Light, Radiation, and Quanta

Abstract

For thousands of years, people observed stars and planets solely by that marvelous instrument, the unaided human eye. And, despite the various forms of electromagnetic radiation populating the environment, such as microwaves, radio waves, ultraviolet and infrared waves, and so forth, the human eye still responds only to the visible portion of the electromagnetic spectrum of radiation. That visible portion is, of course, what we call light; the remainder is usually referred to as radiation. Human eyes are sensitive to only two characteristics of light: its wavelength and its intensity. For the moment, you may think of wavelength as referencing the color of the light; I will present a more technical definition shortly. Intensity means the amount of light and is a measure of intrinsic brightness: a 1000-W bulb is more intense than a 100-W bulb; the sun is far more intense than either. It is usually easier to see a source of greater intensity than it is a weaker one, but just as too weak a source will not lead to seeing, too strong a source cannot be viewed for too long.