Light waves are distinguished by their lengths. The distance from any point on a wave to the next identical point, such as from crest to crest, is called the wavelength (Figure 2.2).

The human eye responds to waves that have extremely short wavelengths. Physicists and astronomers measure these waves in nanometers, nm, or the angstrom unit, A, after Swedish physicist Anders J. Angstrom (1814-1874), who first measured wavelengths of sunlight. One nm is 10-9 m, and one angstrom is 0.10 nm.

The diameter of a human hair is about 50,000 nm (500,000 A)!

Visible light has wavelengths of 4000 A to 7000 A. The varying wave-

Figure 2.2. Wavelength measured from crest to crest or trough to trough.

lengths of visible light are perceived as different colors. The arrangement of the colors according to wavelength is called the visible spectrum.

Refer to Figure 2.3. Which color light has (a) the shortest wavelength?

___(b) the longest wavelength?___(c) To which wavelength

(color) is the eye most sensitive?_

Answer: (a) Violet; (b) red; (c) 5550 A (yellow-green).

o 80

cd E

"5 60


Violet Blue Green Yellow Orange Red

05 20

4000 4500

5000 5500 6000 Wavelength (angstroms)

6500 7000

Figure 2.3. Relative sensitivity of the human eye to different colors and wavelengths of visible light.

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