Electromagnetic Waves
Light is one of a group of waves called electromagnetic
waves (EM). Electromagnetic waves are a disturbance
of the electrical and magnetic fields of space. Because
these waves are not a disturbance of matter,
electromagnetic waves can travel through empty space.
The current theory of how light waves form is that light is given off
when electrons move from a higher energy level to a lower energy
level.
When atoms absorb energy (heat, light, other radiation), the electrons
move out to other levels. This is called being “excited”. Excited atoms
are unstable and want to get rid of this extra energy. The atom does
this by giving off light when the electron jumps back to a lower
energy level.
Albert Einstein proposed that light (and other EM) acts as both a wave and a particle. The evidence for this
theory was that light could knock electrons off of atoms in a photo cell, as if the light were tiny particles. He
called the unit of 1 light particle a “photon”.
All electromagnetic waves travel at the same speed—about 300,000,000 m/sec in empty space. When traveling
through matter, the waves slow down. The change in the speed of light is why refraction (or bending) of light
waves happens.
SPECTRUM
Electromagnetic waves form a continuous set of waves with different wavelengths and frequencies. This
continuum is called the electromagnetic spectrum.
meaning:
Radio
How do I remember this? Try:
Rabbits
Meet
In
Very
Unusual
eXpensive
Gardens
Microwaves
Infra-Red
Visible light
Ultra-violet
X-rays
Gamma rays
Radio Waves: Radio waves have the longest wavelengths and the lowest frequencies. AM radio waves have
longer wavelengths than FM waves. AM means amplitude modulation—the sound signal is sent by making the
same frequency “louder” or “softer”—more or less amplitude. Because AM waves are very long, they are
reflected by the upper layers of the atmosphere (ionosphere) at night, which is why you can get AM from a long
distance at night. FM stands for frequency modulation. The signal is sent by changing the frequency of the
waves slightly. FM signals have less noise, but do not travel as far. Television signals use FM to send the sound
and AM to send the video signal, but TV uses shorter wavelength that radio does.
Microwaves: Microwaves have higher frequencies and shorter wavelengths than radio waves. Microwaves are
also higher energy. Cell phones and satellites use microwaves to send and receive signals. Water and oils absorb
some wavelengths of microwaves, so microwave ovens heat food this way. Studies show that the microwaves
from cell phones cause parts of the brain to become warmer—as if you are cooking your grey matter when you
talk. Microwaves are also used for radar.
Infrared: Infrared Rays (IR) are shorter than microwaves and just below visible light. All objects above
absolute zero give off IR. The hotter the object is, the more IR it gives off. IR can be used for space heaters as
well as TV and other remote controls. IR cameras can be used to give night vision. Some snakes use pits on
their noses which detect IR to hunt animals at night.
Visible light: White light is a mixture of different colors, each color has a different wavelength. The colors of
visible light from longest wavelength to shortest wavelength are:
Red
Orange
Green
Blue
Indigo
Violet
Which can be remembered by the name: ROY G. BIV
Each wavelength bends (or is refracted) at a slightly different angle
depending on the wavelength. The shorter wavelengths bend the most. If a
mixture of colors is passed through an object which causes a large amount
of refraction, the colors will sort out by wavelength giving a rainbow effect.
A piece of glass shaped like a prism will have this effect and so will a
raindrop.
R
O
Y
G
B
V
I
Ionizing Radiation: EM with wavelengths shorter than visible light has the ability to knock electrons off of
atoms creating ions—particles which have lost or gained electrons. EM which can create ions is called ionizing
radiation. Ionizing radiation in cells can cause damage to DNA. Damage to DNA leads to higher rates of
cancer. Light and radio waves lack the power to create ions and so do not increase cancer rates. Damage to
DNA can also cause mutations. Mutations in gametes (egg or sperm cells) can cause birth defects.
Ultraviolet: UV radiation is slightly shorter wavelength than violet in the visible spectrum. Sunlight also
contains large amounts on UV. Some insects see using UV, flowers often have patterns like targets when filmed
using UV. When your skin is exposed to UV, your body makes vitamin D, which you need in order to absorb
calcium for your bones. UV damages DNA in skin cells. Cells respond to broken DNA by producing more of a
pigment called melanin which absorbs UV and protects against further damage. Melanin is the pigment
responsible for skin color in humans. Repeated exposure to UV increases the chances of skin cancer. Because
bacteria can be destroyed by UV, it can be used to sterilize things.
X-rays: X rays have a shorter wavelength than UV and can pass through many substances, like skin and
muscle. X rays are absorbed by metals. Calcium is a metal, so bones and teeth which contain calcium absorb Xrays and show up as white on a medical X-ray. Because X-rays are ionizing radiation, they can damage DNA
and increase cancer rates. At a hospital or dentist’s office, the X-ray technician always goes behind a shielded
wall when the machine is turned on. X-rays can increase cancer rates, but cancer cells can be killed by X-rays,
so X-rays are used to treat some forms of cancer. It would be possible the be cured of one form of cancer by Xrays, which cause another form of cancer.
Gamma rays: Gamma rays have the most energy of any EM. Gamma rays are produced by radioactive decay
when the nucleus of an atom changes. Gamma rays penetrate the farthest of any ionizing radiation and can
cause damage to DNA of cells. Because of this gamma rays can be used to kill bacteria and sterilize things.
Gamma rays can be used to preserve food which has not been cooked. Gamma rays are also used to sterilize
bandages after they have been packaged. Like X-rays, gamma rays can either cause cancer or can be used to
treat some forms of cancer.
Gamma rays are given off by the dust (or fallout) after a nuclear explosion. Gamma rays can cause radiation
sickness, where the fast growing cells of the body die—like someone with chemotherapy. Only several feet of
concrete can stop gamma rays. During the Cold War people were concerned that the Soviet Union would use
nuclear weapons on the USA. Fallout shelters were designated as safe places from radiation. Fallout shelters
are usually the basement of buildings with very thick walls which could stop the gamma rays given off by the
fallout following a nuclear explosion.