Focusing Light

90
Chapter 3 Sensation and Perception
Amplitude
Wavelength
Baseline
FIGURE 3.4
The Dimensions of a Wave
Wavelength is the distance from one peak
of a wave to the next. Frequency is the
number of complete waves, or cycles, that
pass a given point in a given amount of
time, such as one second. Amplitude is the
height of a wave from baseline to peak.
given amount of time. Amplitude is the height of the wave from baseline to peak (see
Figure 3.4). Different wavelengths, frequencies, and amplitudes create different visual
and sound experiences. Let’s now consider how these physical properties of light and
sound waves become sights and sounds.
Seeing
䉴 Why do some people need eyeglasses?
Soaring eagles have the incredible ability to see a mouse move in the grass from a mile
away. Cats have special “reflectors” at the back of their eyes that help them to see even
in very dim light. Nature has provided each species with a visual system uniquely
adapted to its way of life. The human visual system is also adapted to do many things
well. It combines great sensitivity with great sharpness, enabling us to see objects near
and far, during the day and night. Our night vision is not as good as that of some animals, but our color vision is excellent. Not a bad tradeoff; after all, being able to experience a sunset’s splendor seems worth an occasional stumble in the dark.
Light
amplitude The distance between the
peak and the baseline of a wave.
visible light Electromagnetic radiation
that has a wavelength of about 400
nanometers to about 750 nanometers.
light intensity
A physical dimension of
light waves that refers to how much
energy the light contains and that
determines its brightness.
light wavelength A physical dimension of light waves that refers to their
length and that produces sensations of
different colors.
cornea The curved, transparent, protective layer through which light rays
enter the eye.
pupil An opening in the eye, just
behind the cornea, through which light
passes.
iris The part of the eye that gives it its
color and adjusts the amount of light
entering it.
lens The part of the eye directly
behind the pupil.
retina The surface at the back of the
eye onto which the lens focuses light rays.
accommodation
The ability of the
lens to change its shape and bend light
rays so that objects are in focus.
Light is a form of energy known as electromagnetic radiation. Most electromagnetic
radiation, including x-rays, radio waves, television signals, and radar, is invisible to the
human eye. In fact, as shown in Figure 3.5, the range, or spectrum, of visible light is
just the tiny slice of electromagnetic radiation that vibrates at wavelengths from just
under 400 nanometers to about 750 nanometers. (A nanometer is one-billionth of a
meter.) It is correct to refer to light as either light waves or light rays.
Sensations of light depend on the intensity and wavelength of light waves. Light
intensity, which refers to how much energy the light contains, determines the brightness of light. And what color you sense depends mainly on light wavelength. At a
given intensity, different wavelengths produce sensations of different colors. For
instance, 440-nanometer light appears violet blue, and 700-nanometer light appears
orangish red.
Focusing Light
The eye transduces light energy into neural activity. First, accessory structures of the
eye modify incoming light rays. The light rays enter the eye by passing through the
curved, transparent, protective layer called the cornea. As shown in Figure 3.6, the
light then passes through the pupil, the opening just behind the cornea. The iris,
which gives the eye its color, adjusts the amount of light allowed into the eye by constricting to reduce the size of the pupil or dilating to enlarge it. Directly behind the
pupil is the lens. Both the cornea and lens of the eye are curved so that they bend
light rays. (A camera lens works the same way.) This bending process focuses light
rays coming from various angles into a sharp image on the inner surface at the back
of the eye. This surface is called the retina. Light rays from the top of an object are
focused at the bottom of the image on the retinal surface. Light rays from the right
side of the object end up on the left side of the retinal image (see Figure 3.7). The
brain rearranges this upside-down and reversed image so that we can see the object
as it is.
The muscles that hold the lens adjust its shape so that either near or far
learn objects can be focused on the retina. To illustrate this for yourself, try readby
doing ing the next sentence while holding the book as close to your face as possible. To maintain a focused image at close range, your muscles have to tighten your
lenses, making them more curved. This ability to change the shape of the lens to bend
light rays is called accommodation. As the lens loses some of its flexibility over the
2
FIGURE
3.5
The Spectrum of Electromagnetic Energy
The human eye is sensitive to only a narrow range of electromagnetic wavelengths. To detect energy outside this range, we rely on radios, cell
phones, TV sets, radar detectors, infrared night-vision scopes, and other electronic instruments that can “see” this energy, just as the eye sees
visible light.
Wavelength in meters
10–14
10–13
10–12
Cosmic rays
10–11
10–10
Gamma
rays
10–9
X-rays
10–8
10–7
Ultraviolet
10–6
10–5
Visible
10–15
10–4
10–3
10–2
10–1
101
450
500
550
600
650
700
Wavelength in nanometers
FIGURE
3.6
Major Structures of the Eye
As shown in this top view of the eye, light
rays bent by the combined actions of the
cornea and the lens are focused on the
retina, where the light energy is converted
into neural activity. Nerve fibers from the
retina combine to form the optic nerve,
which leaves the back of the eye and continues to the brain.
Cornea
Iris
Lens
Pupil
Muscle
to adjust
the lens
Retina
Muscle
to move
the eye
Fovea
Optic nerve
(to brain)
FIGURE
3.7
The Lens and the Retinal Image
To see objects as they are, your brain must
rearrange the upside-down and reversed
images that the lens focuses on the retina.
If light rays are out of focus when they
reach the retina, glasses usually correct the
problem. In some older people, vision is impaired by cataracts, a condition in which a
“cloudy” lens severely reduces incoming
light. Cataracts can be cleared up with laser
surgery or by replacing the natural lens with
an artificial one (Snellingen et al., 2002).
Lens
103
Microwaves Radar TV FM AM Short
Radio waves waves
Infrared
VISIBLE SPECTRUM
400
102
Retina
750