[tobias89]

How would you calculate wavelength (in nm) given the energy in cm-1. I realize that its e= hc/λ. Everything I found online said to get cm-1 into energy in joules multiply by Plancks and the speed of light. But it doesn't seem to work.
 For example: Energy in cm-1= 14,904
14904 * (3*10^8) * (6.626*10^-34) = 2.96*10^-21 Joules
Plug that into e= hc/λ
2.96*10^-21= 1.9878*10^-25/λ
And you get a ridiculous answer. What am I doing wrong? Thanks!

[Sunil Simha]

Hello tobias89,

Are you aware of a quantity called wave number?

[tobias89]

I am not.  Thank you for the fast reply by the way!

[Corribus]

Tobias, easy way to remember:

Take the value in wavelength.  Divide 10⁷ by this value.  Done!

So, 400 nm: 10⁷/400 = 25000 cm^-1

Same relationship works for the reverse conversion.

Wavenumber is just the reciprocal of the wavelength, but since wavelength is in nm and the wavenumber is in cm (inverse), there's a factor of 10⁷ for the conversion.

[tobias89]

Ohhhhh okay! That makes since now! Thank you guys so much!