# I don't understand per cm

## Main Question or Discussion Point

I don't understand "per cm"

Chemists seem to use the "per cm" $$(cm^{-1})$$ scale for spectroscopy. I'm use to dealing with THz.
Can someone please point me to a resource or explain where per cm is used and how to convert between different common units??
I know you just take the wavelength, put in cm form and inverse - but I don't think its as simple as that for bandwidths...

mgb_phys
Homework Helper
Chemist use wavenumber (per cm) for frequency - you have to get used to it.
The main advantage is that for most optical-ir you have a convenient size number to deal with rather than having 10^12 Hz type units.

It makes less sense for bandwidths but it's still the same number of Hz.

Last edited:
symbolipoint
Homework Helper
Gold Member
Does the cm to the negative one indicate wavenumber? Would this be "cycles per centimeter"? ...can't remember too well myself. Check a handbook or an analytical chemistry textbook.

Chemist use wavenumber (per cm) for frequency - you have to get used to it.
The main advantage is that for most optical-ir you have a convenient size number to deal with rather than having 10^12 Hz type units.

It makes less sense for bandwidths but it's still the same number of Hz.
Ok so for bandwidths i can get $$\Delta f$$ but NOT $$\Delta \lambda$$?

Anyone know where these units started from? Even for IR the numbers are still fine,
eg. 1550nm corresponds to ~2Thz.
THAT IS A MUCH NICER looking number than: 6452cm^-1.

I mean "cm"?? C'mon, surely there is a better explanation. - I just don't like getting use to weird units.

mgb_phys
Homework Helper
It started in the 30s when CGS rather than MKS was used.
Frequency makes more sense if you are talking about bonds resonating, wavelenght makes more sense when talking about widths of spectra.
Really, using metres ( a fraction of the Earth's circumference) makes no more sense when talking about wavelegths of light!

ps. The units are much worse in astronomy.

Really, using metres ( a fraction of the Earth's circumference) makes no more sense when talking about wavelegths of light!

ps. The units are much worse in astronomy.
Good point...Now I need to spend time getting use to relative numbers in per cm cm -1 is used for infrared spectroscopy. it indicates the inverse of frequency.

ummmmmmm.... its actually the inverse of wavelength, NOT frequency; hence the units of cm^-1 and not s.

Wavenumber is used simply because, as other have said, the numbers turn out pretty nice (especially for the IR region).