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millimeter in length and the longest radio waves being hundreds of kilometers long. Humans are
blind to almost all of this energy—our eyes detect only the range from about 400 to 700
billionths of a meter, the part of the electromagnetic spectrum known as the visible spectrum.
Figure 4.6 The Electromagnetic Spectrum
Only a small fraction of the electromagnetic energy that surrounds us (the visible spectrum) is detectable by the
human eye.
The Sensing Eye and the Perceiving Visual Cortex
As you can see in Figure 4.7 "Anatomy of the Human Eye", light enters the eye through
the cornea, a clear covering that protects the eye and begins to focus the incoming light. The
light then passes through the pupil, a small opening in the center of the eye. The pupil is
surrounded by the iris, the colored part of the eye that controls the size of the pupil by
constricting or dilating in response to light intensity. When we enter a dark movie theater on a
sunny day, for instance, muscles in the iris open the pupil and allow more light to enter.
Complete adaptation to the dark may take up to 20 minutes.
Behind the pupil is the lens, a structure that focuses the incoming light on the retina, the layer of
tissue at the back of the eye that contains photoreceptor cells. As our eyes move from near
objects to distant objects, a process known as visual