Electromagnetic Radiation
Electromagnetism: Waves, Frequency and Energy.
Visible light, X Rays, microwaves and radio waves are all different manifestations of the same phenomena: electromagnetic radiation. Electromagnetic radiation is generated by the interaction of a magnetic field and an electron at the atomic level and the result is a discrete packet of energy called a photon, which vibrates at a certain frequency. The difference in their properties relates to the difference in their frequency. Higher frequency electromagnetic waves are more energetic.
The image below shows the range of wavelengths of electromagnetic waves, and their associated properties on the X axis. (We'll discuss the Y axis later). The speed of electromagnetic waves is constrained to the speed of light (186,000 Miles per second or 299,000 kilometres per second). Since the velocity of a wave is defined as it's frequency multiplied by it's wavelength, it follows that higher frequency waves will have shorter wavelengths. Higher frequency waves such as X rays are far more energetic than low frequency radio waves, and this gives rise to a dramatic difference in the properties of waves across the spectrum.
The relationship between energy and frequency of a wave can be described by a very simple but very important expression which was devised in 1900 by German physicist Max Planck:
E = hv
where:
- E is energy
- h is the Planck Constant, 6.26 x 10-34
- v is the frequency of the photon
At the very short wavelength end of the spectrum (left hand side of the X axis in the image above), photons have high frequencies hence high energies. This is where we find X rays, which are energetic enough to penetrate solid materials.
Between 400nm and 700nm we find the visible part of the electromagnetic spectrum. Blue light has a higher frequency, with a wavelength of 400nm and is more energetic than red with a lower frequency hence slightly longer wavelength at 700nm.
Beyond red light we have infrared then microwave radiation. Radio waves have a much longer wavelength and lower frequency than visible light. Note that the wavelength scale on the X axis is logarithmic and not linear. Long Wave Radio waves are hundreds of metres long, therefore they have very low frequencies and hence very low energy.
The Y axis represents atmospheric opacity. It is given as a percentage, where 100% means no electromagnetic energy in the form of photons can reach the Earth, and 0% means there is no impedance to photons reaching Earth. Due to atmospheric properties most of the electromagnetic waves in the ultraviolet range are scattered and do not make it to the Earth's surface so they are difficult to detect.
There is a narrow part of the spectrum where visible light reaches Earth with relatively little atmospheric disturbance. Past the optical wavelengths we find that longer wavelength, low frequency infrared radiation is effectively stopped by the atmosphere. However, when we move along the spectrum to even lower frequencies of microwave radiation and radio waves, we find that these waves effectively penetrate the atmosphere.
For wavelengths between about 5cm and 15m, the opacity is virtually zero so these are suitable for study using ground based equipment provided there is not too much radio interference nearby.
For wavelengths outside the optical and radio wavelengths, atmospheric opacity makes observation virtually impossible hence the need for space based observatories designed to see in the X ray, infra red and microwave frequencies.
Electromagnetic waves are produced by a variety of different physical interactions, four of which I will describe here. They are called:
- Thermal
- Bremsstrahlung
- Synchrotron
- Quantum Emission
Understanding these will help to understand the various projects that amateur radio astronomers can undertake.
Thermal radiation is the result of subatomic particles in matter moving around as they are heated up, generating electromagnetic radiation. Any particle that is above zero degrees Kelvin will generate some thermal radiation. Stars are a good astronomical source of thermal radiation, and their temperature can allow us to classify them.
Bremsstrahlung radiation means 'braking radiation' in German and is a type of electromagnetic radiation that is generated when a fast moving electron is deflected as it approaches a positive nucleus of an ion. Hot, gaseous nebulae are a good source of Bremsstrahlung radiation, as are X ray stars.
Synchrotron radiation occurs in a similar fashion to Bremsstrahlung except in this case it is a magnetic source that deflects the electron rather than a positive ion. Some planets and other astronomical phenomena have very strong magnetic fields. When a stray fast moving electron is captured by this field it goes into a spiral spin, and as it spins faster and faster it gives off ripples of electromagnetic radiation. Astronomical sources are Jupiter and M87 ( a galaxy in Virgo), and the crab nebula (M1).
The fourth type of electromagnetic source I want to mention is Quantum Emission. This occurs when the polarity of an electron orbiting around the nucleus of an atom flips relative to the nucleus. This generates a quantum packet of energy in the form of a photon, which has a frequency of 1420MHz. These quantum flipping events are very rare, occurring on average once every 10 million years, but because of the abundance of hydrogen atoms in the Universe, the night sky in the galactic plane is filled with quantum emission radiation that amateur radio astronomers are able to measure.
