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A Closer Look

Th e Rainbow
rainbow is a spectacular, natural dis- raindrops into a spherical shape as they fall
A lay of color that is supposed to have through the air. Sunlight
p
a pot of gold under one end. Understand- Box Figure 7.7 shows one thing that First
refraction
ing the why and how of a rainbow requires can happen when a ray of sunlight strikes Reflection
information about water droplets and a single spherical raindrop near the top of
Enlarged raindrop
knowledge of how light is reflected and the drop. At this point, some of the sunlight
refracted. This information will also explain is reflected, and some is refracted into the
why the rainbow seems to move when you raindrop. The refraction disperses the light
Second
move—making it impossible to reach the into its spectrum colors, with the violet light
refraction
end to obtain that mythical pot of gold. being refracted most and red the least. The
Rainbow ray
First, note the pattern of conditions refracted light travels through the drop to
that occur when you see a rainbow. It the opposite side, where some of it might be Observer
usually appears when the Sun is shining reflected back into the drop. The reflected
BOX FIGURE 7.7 Light is refracted
low in one part of the sky and rain is fall- part travels back through the drop again,
when it enters a raindrop and when it
ing in the opposite part. With your back leaving the front surface of the raindrop. As
leaves. The part that leaves the front surface
to the Sun, you are looking at a zone of it leaves, the light is refracted for a second of the raindrop is the source of the light in
raindrops that are all showing red light, time. The combined refraction, reflection, thousands upon thousands of raindrops from
another zone that are all showing vio- and second refraction is the source of the which you see zones of color—a rainbow.
let light, with zones of the other colors zones of colors you see in a rainbow. This
between (ROYGBV). For a rainbow to also explains why you see a rainbow in the
form like this requires a surface that part of the sky opposite from the sun. one color, and all drops showing this color
refracts and reflects the sunlight, a condi- The light from any one raindrop is one are on the arc of a circle. An arc is formed
tion met by spherical raindrops. color, and that color comes from all drops because the angle between the sunlight and
Water molecules are put together in on the arc of a circle that is a certain angle the refracted light of a color is the same for
such a way that they have a positive side and between the incoming sunlight and the each of the spherical drops.
a negative side, and this results in strong refracted light. Thus, the raindrops in the There is sometimes a fainter second-
molecular attractions. It is the strong attrac- red region refract red light toward your eyes ary rainbow, with colors reversed, that
tion of water molecules for one another that at an angle of 42°, and all other colors are forms from sunlight entering the bottom
results in the phenomenon of surface ten- refracted over your head by these drops. of the drop, reflecting twice, and then
sion. Surface tension is the name given to Raindrops in the violet region refract violet refracting out the top. The double reflec-
the surface of water acting as if it were cov- light toward your eyes at an angle of 40° and tion reverses the colors, and the angles are
ered by an ultrathin elastic membrane that the red and other colors toward your feet. 50° for the red and 54° for the violet. (See
is contracting. It is surface tension that pulls Thus, the light from any one drop is seen as Figure 1.15 on p. 18 .)

The pattern of bright lines and dark zones is called an inter- 3, 4, and so on wavelengths. Similarly, zones of darkness occur

ference pattern (Figure 7.19B). The light moved from each slit above and below the center bright line at positions representing
1
1

1
1
in phase, crest to crest and trough to trough. Light from both differences in paths of ⁄2, 1 ⁄2, 2 ⁄2, 3 ⁄2, and so on wavelengths.
slits traveled the same distance directly across to the screen, so Young found all of the experimental data such as these in full
both beams arrived in phase. The crests from the two slits are agreement with predictions from a wave theory of light. About

superimposed here, and constructive interference produces a 15 years later, A. J. Fresnel (pronounced “fray-nel”) (1788–1827)

bright line in the center of the pattern. But for positions above demonstrated mathematically that diffraction as well as other
and below the center, the light from the two slits must travel behaviors of light could be fully explained with the wave theory.
different distances to the screen. At a certain distance above In 1821, Fresnel determined that the wavelength of red light was

–7
–7
and below the bright center line, light from one slit has to travel about 8 × 10 m and of violet light about 4 × 10 m, with
a greater distance and arrives one-half wavelength aft er light other colors in between these two extremes. The work of Young

from the other slit. Destructive interference produces a zone and Fresnel seemed to resolve the issue of considering light to
of darkness at these positions. Continuing up and down the be a stream of particles or a wave, and it was generally agreed
screen, a bright line of constructive interference will occur at that light must be waves.
each position where the distance traveled by light from the two
slits differs by any whole number of wavelengths. A dark zone

of destructive interference will occur at each position where the POLARIZATION
distance traveled by light from the two slits differs by any half- Huygens’ wave theory and Newton’s particle theory could

wavelength. Thus, bright lines occur above and below the center explain some behaviors of light satisfactorily, but there were

bright line at positions representing differences in paths of 1, 2, some behaviors that neither (original) theory could explain.

190 CHAPTER 7 Light 7-14