◾ BASIC PRINCIPLES OF LIGHT AND VISION
As you can see in Figure 1.1, the visible light is just a small part of the electromagnetic spectrum to which the human eye is sensitive, and includes waves with lengths from around 380 nanometers (9-10 m) to about 780 nanometers. On the lower-energy side, the spectrum starts with radio waves that we use to transfer images and sounds (like radio and television); continues to microwaves, used in devices such as radar and the microwave oven; and further down one ¨nds the infrared waves that we perceive as heat. On the higher-energy side of the visible applications with shorter wavelengths, we ¨nd the ultraviolet radiation (see more on UV radiation in the Nonvisible Applications section of Chapter 3). Žen we ¨nd X-rays used in medicine for bone outlining, then gamma rays, and ¨nally cosmic rays. Di¥erent regions of the electromagnetic spectrum are presented in the ¨gure.