# Speed of light in pure Hydrogen

Hello Everyone

Can anyone tell me what is the speed of light in hydrogen or may be where can I find that for all the elements.
Has these datas available already in or books.

ZapperZ
Staff Emeritus
Hello Everyone

Can anyone tell me what is the speed of light in hydrogen or may be where can I find that for all the elements.
Has these datas available already in or books.

This is a very vague question, because it depends on the gas DENSITY. I could have only one hydrogen atom in a vacuum and the speed of light would not change by any noticeable means.

Zz.

Hi Zz. !!
Thanks ! Yes of course ! Say we are in space with 1m3 of H2 trapped inside a chamber. And now if we pass the light thru it, than can we know the difference in the speed of light compared to its original ?

ZapperZ
Staff Emeritus
Hi Zz. !!
Thanks ! Yes of course ! Say we are in space with 1m3 of H2 trapped inside a chamber. And now if we pass the light thru it, than can we know the difference in the speed of light compared to its original ?

You've just given me the volume of the gas, not its density.

Zz.

Oops !!! Lets assume the density is 0.0899 kg/m3. How should I proceed now ??!!!

mgb_phys
Homework Helper
At stp the refractive index of hydrogen (H2) is 1.000 132 (for Sodium D line probably)
The refractive index increases linearly with pressure (for an ideal gas) but i don't have the constant for hydrogen

Redbelly98
Staff Emeritus
Homework Helper
The refractive index increases linearly with pressure (for an ideal gas) but i don't have the constant for hydrogen

Wouldn't (n-1) simply be proportional to the gas density?

mgb_phys
Homework Helper
Wouldn't (n-1) simply be proportional to the gas density?

Yes sorry, I meant (n-1) rather than n. We tend to assume (n-1) .
But the poster presumably still needs the constant.

Hello again, So I calculated the light speed in H2 and its value is this 299960405.22 m/s. Almost equal to its speed in vacuum. This is how I did, I got the refractive index for H2 and its 1.000132. And 1/1.000132 will give us 0.99986.... when multiplied with the Speed of light in vacuum ie, 3x10^8 m/s we will get the speed of light in H2. Please correct me if am wrong. Thanks !!!!

Redbelly98
Staff Emeritus
Homework Helper
There is a problem that your answer is actually greater than c! The issue here is with the accuracy of your answer (i.e. number of significant figures)

If you're using 3x108 m/s for c, that's only 1 significant digit of accuracy, so the answer for H2 would still be 3x10^8 m/s.

Even if you use 3.00x108 m/s, that still won't change significantly when divided by 1.000132.

Better yet: use 2.997925x108 m/s. That has the same number of significant figures (7) as the refractive index.

Okay, Thanks !! I will try with this and let see what comes up !!