I don't understand per cm

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In summary, chemists use the "per cm" scale for spectroscopy. This scale is based on wavelength, and is easier to deal with than frequency units. 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. However, for bandwidths, frequency is the preferred unit.
  • #1
n0_3sc
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I don't understand "per cm"

Chemists seem to use the "per cm" [tex] (cm^{-1}) [/tex] 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...
 
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  • #2
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.
 
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  • #3
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.
 
  • #4
mgb_phys said:
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 [tex] \Delta f [/tex] but NOT [tex] \Delta \lambda[/tex]?

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.
 
  • #5
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.
 
  • #6
mgb_phys said:
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 :rolleyes:
 
  • #7
cm -1 is used for infrared spectroscopy. it indicates the inverse of frequency.
 
  • #8
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).
 

1. What does "per cm" mean in scientific measurements?

In scientific measurements, "per cm" refers to the unit of length centimeter. It is often used to express a rate or ratio of a certain quantity per centimeter of distance.

2. How is "per cm" different from other units of measurement?

"Per cm" is a specific unit of length, while other units such as grams, seconds, or liters refer to different quantities. "Per cm" is used when expressing a rate or ratio of a certain quantity over a distance of one centimeter.

3. Can you give an example of "per cm" in a scientific context?

One example of "per cm" in a scientific context is when measuring the density of a substance. Density is often expressed in units of mass per volume, such as grams per cubic centimeter (g/cm3).

4. How do I convert from "per cm" to other units of measurement?

Converting from "per cm" to other units of measurement will depend on the specific quantity being measured. It is important to understand the relationship between the units and use conversion factors to make the conversion.

5. Why is "per cm" used in scientific measurements?

"Per cm" is used in scientific measurements because it allows for a more precise and standardized way of expressing rates or ratios over a specific distance. It also makes it easier to compare measurements across different scales and units.

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