# Why do we have to do this?, or Making a Graph for our Spec

We still need to know the rate at which light gets absorbed ("e"), so we can interpret what the spec is telling us. But how do we get that rate? Just like there are lots of possible rates for paycheck vs. time spent working (depending on what your job is), there are also lots of possible rates for absorbance vs. concentration (depending on what kind of solution you're testing).

The only way to find the correct rate is by measuring it. And before we can even start measuring, we need to know what wavelength of light to use. This depends on what you're testing. Pure water (at least in small amounts) doesn't absorb any light, so all wavelengths (colors) are equally able to get through. Mixed together, they look like white ( = colorless) light. Gunky fish tanks appear greenish because whatever is in there absorbs the other colors of light. The more gunk that's in there, the more non-green light gets absorbed -- and correspondingly, what does get through looks greener.

# The Virtual Spec

Choose a solution:

Now, let's think about this a little differently - the light does not actually have more green at all, it has the same amount it always had. What is really happening is that you are seeing LESS of the red and blue colors so that the green becomes more prominent. In other words, the amounts of the colors you DON'T see are changing in proportion to the amount of gunk! So, if you want to measure the concentration of gunk in green water you need to set your spec to measure the absorption of one of the other (non-green) colors.

Likewise, red coolaid absorbs all the light except red. To test the concentration of coolaid, you would need to set your spec to measure something other than a red wavelength.

So in general, before you use a spec, you have to figure out what wavelength works the best. How do you do that? More or less by trial and error. But after you've figured out the best wavelength, you can proceed to the next step, actually measuring e...