called optical illusions have little to do with the
physics of light, but they take shape in the mind of
the observer. Such illusions are more appropriately
called visual (or in other senses, auditory, tactile,
etcetera) or cognitive, depending on whether they
arise from sensory or cognitive brain regions. One
example of a visual (rather than an optical) illusion
is our perception of movement in certain stationary
images, which occurs when our visual brain circuits
respond to local brightness changes in an image as
they would do to actual motion.
As for mirages themselves, they result from the
quantum-electrodynamics phenomenon that pho-
tons always take the path of minimum time. Thus,
when air temperature is constant, light travels in a
straight line. But in the presence of a steady tem-
perature gradient, light follows a curved path from
hotter to cooler air. In the specific case of a vertical
temperature gradient, light curves from the sky to
the observer’s eye. One familiar example of such a
situation is seeing the sky reflected on the road
ahead, which we may interpret as water on the
ground. This kind of “mirage effect” reflection was
central to the development of underwater cloaking
technology in the past decade.
“Although I was aware of the existence of the
Mirage, I could not prevail on myself to believe that
the images... were only reflected,” wrote Madden,
and so it remains today. As mesmerizing now as
200 years ago, mirages still puzzle our senses and
remind us that there is much more to vision than
meets the eye.
ILLUSIONS
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