Activity 7
Just the Fax

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Background Information

The fax machine scans an image with an array of solid-state detectors that sense how each part of the image reflects light. The signal from these detectors is sent over telephone lines and is reconstructed to make the transmitted fax. Most faxes are printed on thermal paper, which darkens when exposed to heat from an array of wires. Currents through these wires produce the heat that makes the pixels as the paper moves over the wires. The resolution of a fax is 8 pixels per millimeter, or 200 pixels per inch.

Resolution is a concept related to the perception of detail. If students examine the fax image with a magnifier, they can see graininess. Curved lines appear to be jagged, made of many steps. But when the students back away, in Step 5, the jagged curves disappear and look perfectly smooth. The same process occurs in looking at a newspaper photograph up close, where the individual dots are visible, and from a normal distance, where the individual dots cannot be seen. From close-up, a viewer can resolve the dots. The explanation for resolution lies in the physics and biology of vision. Resolution depends on the size of the light-sensitive cells in the retina and also on how light spreads out, through diffraction, as it enters the pupil. Also, any limitation on focusing makes it correspondingly more difficult to resolve detail.

In this activity, students create a fax image from a stream of data. The data, on page 286, is divided into groups of three rows. Each group contains 160 ones and zeroes. The students prepare graph paper with 40 squares per row. Every four rows of the image contains 160 pixels. Consequently, three rows of data correspond to four rows of the image (160 ones and zeroes corresponds to 160 pixels). You can assign one group of three rows to one group of students.