and so i have our incident ray, so in the vacuum it's traveling at v1--and in the case of a vacuum, it's actually going at the speed of light, or the speed of light in a vacuum, which is c, or 300,000 kilometers per second, or 300 million meters per second--let me write that-- so c is the speed of light in a vacuum, and that is equal to 300-- it's not exactly 300, i'm not going into significant digits-- this is true to three significant digits--300 million meters per second.