Figure 42.1: Diagram of an electromagnetic wave. The electric and magnetic vectors are perpendicular, and
they peak together and go to zero together. The wave travels in theEBdirection (to the right in this figure).
(Credit: NOAA.)
Gamma raysare the shortest-wavelength, highest-frequency, highest-energy waves. They are generally
associate with nuclear processes (such as nuclear fission and fusion), and with other high-energy reactions
such as matter-antimatter annihilation.
X-raysare familiar for their medical uses. Human tissue is transparent in X-rays, but human bone is
opaque. By viewing an image of the human body in X-rays, one may create images of the human skeleton.
X-rays are generally less energetic than gamma rays, and are generally produced by atomic reactions.
Ultraviolet lightis light whose wavelength is shorter than can be seen by the human eye (although some
animals can see in ultraviolet light). The Sun emits a significant amount of ultraviolet light, which can cause
sun tans and sunburns in humans.
Visible lightis light whose wavelengths are visible to the human eye. Violet light is the shortest wave-
length (about 400 nm) and highest frequency and energy; red light is the longest wavelength (about 700 nm)
and lowest frequency and energy. The order of the colors of visible light (from longest to shortest wavelength)
is given by the mnemonic “ROY G. BIV”: Red, Orange, Yellow, Green, Blue, Indigo, Violet.
Infrared lightis light whose wavelength is longer than can be seen by the human eye—although some
animals like the pit viper can see in infrared light. Bodies that we consider “warm” (say, somewhat above
room temperature) emit significant amounts of infrared light. For example, the human body can be seen to
be “glowing” in infrared light, although it does not glow significantly in visible light. This is the principle of
the night-vision scope, which is a device that converts infrared light to visible wavelengths, so that the user
can detect this glowing of warm bodies.
Submillimeter wavesare electromagnetic waves whose wavelength is between 0.1 to 1 mm. These waves
are of some astronomical interest, and have a few applications in medicine.
Microwavesare electromagnetic waves whose wavelengths are typically measured in centimeters. Mi-
crowaves are familiar in their use in microwave ovens: the oven emits microwaves designed to resonate with
the water molecules in food, thereby “shaking” the water molecules and heating the food. Microwaves also
find uses in communications (high-frequency radio).
Radio wavesare the longest-wavelength ( >1m), lowest-frequency, lowest-energy electromagnetic
waves. They are used in radio astronomy and in radio communications. These wavelengths includes AM and
FM radio and broadcast television. (FM radio lies between TV channels 6 and 7.)