The quantitative aspects of flow cytometry are based upon the measurement of light, be this fluorescent, scattered or absorbed. Because of this fundamental dependence on light some of its properties and its behaviour on interaction with matter will be considered briefly.

Snell's Law

A light beam which encounters a surface at an angle where there is a change in refractive index at that surface (also termed a dielectric interface) will exhibit two phenomena. Firstly, some of the light will be reflected symetrically about the perpendicular to the surface at an angle equal to the incident angle. Second, the remainder of the light will cross the dielectric interface and enter the second medium. However, the angle at which the beam traverses the second medium is not the same as the incident angle. This is refraction and both phenomena are depicted in figure 3.1 where the beam passes from a low (A) to a high (B) refractive index material.

The first descriptive observation of refraction appears in Plato's Republic (c. 3 70 bc) with the following quotation attributed to Socrates '… the same object appears straight when looked at out of water and crooked when in water…’ (from Herzberger, 1966). Amazingly, the first measurements of refraction were performed by Ptolemy of Alexandria (c. ad 150). He constructed a circular disc, looking much like a clock face, with two equal-length pointers free to rotate about the center and the circumference was marked out with 360 equal divisions.