ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Photocopier in an office. Photocopiers produce black-and-white copies of documents. The document is placed on top facing down. Beneath is a positively charged photoconductive drum. A powerful light scrolls across the document. Photons are only reflected back from the light areas. These hit the drum. Because of its photoconductive properties, the drum loses the positive charge in those areas which are hit. So only the parts corresponding to dark areas on the paper remain positively charged. A fine, black negatively-charged powder called toner is added, which adheres to the positively charged areas. Paper is rolled over this and heated so that it bonds to the toner. A copy is made.
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