Refraction describes how a wave changes its direction of travel as it moves from one medium to another. Another way that waves refract is when they move in a medium, such as air, that varies in density or temperature.
Diffraction is the process of a wave bending as it travels past the
edge of, or even around an obstacle. The fact that you can hear a
friend call you from another side of a building is an example of diffraction
of sound waves. Sound waves and waves that have longer wavelengths,
or lower frequencies, diffract better than high frequency waves.
Light waves do not diffract very much. Yet, white light is composed
of all of the colors of the rainbow. The reason that we are able
to see all of these colors is that they have different wavelengths.
Red waves bend more than orange waves. Orange bends more than yellow, and
so on. Violet has the shortest wavelength and as a result it bends
the least. When white light travels through a medium at an angle
it gets separated into its component colors.
As waves travel between media they may be absorbed or reflected by,
or transmitted through the medium that they are entering. Factors
affecting the propagation of waves through media depend on both the medium
of travel and the type of wave.
Transverse - waves that travel perpendicular to the direction of motion
We often call this snake-like of serpentine motion.
Examples of these include the first waves of energy sent out by earthquakes.
(primary waves)
Longitudinal - waves that travel parallel to the direction of motion,
These waves travel as compressions and rarefactions in the medium that
they are traveling in. A compression
has a slightly higher than normal density or pressure. A rarefaction
has a lower pressure than normal.
Examples: sound waves and secondary waves of earthquakes.
Matter waves - particles that show wave like tendencies
A particle moving close to the speed of light can diffract or bend along
a different path of travel as it moves around
the edges of objects. Matter waves also have the ability to interfere
with each other. This might be best described as
waves that cancel each other out or add together to produce a new wave
that is a combination of the two interfering
waves.
Electromagnetic waves - waves of energy emitted from any object that
is above absolute zero in temperature.
These waves are referred to in a variety of ways, such as, light, rays,
radiation, photons or energy. Their
different forms include radio waves, microwaves, infrared, visible light,
ultra-violet, x-rays and gamma
rays.
Tetra (T) 1012 1,000,000,000,000
Giga (G) 109
1,000,000,000
Mega (M) 106 1,000,000
Kilo (k) 103
1,000
Wavelength describes the distance over which a wave begins to repeat itself. It is measured in meters. Again, there is a huge range over which electromagnetic wavelengths vary. The longest of waves are radio waves, at a distance of up to 10,000 meters (6.2 miles). The shortest of waves are Gamma waves. Their wavelengths are as short as 10-10 meters. That is 0.000,000,001 meters in length. Again, System International prefixes are used to show these small numbers.
pico (p) 10-12
0.000 000 000 001
nano (n) 10-9
0.000 000 001
micro (m) 10-6
0.000 001
milli (m) 10-3
0.001
centi (c) 10-2
0.01
Visible light is also measured in a unit called “Angstroms”. (Å)
to describe 10-10 meters in length. This unit is not commonly used
any more.
ENERGY OF ELECTROMAGNETIC WAVES
IONIZING OR NON-IONIZING
Electromagnetic waves range over a very large spectrum of frequencies and wavelengths. We can also look at them with respect to their energy levels. Higher frequency waves posses more energy. Some types of this energy are harmful to cells. The distinction of whether a wave is harmful or not is its classification as ionizing or non-ionizing. Ionizing waves have enough energy to cause atoms and molecules to become ions. In humans healthy cells that absorb ionizing radiation can mutate. Often these mutations lead to forms of cancer. Gamma rays and x-rays are both ionizing forms of radiation. Ultra-violet radiation is non-ionizing to humans. Yet, it has enough energy to ionize atoms and molecules in our atmosphere. All of the waves from ultra-violet through radio waves are considered to be non-ionizing.
Most people think of the emission of particles during nuclear reactions when the word radiation is mentioned. One definition of the word radiation is the “spontaneous emission of particles from the nucleus of an atom”. These particles are protons (gamma decay), Helium atoms (alpha decay), and electrons (beta decay). The release of this gamma ray and x-ray energy is essential to the need of the conservation of mass in a reaction. These ionizing forms of radiation are very harmful to us.
TYPES OF ELECTROMAGNETIC WAVES
GAMMA RAYS (g)
Emitted from the nuclei of atoms during radioactive decay. Also emitted during high-speed collisions of particles. Gamma rays are ionizing waves. Below is an example of the chemistry of how gamma radiation is emitted.
n0à p+ + e- + g a neutron decays into a proton and an electron and emits gamma radiation
e- + e+ à 2g an electron collides with a positron (a positively charged electron) and are
annihilated to produce gamma radiationUses: cancer treatment and therapy, sterilization of foods and medical equipment (Gamma radiation kills microbiological organisms.
No lasting radiation remains on the product.)Sources: cobalt 60, the inner core of the sun,
X-RAYS
Emitted when an electron moves from certain excited states back down to its ground state, or when an electron that is moving very quickly is stopped suddenly. An example of this is cathode ray tube (CRT) monitors. In them a beam of electrons hits a screen that is coated with a phosphorescent material. When this happens some of the energy of the electron transfers to the material and makes the screen glow. The remainder of the energy is radiated in the form of a x-ray photon.
There are two groups of x-rays long wavelength or soft x-rays and shorter wavelength or hard x-rays.Uses: radiography (x-ray photography) killing cancerous cells (hard x-rays), to examine materials in industry
for defects (i.e.: castings and welds)Sources: emission by heavy atoms after bombardment by an electron
ULTRAVIOLET
Type of wave above the color Violet. Bees see in ultraviolet. These energetic waves, when incident to human skin, cause the melanoma in our skin to produce more pigment in an effort to protect itself. We call this effect sun tanning.
There are three groups of ultraviolet waves UV A, UV B, and UV C. The “A” type has the longest wavelength and is the least harmful of these three groups. UV B, and UV C are absorbed by DNA in cells. The human eye is also very susceptible to the harmful effects of ultraviolet rays. Snow blindness is an example of an extreme case of damage done by ultraviolet radiation to the eye.
Fortunately, a very small amount of the sun’s ultraviolet energy actually reaches the Earth’s surface.Uses: ultraviolet waves help the body to produce vitamin D, to kill bacteria on objects, sun tanning
Sources: Ultra hot objects 5000°C or more, Mercury vapor lamps, electric arcs (high current sparks),
emission of a photon of light (UV) when an electron goes through a small drops in energy levelsVISIBLE LIGHT
White light coming from the sun contains every color that we know. A rainbow is an example of white light that has been separated into a continuous spectrum of colors. A prism or diffraction grating, CD-ROM disks, and oil drops, accomplish the same feat.
The names of colors are assigned in the order of their wavelengths. There is actually a range of wavelengths for each color. The definition of a color is often arbitrary and based on an individual’s pleasure. See table 6.2 (K.Y.Kondratiev, “Radiation in the Atmosphere”, Academic Press, 1969)Uses: communications with fiber optics and lasers (red, green, and blue) light bulbs and gas discharge tubes (fluorescent)
Sources: very hot objects, luminous objects
INFRARED
Infrared radiation is usually thought of, and most easily understood, as heat. Yet, not all infrared energy is heat. Near infrared energy is the one most close to visible light. These waves are not hot and do not warm objects that they hit. Remote controls for electronic devices (TV’s, VCR’s) Use infrared waves as a method of data transfer.
Far infrared energy is heat energy. All objects that have warmth radiate infrared waves. Infrared energy is easily absorbed and re-radiated. Our sun emits very strongly in the infrared range. Rattlesnakes and some other families of snakes can sense, or “see” in infrared. This Trait allows them to sense prey and foes at night and in darkened burrows.Uses: surveillance by police, fire and military, therapy of muscles, photography with limited light
Sources: Humans 310K, the sun 5730K
MICROWAVES
These are waves are from one millimeter to one decimeter in length. Water molecules absorb microwave radiation. Microwave ovens use this simple principle to heat food. Many stars emit microwave radiation due to the fact that they are young stars, and not warm enough to “burn” yet.
Perhaps the best known use of microwaves is in communications. As a radio wave, a microwave travels at the speed of light. Microwave frequencies used in telecommunications are 6-11,000 MHz for distances up to 400 km. With a series of relay towers, a microwave signal can cross the continental United States in 0.016 seconds. This point to point communication is the reason that we must have so many towers along our road systems. Hills and valleys block the line of sight of some towers. Modern microwave relay towers only need to be placed at 40-50 km intervals to boost the signal. Cell phone services also use relay towers to send information to communications satellites that relay messages back to ground based transfer stations. Just three communications satellites correctly positioned 22,000 miles away from Earth could provide a worldwide telecommunications network.
Microwaves are also a convenient way to transmit energy to be used as a power source. There have been proposals for satellites the convert solar power into microwaves that are beamed down to Earth and converted into electricity.Uses: communications (point to point) power transmission
Sources: special electric circuits, many stars, microwave ovens
RADIO WAVES
Radio waves have the longest wavelength of all electromagnetic waves. They start at 10 centimeters and extend up to over 100,000 meters in length for one single wave. The actual extent of the length of radio waves is unknown. (The same is true for gamma waves.) Radio waves are the only cosmic waves the reach the surface of the Earth. They are responsible for the noise that we hear on our radios or the snow that we see on television channels on which signals are not broadcast.
Radio waves are divided into smaller groups or bands depending on the frequency. The higher the frequency of a wave the more the need for line of sight transmission and reception. Lower frequency waves allow for over the horizon reception.Frequency Range Wavelength Range Name(abr.)
30 GHz - 300 GHz 1 cm - 1 mm Extremely High Frequency (EHF)
3 GHz - 30 GHz 10 cm - 1 cm Super High Frequency (SHF)
300 MHz - 3 GHz 1 m - 10 cm Ultra High Frequency (UHF)
30 MHz - 300 MHz 10 m - 1 m Very High Frequency (VHF) (FM Radio)
3 MHz - 30 MHz 100 m - 10 m High Frequency (HF) (TV)
300 kHz - 3 MHz 1 km - 100 m Medium Frequency (MF) (AM Radio)
30 kHz - 300 kHz 10 km - 1 km Low Frequency (LF) (International AM radio)
3 kHz - 30 kHz 100 km - 10 m Very Low Frequency (VLF)
20 Hz - 3 kHz greater than 100 km Extremely Low Frequency (ELF)Uses: Medium range communications
Sources: transmitters, sparks from brushed (and unsuppressed) motors,The National Telecommunications and Information Administration works under the guise of the U.S. Department of Commerce to regulate the uses of radio waves in the United States. They have broken these ranges of frequencies down into further smaller groups that are really just a range of frequencies. We adjust the frequency that we receive on our televisions and radios. We also call this a channel.
Letters used to be assigned to a range of frequencies. The use of these designations, L, S, C, X, and K, for a range of frequencies has been discontinued due to the explosion of the use of radio frequencies. They were, quite simply, too large a range of frequencies to use anymore.SOURCES OF ELECTROMAGNETIC SPECTRA
When an atom releases energy we can “see” that energy. Occasionally, the energy is visible light. Yet in order for the object to produce the light it must “glow” or phosphoresce. Not all objects glow. Yet, all objects give off, or radiate, energy.
I order for an object to radiate energy it must have molecules of atoms that are moving. Atoms that are not moving are said to be at absolute zero (0K or -273°C). This is the coldest that an object can be. If an object has warmth then is radiating energy. The hotter that an object is the stronger and more varied the forms of energy that it radiates are.
Cold objects radiate mostly radio waves. The interstellar dust clouds in the Milky Way galaxy are estimated to be a few degrees above absolute zero. As a body warms to a few hundred degrees above 0K it begins to radiate strongly in the infrared (below red) range. Humans body temperatures are 98.6°F, 37°C, or 319.15k. Some surveillance cameras record images in infrared. Infrared cameras are also used to study stars that are in the beginning stages of formation, or later stages of death. (Kaler; Astronomy 9/2000)![]()
Objects that begin to glow in the visible spectrum are said to be red hot. Red is the longest frequency of light and lowest energy wave that we can detect as visible. Objects begin to be red hot at 1000K. At a few thousand Kelvin bodies produce all of the previous forms of energy plus all visible light and some ultra violet (above violet) radiation. Cool stars, 3500 K produce mostly infrared radiation. Hot stars, 5000 K give off mostly ultra-violet energy. The region of our sun (the photosphere) that produces the light that we see is estimated to be 5730K (10,000 °F). As we know from experience the sun's rays are warm (infrared) but our sun radiates most strongly in the ultra-violet range. Occasionally, during explosive outbursts, it releases x-ray radiation. The temperatures necessary to produce these very energetic waves are upwards of 500,000°C (900,000°F). The sun’s outer atmosphere is capable of temperatures this high. The inner core is estimated to be at 15,000,000 °C (27,000,000°F). At this temperature the most energetic form of electromagnetic radiation are produced. Surprisingly the sun radiates very little Gamma ray energy.
Much of the energy from the sun that reaches our atmosphere is absorbed. We are fortunate that our atmosphere is such a good protector for us. All ionizing radiation, gamma rays and x-rays are blocked before they reach the surface. Much of the ultra-violet radiation from the sun is also absorbed. All short wavelengths up to 320 nanometers are absorbed or reflected. The deepest penetrating of these waves is in the UV range. Atmospheric ozone in the ozone layer, 30 km to 80 km above the Earth, is the greatest absorber of ultraviolet radiation, specifically UV C and UV B waves. The importance of this tri-atomic (O3) upper stratospheric gas cannot be stated strongly enough. It protects virtually all life forms here on Earth.
The graph (####) shows penetration depth as a function of wavelength for fat, blood and muscle tissue. These wavelengths are non-ionizing microwaves. Our telecommunications industry works with microwaves between the frequencies of 0.1 to 100 gigahertz. The latest cordless phones transmit and receive on 900 megahertz and 2.4 gigahertz. The higher the frequency a wave is, the more likely it is to travel through a material that is between the transmitter and the receiver. As our atmospheric gasses absorb radiation they become more energetic. They may travel faster, vibrate more or rotate faster. When they are unable to hold onto this energy they release it by giving up an electron and become a positive ion. Occasionally, absorbing atoms and molecules release an electron and a different form of energy. This energy then travels along reflecting and refracting until it is absorbed or travels back into space.
Nitrogen and oxygen gasses as well as molecular oxygen absorb most of the wavelengths up to 200 nm at altitudes between 200 km and 80 km. (Tascione, 6.2) The absorption of this energy is the cause of the Ionosphere. The ionosphere is a region of charged ions (positive) and electrons our atmosphere from 80 km up to 1000 km in altitude.
The solar radiation that is transmitted through our atmosphere and reaches the surface are visible light, infrared and some radio waves.