This Demonstration shows the light amplitude vectors and their relationship to the thickness of a glass slide and its reflectance. The surface reflection amplitude vectors plotted are based on the time it takes for red light to bounce off the surface and the bottom of the glass slide. Total amplitudes can be composed of a vector sum of partial amplitudes. Probabilities are then calculated from the squares of the total amplitudes. For example, the norm of the difference of the two vectors is squared to calculate the reflectance (which is a kind of probability) of the glass slide. A visual representation of the relationship between the thickness of a glass slide and its light reflectance can thereby be constructed.