32.9 - Dispersion and prisms
Dispersion: Refraction that
causes light to separate into
its various wavelengths.
Humans perceive different wavelengths of light as
different colors. When we see light of a single
wavelength, we perceive it as a pure color such as
red, green or violet. The light that comes from the Sun
and from standard light bulbs, which we perceive as
white light, consists of a mixture of many different
wavelengths of electromagnetic radiation.
Prisms, such as the one shown to the right, separate
white light into its many wavelengths. This is called dispersion, or to be more specific,
chromatic dispersion, since dispersion can also occur with non-visible electromagnetic
radiation and mechanical waves as well. Sir Isaac Newton famously used a pair of
prisms to disperse white sunlight into colors and recombine the colors into white light.
Prisms disperse light because the index of refraction depends on the light’s wavelength.
The index of refraction for a given material is greater for waves of shorter wavelength,
so blue and violet light refract more than red or orange light. When white light is incident
upon a prism, the different wavelengths that make it up are refracted at different angles,
resulting in a rainbow of colors.
The rainbows we see in the sky are also caused by dispersion. Light disperses as it
enters a spherical raindrop. Total internal reflection occurs inside the raindrop, and the
light then refracts again when leaving the drop, dispersing even more. You see this
illustrated in a raindrop to the right.
Why does this process cause rainbows? Raindrops reflect back nested “cones” of light
of different colors. In Concept 3 you see how red light reflected back from every drop in
the outermost portions of the rainbow reaches the observer, but the violet light from
those drops passes him by. You also see how the violet light from the innermost
portions of the rainbow reaches him, but not the red light. Other topics in rainbow theory
are still being researched. For instance, physicists, although vaguely alluding to
quantum effects and the promise of string theory, have to date offered no convincing
explanation for the pot of gold at the end of the rainbow.
A rainbow provides an example of dispersion.Dispersion
Creation of a spectrum by refraction
Different wavelengths refract differentlyRainbow: inside a raindrop
Refraction plus total internal reflection
Colors refract at different anglesObserver views rainbow
Red light from outer drops reaches him
Violet light from inner drops reaches him