554
UNIT 3
Regulation and Integration of the Body
15
one transparent medium into another with a different density,
its speed changes. Light speeds up as it passes into a less dense
medium and slows as it passes into a denser medium. Because of
these changes in speed, bending or
refraction
of a light ray occurs
when it meets the surface of a different medium at an oblique
angle rather than at a right angle (perpendicular). Te greater this
angle, the greater the amount of bending.
Figure 15.11
shows
refraction: A straw in a glass of water appears to break at the air-
water interface.
A lens is a transparent object curved on one or both surfaces.
Since light hits the curve at an angle, it is refracted. If the lens
surface is convex, that is, thickest in the center like a camera
lens, the light rays bend so that they converge or intersect at a
single point called the
focal point
(Figure 15.12a)
. In general,
the thicker (more convex) the lens, the more the light bends and
the shorter the focal distance (distance between the lens and
focal point). Te image formed by a convex lens, called a
real
image
, is inverted—upside down and reversed from leF to right
(±igure 15.12b).
Concave lenses, which are thicker at the edges than at the
center, diverge the light (bend it outward) so that the light rays
move away from each other. Consequently, concave lenses pre-
vent light from focusing and extend the focal distance.
Focusing Light on the Retina
As light passes from air into the eye, it moves sequentially
through the cornea, aqueous humor, lens, and vitreous humor,
and then passes
through the entire neural layer of the retina
to
excite the photoreceptors that abut the pigmented layer (see
light into a
visible spectrum
, or band of colors (±igure 15.10b).
(A rainbow that appears during a summer shower represents
the collective prismatic effects of all the tiny water droplets sus-
pended in air.) Red wavelengths are the longest and have the
lowest energy, while violet wavelengths are the shortest and
most energetic.
Like sound, light can reflect, or bounce, off a surface. Tis
reflection
of light by objects in our environment accounts for
most of the light reaching our eyes. Objects have color because
they absorb some wavelengths and reflect others. A red apple
reflects mostly red light, while grass reflects more of the green.
Tings that look white reflect all wavelengths of light, whereas
black objects absorb them all.
Refraction and Lenses
Light travels in straight lines (rays) and
is blocked by any nontransparent object. When light travels in
a given medium, its speed is constant. But when it passes from
Light absorption (percent of maximum)
400
450
500
550
600
650
700
100
0
50
Wavelength (nm)
Visible light
(b)
(a)
Blue
cones
(420 nm)
Rods
(500 nm)
Green
cones
(530 nm)
Red
cones
(560 nm)
10
–5
nm 10
–3
nm
X rays
UV
Infrared
Micro-
waves
Radio waves
10
3
nm
10
3
m
10
6
nm
(10
9
nm =)
1 m
1 nm
Gamma
rays
Figure 15.10
The electromagnetic spectrum and photorecep-
tor sensitivities.
(a)
The electromagnetic spectrum, of which visible
light constitutes only a small portion. (nm
5
nanometers.)
(b)
Sensi-
tivities of rods and the three cone types to the different wavelengths
of the visible spectrum.
Figure 15.11
Refraction.
A straw standing in a glass of water
appears to be broken at the water-air interface. This occurs because
light bends toward the perpendicular when it travels from a less
dense medium (such as air) to a more dense medium (such as
water).
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