In two of the trials, the box was dark for 12 minutes and then illuminated for 48 minutes with either long-wavelength red light or short-wavelength blue light directed at the subjects' corneas. (Light color is determined by its wavelength.) The box was dark in a control experiment.

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At the moment, the earth-observation satellites which employ microwaves generally use short-wavelength radiation, because this provides more detailed images.

ECONOMIST: Archaeology: What lies beneath | The

Researchers said that result doesn't rule out the possibility that short-wavelength light also could be an alerting stimulus in the afternoon.

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NASA's Jet Propulsion Laboratory, in Pasadena, California, used short-wavelength microwaves in 1992 to reveal caravan trails leading to the lost city of Ubar in the Arabian peninsula.

ECONOMIST: Archaeology: What lies beneath | The