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3秒自动关闭窗口Electromagnetic Spectrum - Introduction
The Electromagnetic Spectrum
The electromagnetic (EM)
is the range of all types of EM .
Radiation is energy that travels and spreads out as it goes – the
that comes from a lamp in your house and the
waves that come from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are , , ,
You know more about the electromagnetic spectrum than you may think. The image below shows where you might encounter each portion of the
in your day-to-day life.
The electromagnetic spectrum from lowest energy/longest
(at the top) to highest energy/shortest wavelength (at the bottom). (Credit: NASA's Imagine the Universe)
Radio: Your radio captures radio waves emitted by radio stations, bringing your favorite tunes. Radio waves are also emitted by
and gases in space.
Microwave: Microwave radiation will cook your popcorn in just a few minutes, but is also used by
to learn about the structure of nearby .
Infrared: Night vision goggles pick up the infrared light emitted by our skin and objects with heat. In space, infrared light helps us map the
between stars.
Visible: Our eyes detect visible . Fireflies, light bulbs, and stars all emit visible light.
Ultraviolet: Ultraviolet radiation is emitted by the Sun and are the reason skin tans and burns.
"Hot" objects in space emit UV radiation as well.
X-ray: A dentist uses X-rays to image your teeth, and airport security uses them to see through your bag.
Hot gases in the
also emit X-rays.
Gamma ray: Doctors use gamma-ray imaging to see inside your body.
The biggest gamma-ray generator of all is the Universe.
Is a radio wave the same as a gamma ray?
Are radio waves completely different physical objects than gamma-rays? They are produced in different processes and are detected in different ways, but they are not fundamentally different.
Radio waves, gamma-rays, visible light, and all the other parts of the electromagnetic spectrum are electromagnetic radiation.
Electromagnetic radiation can be described in terms of a stream of mass-less particles, called , each traveling in a wave-like pattern at the . Each photon contains a certain amount of energy.
The different types of radiation are defined by the the amount of energy found in the photons. Radio waves have photons with low energies, microwave photons have a little more energy than radio waves, infrared photons have still more, then visible, ultraviolet, X-rays, and, the most energetic of all, gamma-rays.
Measuring electromagnetic radiation
Electromagnetic radiation can be expressed in terms of energy, wavelength, or . Frequency is measured in cycles per second, or .
Wavelength is measured in . Energy is measured in . Each of these three quantities for describing EM radiation are related to each other in a precise mathematical way.
But why have three ways of describing things, each with a different set of physical units?
Comparison of wavelength, frequency and energy for the electromagnetic spectrum. (Credit: NASA's Imagine the Universe)
The short answer is that scientists don't like to use numbers any bigger or smaller than they have to.
It is much easier to say or write "two kilometers" than "two thousand meters." Generally, scientists use whatever units are easiest for the type of EM radiation they work with.
Astronomers who study radio waves tend to use wavelengths or frequencies. Most of the radio part of the EM spectrum falls in the range from about 1 cm to 1 km, which is 30 gigahertz (GHz) to 300 kilohertz (kHz) in frequencies. The radio is a very broad part of the EM spectrum.
Infrared and optical astronomers generally use wavelength. Infrared astronomers use microns (millionths of a meter) for wavelengths, so their part of the EM spectrum falls in the range of 1 to 100 microns.
Optical astronomers use both
(0. cm, or 10-8 cm) and nanometers (0.0000001 cm, or 10-7 cm). Using nanometers, violet, blue, green, yellow, orange, and red light have wavelengths between 400 and 700 nanometers. (This range is just a tiny part of the entire EM spectrum, so the light our eyes can see is just a little fraction of all the EM radiation around us.)
The wavelengths of ultraviolet, X-ray, and gamma-ray regions of the EM spectrum are very small. Instead of using wavelengths, astronomers that study these portions of the EM spectrum usually refer to these photons by their energies, measured in electron volts (eV). Ultraviolet radiation falls in the range from a few electron volts to about 100 eV. X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then are all the photons with energies greater than 100 keV.
Why do we put telescopes in ?
The Earth's
stops most types of electromagnetic radiation from space from reaching Earth's surface. This illustration shows how far into the atmosphere different parts of the EM spectrum can go before being absorbed. Only portions of radio and visible light reach the surface.
(Credit: STScI/JHU/NASA)
Most electromagnetic radiation from space is unable to reach the surface of the Earth.
Radio frequencies, visible light and some ultraviolet light makes it to sea level.
Astronomers can observe some infrared wavelengths by putting telescopes on mountain tops.
Balloon experiments can reach 35 km above the surface and can operate for months.
Rocket flights can take instruments all the way above the Earth's atmosphere, but only for a few minutes before they fall back to Earth.
For long-term observations, however, it is best to have your detector on an orbiting
and get above it all!
Updated: March 2013Navigation::
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Electromagnetic waves - reflection, refraction, diffraction
- a summary or tutorial about the basics of the way in which electromagnetic waves are reflected, refracted and diffracted
In this section
As electromagnetic waves, and in this case, radio signals travel, they interact with objects and the media in which they travel. As they do this the radio signals can be reflected, refracted or diffracted. These interactions cause the radio signals to change direction, and to reach areas which would not be possible if the radio signals travelled in a direct line.
Reflection
Reflection of light is an everyday occurrence. Mirrors are commonplace and can be seen in houses and many other places. Shop windows also provide another illustration for this phenomenon, as do many other areas. Radio waves are similarly reflected by many surfaces.
When reflection occurs, it can be seen that the angle of incidence is equal to the angle of reflection for a conducting surface as would be expected for light.
When a signal is reflected there is normally some loss of the signal, either through absorption, or as a result of some of the signal passing into the medium.
A variety of surfaces can reflect radio signals. For long distance communications, the sea provides one of the best reflecting surfaces. Other wet areas provide good reflection of radio signals. Desert areas are poor reflectors and other types of land fall in between these two extremes. In general, though, wet areas provide better reflectors.
For relatively short range communications, many buildings, especially those with metallic surfaces provide excellent reflectors of radio energy. There are also many other metallic structures such as warehouses that give excellent reflecting surfaces. As a result of this signals travelling to and from cellular phones often travel via a variety of paths. Similar effects are noticed for Wi-Fi and other short range wireless communications. An office environment contains many surfaces that reflect radio signals very effectively.
Refraction
It is also possible for radio waves to be refracted. The concept of light waves being refracted is very familiar, especially as it can be easily demonstrated by placing a part of stick or pole in water and leaving the remaining section in air. It is possible to see the apparent change or bend as the stick enters the water. Mirages also demonstrate refraction and a very similar effect can be noticed on hot days when a shimmering effect can be seen when looking along a straight road. Radio waves are affected in the same way. It is found that the direction of an electromagnetic wave changes as it moves from an area of one refractive index to another. The angle of incidence and the angle of refraction are linked by Snell's Law that states:
n1 sin (theta 1)
n2 sin (theta 2)
For radio signals there are comparatively few instances where the signals move abruptly from a region with one refractive index, to a region with another. It is far more common for there to be comparatively gradual change. This causes the direction of the signal to bend rather than undergo an immediate change in direction.
Diffraction
Radio signals may also undergo diffraction. It is found that when signals encounter an obstacle they tend to travel around them. This can mean that a signal may be received from a transmitter even though it may be "shaded" by a large object between them. This is particularly noticeable on some long wave broadcast transmissions. For example the BBC long wave transmitter on 198 kHz is audible in the Scottish glens where other transmissions could not be heard. As a result the long wave transmissions can be heard in many more places than transmissions on VHF FM.
To understand how this happens it is necessary to look at Huygen's Principle. This states that each point on a spherical wave front can be considered as a source of a secondary wave front. Even though there will be a shadow zone immediately behind the obstacle, the signal will diffract around the obstacle and start to fill the void. It is found that diffraction is more pronounced when the obstacle becomes sharper and more like a "knife edge". For a radio signal a mountain ridge may provide a sufficiently sharp edge. A more rounded hill will not produce such a marked effect. It is also found that low frequency signals diffract more markedly than higher frequency ones. It is for this reason that signals on the long wave band are able to provide coverage even in hilly or mountainous terrain where signals at VHF and higher would not.
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