## The Electromagnetic Spectrum

The quantum description of electromagnetic radiation emphasises its wave-particle duality. Thus energy is transferred in packets or quanta called photons and the energy of a photon of frequency v (Ev) is given by hc

where h is Planck's constant, c is the velocity of light and A is the wavelength. When photons of high energy (short wavelength) are involved, the particle or photon nature of the radiation is most conveniently stressed. Alternatively, at long wavelength (low energy), the wave description becomes more appropriate. However, Equation (1) always applies. The names given to the different regions of the spectrum are indicated in Table 13.1 along with the appropriate wavelength, frequency and energy ranges.

More specialised units are typically used in the different ranges. Thus, for X-and gamma rays, photon energy units are often used where the electron volt (eV) is common. Here 1 eV = 1.6 x 10~19 J and the X-ray range is ~ 100 eV to 100 keV. While energy can still be important, particularly when considering interactions with detectors, UV and optical photons are more usually described by wavelength where the overall range stretches from 10 nm to 700 nm. Units of nm are used in the near-IR, giving way to pm for longer wavelengths. However, as we approach and enter the microwave range, mm and cm wavelength units give way to frequency units — THz, GHz and MHz. Finally, in the radio spectrum, both frequency and wavelength units are in common use.

Photons of electromagnetic radiation can interact with matter in a variety of ways, depending on their energy, as indicated schematically in Figure 13.1 (see also Chapter 8). At lower photon energies, the photoelectric effect (a) involves the removal of a bound electron by the incoming photon (ionisation) where the

Table 13.1 Regions of the electromagnetic spectrum.

Wavelength (m) Frequency (Hz) Energy (J)

Table 13.1 Regions of the electromagnetic spectrum.

Wavelength (m) Frequency (Hz) Energy (J)

 Radio > 1 x 10"1 < 3 x 109 < 2 x 1024 Microwave 1 x 10 "3 - 1 x 10" 1 3 x 109 - 3 x 1011 2 x 10"24 - 2 x 10" 22 Infrared 7 x 10 "7 - 1 x 10" 3 3 x 1011 - 4 x 1014 2x 10"22 - 3 x 10" 19 Optical 4 x 10 "7 - 7 x 10" -7 4 x 1014 - 7.5 x 1014 3x 10"19 - 5 x 10" 19 UV 1x 10 "8 - 4 x 10" -7 7.5 x 1014 - 3 x 1016 5x 10"19 - 2 x 10" 17 X-ray 1x 10" 11 - 1 x 10 8 3 x 1016 - 3 x 1019 2x 10"17 - 2 x 10" 14 Gamma-ray < 1 x 10"11 > 3 x 1019 > 2 x 10"14
Figure 13.1 Schematic illustration of the processes for photon interaction with matter, (a) photoelectric absorption, (b) Compton scattering, (c) pair production.

photon energy, Ev > EB, the electron binding energy. When Ev has increased above EB for electrons in the target atoms, the photons instead undergo Compton scattering with electrons (b). Here the photon energy is reduced — unlike in the case of Thomson scattering — and the electron emerges with the energy lost by the incoming photon, expressed as a wavelength change as

where a is the angle between the scattered electron and the incoming photon. This process operates at hard X-ray and lower gamma-ray energies. Finally, when the photon energy has increased to more than twice the electron rest energy, or 2mec2 = 1.022 MeV, the process of pair production can occur (c). Here the incident photon can produce an electron-positron pair in the Coulomb field of a nucleus. In practice, use of pair production in gamma-ray detectors becomes practical for Ev > 20 MeV. All three of the above processes are important for photon detectors, depending on the photon energy involved. See Longair (1992) for a discussion of their behaviour with Ev.

Figure 13.2 The transmission of the Earth's atmosphere for photons of the electromagnetic spectrum as a function of wavelength.

Photoelectric absorption has a crucially important role in the Earth's atmosphere, as is apparent in Figure 13.2. Here the transmission or transparency of the atmosphere is plotted against wavelength. Although the plot extends only to 0.1 nm at the short wavelength end — a photon energy of 12.4 keV — the atmosphere remains opaque to all higher-energy photons.

While X-rays with Ev > 50 keV can penetrate to ~ 30 km above the Earth's surface, where they can be studied by balloon-borne detectors, practically speaking it is necessary to place instruments in spacecraft that remain outside the atmosphere for long periods so that the radiation which cannot penetrate to the Earth's surface may be studied. Thus Figure 13.2 indicates the compelling need for observations of electromagnetic radiation from space. Furthermore, even the narrow though important visible spectral range (400-1000 nm) is affected by atmospheric turbulence or 'seeing' conditions. These limit the useable angular resolution of ground-based optical telescopes to ~ 1 arc sec. While this can be reduced by about a factor of 10 using current active or adaptive optics techniques, such performance is at present available only in the IR range at A > 1 ¡m. We will see later that the Hubble Space Telescope (HST), with angular resolution of ~ 0.03 arc sec, operating at visible wavelengths in near-Earth orbit with sky background per resolution element reduced by ~ 1000 and free from atmospheric turbulence, has enormously advanced our understanding of the Universe.

## Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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