THE NATURE OF LIGHT
Light consists of particles 
called photons, each one of 
which can be regarded as a 
packet of electromagnetic 
waves. For a beam of 
electromagnetic energy to be 
light, and not X-rays or radio 
waves, is a matter of the 
wavelength--the distance from 
one wave crest to the next--and 
in the case of light this 
distance is about 5 x 10-7 
meters, or 0.0005 millimeter, or 
0.5 micrometer, or 500 
nanometers.

    Light is defined as what we 
can see. Our eyes can detect 
electromagnetic energy at 
wavelengths between 400 and 
700 nanometers. Most light 
reaching our eyes consists of a 
relatively even mixture of 
energy at different wavelengths 
and is loosely called white 
light.