I set up a way of thinking about the colour filter. The arrow on the left is the white light made of red, green and blue. Shone at a cuvette full of copper sulfate, let's say half of the red and half of the green is absorbed so the other half is transmitted. The blue light is unaffected so any blue that goes into the cuvette must come out. Io is the abbreviation for the intensity of the light going in and I is the abbreviation for the light coming out. So in my way of doing things, the total going in is 3 and the total coming out is 2.
Now let's insert an orange filter that absorbs all of the blue light.
Then let's apply the formula for the absorbance which is based on logarithms.
Notice that the absorbance value is much higher with the filter in place because the transmitted blue light has been removed. That's why we need a filter in a colorimeter.
