# Homework Help: Light absorption through water

1. Aug 3, 2010

### dobbinatrix

1. The problem statement, all variables and given/known data
I am currently doing a biology honours project at university and have been stumped by a fairly basic physics hurdle. I need to know the equation to calculate the amount of light absorbed by water/unit depth. I have tried to find the equation all over but cannot find anything. Im sure i did this in A-Level physics but i really cant remember -_-

Any help would be greatly appreciated
Thanks

2. Relevant equations

3. The attempt at a solution

2. Aug 3, 2010

### ehild

The light absorption in water depends on the wavelenght of the light. It is fairly law for the visible light but strong for UV and infrared. See at
http://upload.wikimedia.org/wikipedia/en/7/72/Water_absorption_spectrum.png [Broken].

If you have the absorption coefficient alpha in reciprocal centimeters, multiply it by the thickness (in cm units) and you get the "absorbance" (A) of the water layer of the given thickness. The intensity of the light decreases with the ratio exp(-A) or 10-A, depending on the way alpha was given.
If the intensity of the incident light is I0, it decreases to I0*10-A after travelling through a distance d in water.

ehild

Last edited by a moderator: May 4, 2017
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