Nats 101 S04 #14
Reading: T&H 157-168
The Electromagnetic spectrum
All electromagnetic radiation is created in the same way, by accelerating
Know the electromagnetic spectrum, and the relative positions of the various
types of radiation
The electromagnetic spectrum
All radiation is produced in the same way, by accelerating electrical charges.
By analyzing the electromagnetic equations, Maxwell realized that there
were more types of radiation than just visible light. He predicted the
This was a test of his theory, and was verified by Hertz in the late 1880’s when he discovered radio waves.
The entire electromagnetic spectrum is a continuum of waves that can be
thought of in terms of frequencies, wavelengths, or energy; it does not
The gross properties of different types of radiation can be understood in terms of the differences between
their respective wavelengths, or between their respective energies.
It does not really matter which way you look at it.
Units of length:
mm = micron = 1/1000 mm
nm = nanometers = 10-9 m = 1/1000 mm
Å = angstroms = 10-10 m = 0.1 nm = 1/10,000 mm
Radio waves: ¥ > l
Oscillating electrons along a tall metal antenna produces the waves.
The speed of oscillation determines the wavelength
AM and FM transmissions:
Broadcaster is limited to a very narrow bandwidth.
AM = amplitude modulated, long l, eg 670 kHz (kilo = 103).
Since the radio waves are long they can scatter off the layers of the atmosphere.
FM = frequency modulated, short l, eg 92.1 MHz (Mega = 106).
Since these radio waves are shorter than AM waves they can also carry more energy and are
able to penetrate objects more easily.
Therefore FM radio signals can be recieved indoors more easily. However, they also penetrate the atmosphere easily too, so
FM radio signals are not recieved far from their transmitting source.
TV is a mix of both, AM for sound, FM for picture
- Satellite radio
works at around 2.3 GHz (Giga = 109), a frequency that has more energy than FM.
Microwaves: 1 m > l > 1 mm.
The short wavelength of microwave beams means that it is easy to make a beam that is
narrow and well-defined in its size.
Microwave transmitters, TV, satellite, phone
Radar, and stealth fighters. Modern military radar can detect a fly at a distance of 1 mile. Stealth fighters use paints that absorb microwave, and are constructed with weird angles to reflect the signal in strange ways.
Microwave ovens oscillate H2O molecules.
Infrared: 1 mm > l > 1 mm.
Radiation from heat is in this range.
Our skin is a crude infrared detector
Energy leaks in homes
- Guidance of air-to-air missiles
Visible light: 7500 Å > l > 4000
The range of electromagnetic radiation that our eyes can see.
We do not see all wavelengths with the same sensitivity.
Ultraviolet: 4000 Å > l > 1000
Energy is now high enough to break many chemical bonds.
This is why we can be sunburned.
Used to clean things, e.g. hospital equipment
Black lights – cause fluorescence
X-rays: 1000 Å > l > 1 Å
Wavelength short enough to penetrate most forms of matter.
That is why it is used to examine broken bones.
High energy is deadly
Gamma rays: 1 Å > l > 1/¥
Down to the size of the nucleus
Astronomy uses gamma rays because they must have been given off by high
New type of communication device
uses the whole radio energy spectrum
messages are sent from a transmitter to a reciever at a set of predetermined
wavelengths known only by these two devices
Signals are binary, ie 0 or 1. They are sent millions of a time per second
with a 0 being 10 picoseconds (10 trillionths of a second) early, and a
1 being 10 picoseconds late.
Exciting properties of the devices are that they are secure, small, little
power needed, can provide positional information, high-capacity, can work
through the earth, "sees" through objects.
Commonly known as "impulse" or "ultra-wide band" (UWB) radio or radar
Problems include the FCC because licensing frequencies is a billion dollar
industry. Eg a FM station sold for $110 million in LA recently.
FCC finally granted them a license