wavelength x Frequency.

All of these waves are electromagnetic radiation, so in a vacuum they will all travel the same speed: the speed of light, c. So this equation looks like :

c = λ * f

where c = speed of light, λ = wavelength, and f = frequency.

As either the wavelength or frequency changes, the other will do so also to keep the speed at c. So radio waves have a big wavelength but short frequency, x-rays and such will have high frequencies and small wavelengths.

Now that’s a vacuum, electromagnetic radiation is affected by the medium it is traveling through. You can usually find out how it affects it by looking at a medium’s “index of refraction,” which is basically a measurement of how EM radiation slows down upon entering a medium (which is what causes refraction).

The new speed at which EM will travel through a medium (which will be some percentage of the speed of light, always less) can actually be dependent on the wavelength. So for instance, air might slow down EM radiation in the “visible light” spectrum more or less than it does EM radiation from some other part of the spectrum, like radio. It can be very specific, some media will even completely block parts of the spectrum. This means some example medium (like glass, for example) doesn’t have a set refractive index that you can use for all parts of the EM spectrum, you will have to use different indices depending on what is going through it. So that’s why when you look at a list of these indices, such at this one you will see that it also lists the wavelength of light. Those are the correct indices IF you are using that wavelength.

This is why rainbows and prisms work, actually, as even inside the small visible light part of the spectrum the different wavelengths are affected differently. Because of that, when white light (made of all the colors) enters a new medium such as glass it will spread out into it’s constituent colors (aka wavelengths) as some were slowed more than others.

Because c changes relative to the material is propagates through, does this affect relativity in say our atmosphere? Is time dilation different now since our speed is now closer to c?

Just a side note, c is a constant, it does not change, it is the speed of light in a vacuum. The speed of light in other mediums may be different speeds, but not some new version of “c” technically. If you were to use the equation I used in anything but a vacuum you wouldn’t use the letter c.

I’m not quite sure what you mean by our speed being closer to c now, I don’t know who “our” is and what is moving close to the speed of light.

The equations of relativity are based on the ratio of an objects speed compared to the constant c, comparing two frames of reference where the speed of light in a vacuum is “universal frame of reference,” and so if I understand what you are asking then the answer is no, it would not affect it. I’m sorry I couldn’t understand all of it.

Sorry for the confusion. Let’s say I’m walking at 1 m/s in our atmosphere. Technically the speed of sound through air is slower than c. For relativity it’s relativity, it’s related to v2/c2, does this mean that ratio is smaller now since the speed of light is now slower where I am? Based on your response though, I’m getting that it doesn’t matter where you are, you always use c in a vacuum.

Exactly, the equation uses c, which is only one number ever. If it meant “Speed of light in the medium you’re in” then it wouldn’t be c, it’d probably be another “v” with a subscript to describe it, like v (subscript) EM, or something.

That’s the great thing about relativity, Einstein actually found a relevant reference frame that you can use, because before it was like “well this depends on your frame of reference,” “everything is gonna be different based on how you view it,” blah blah, but then guess what? Bam: c. Speed of light in a vacuum. Just compare things to that, it is a constant and a maximum upper limit. I mean, you can get into superluminal physics with tachyons and all that if you want, but kind of beyond the scope of this issue.