Beer-Lambert Law

If you ever take a colored drink that's not water, like a juice, Gatorade, Kool-Aid, or anything along those lines, you'll notice that there the drink is filled with lots of color. If you try to shine a light through it, it isn't the same as shining a light through water. The colored drink will absorb a lot of the light while not that much light will transmit. This property of solutions is known as absorbance, and it measures how well the solution absorbs light.

The equation for absorbance is given by the Beer-Lambert Law below:

where A is the absorbance, a unitless quantity. Epsilon is a characteristic constant known as the molar absorptivity which differs from substance to substance. The units for molar absorptivity are 1/(M cm), where M is the molar concentration. l is the path length of the light in the solution it passes through. C is the molar concentration of the solution, measured in mol/L.

Usually, epsilon and length don't change so this law states a linear, direct relationship between a solution's absorbance of light and its concentration. This makes sense because if your solution is more concentrated(think a very, very red bottle of Kool-Aid), it will absorb red very well. This helps for solution analysis, especially with regards to concentration.

Diluting a solution(adding water) will make it clearer, which you can observe in real life. If you take a glass of juice and add water to it, the juice will become mixed with the water and get mixed so that it's visually clearer. This means it now absorbs less light, meaning diluting a solution with water decreases its absorbance.