Regulation and Integration of the Body
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
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.
refraction: A straw in a glass of water appears to break at the air-
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
. 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
, is inverted—upside down and reversed from leF to right
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
excite the photoreceptors that abut the pigmented layer (see
light into a
, 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
Like sound, light can reﬂect, or bounce, oﬀ a surface. Tis
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)
The electromagnetic spectrum and photorecep-
The electromagnetic spectrum, of which visible
light constitutes only a small portion. (nm
tivities of rods and the three cone types to the different wavelengths
of the visible spectrum.
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