The xerographic (dry writing) process was invented in 1938 by C. Carlson. In xerography, an image is produced on a photoconductor by exposing it to light. Selenium is commonly used, since its conductivity increases three orders of magnitude upon exposure to light in the range from 400 to 500 nm. What color light should be used to cause selenium to become conductive? (See Fig. 2.2.)

Fig. 2.2

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10-12 10-10 4 X 107 Gamma X rays Ultraviolet rays Wavelength in meters 108 4 x 1077 × 107 10+ Visible 5 X 107 10-² Infrared Microwaves 6 x 10-7 1 10² Radio waves 104 FM Shortwave AM 7 x 107