# Absorption Spectra

1. Sep 16, 2009

### benndamann33

If you're reading a graph of absorption coefficient(cm^-1) versus wavelength(nm), how can you dtermine if the solution is optically dilute? What about optically thick? Thanks
Ben

2. Sep 16, 2009

### Kalvarin

Well as the absorbtion coefficient is per cm, i am guessing the higher it is the more light is being absorbed when travelling through a cm of the substance. So the optically diluter substance is probably the one with the lower coefficient.

3. Sep 16, 2009

### benndamann33

I agree, unfortunately I'm not comparing two spectra, but rather just answering for a variety of spectra is the solution optically dilute? Is it Optically thick? And not sure where the cut off is, can't find a value anywhere

4. Sep 16, 2009

### Kalvarin

K i looked on wikipedia and the equation for absorbtion coefficient is:

I = Io e^-ax

Where I is transmitted intensity, Io is incident intensity, a is the attenuation coefficient and x is the thickness of the substance. Rearranging for a:

a = ln(Io/I)

If we try some values:

When 75% of light is transmitted a = 0.28

When 50% of light is transmitted a = 0.69

When 25% of light is transmitted a = 1.39