Page 27 - (DK) How to be a GENIUS?

Seeing in color
The cone cells in the retina respond to
different strengths of basic colors such
Mental image
as red, green, and blue. The signals they
The cells of the retina
send to the brain represent millions of
dots of these colors. The brain combines
convert light into electrical
the dots to create all the other colors of
signals. These pass to the
the spectrum, as in this simplified diagram.
visual cortex of the brain,
which turns them into an
upright mental image.
Visual cortex The
part of the brain that
processes visual data
Dark adaptation
When you turn the light off in your
room at night, you can’t see much.
However, as the minutes tick by,
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yo you are able to see more and more.
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This is because the sensory cells in
your eyes can adapt to the low light
Optic nerve Bundle of
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nerve fibers linked to level—but it takes time. If you turn
the sensory cells the light back on, you get dazzled
because your eyes have adapted
to the dark. They must readapt to
the light, but they do this much
more quickly.
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There are around 126 million
Th Th er r e e a r r e a r r o u n d 1 2 6 m i l l i o n
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sensory cells in each eye—
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120 million rods and
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Sensory cells
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The image is focused on a
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six million cones.
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sheet of light-sensitive cells
called the retina. Some of the
cells (rods) are very sensitive
to dim light, while others
(cones) detect color.
Strange effects
Bright lights and contrasting patterns can cause strange
optical effects. For example, if you stare at something for
a minute and then close your eyes, you see a negative
afterimage. Each color is replaced by its opposite, so the
yellow and red flowers shown below appear blue and cyan.
This is a side effect of the way your brain processes color.
Blind spot
The point where the optic nerve leaves the eye cannot
detect light, but your brain invents information to fill the
gap. You can test this using the diagram above. Hold the
book at arm’s length, close your right eye, and focus on the
cross. Slowly move the book toward you. The center of
the wheel will disappear when it falls on your blind spot—
but your brain will fill the gap with spokes of the wheel.
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