# Speed of light in pure Hydrogen

1. Sep 29, 2008

### Arnaho

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.

2. Sep 29, 2008

### ZapperZ

Staff Emeritus
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.

3. Sep 29, 2008

### Arnaho

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 ?

4. Sep 29, 2008

### ZapperZ

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

Zz.

5. Oct 3, 2008

### Arnaho

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

6. Oct 3, 2008

### mgb_phys

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

7. Oct 3, 2008

### Redbelly98

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

8. Oct 6, 2008

### mgb_phys

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

9. Oct 6, 2008

### Arnaho

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 !!!!

10. Oct 6, 2008

### Redbelly98

Staff Emeritus
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.

11. Oct 8, 2008

### Arnaho

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