The following glossary includes terms that are used in this student text. You will probably want to study the terms and their definitions before you read the student text.

Isotropy (I-'sä-tr&-pE)
Isotropy is the state of having the same properties in all directions.

Homogeneous ('hO-(")mO-'jE-nE-&s)
Homogeneous is the quality of consisting of similar parts or elements.

Heterogeneous ("he-t&-r&-'jE-nE-&s)
Heterogeneous is the quality of being composed distinctly different parts or elements.

Light-year ('lIt-"yir)
A light year is the distance that light can travel in one year, which is approximately six trillion miles.