## Electromagnetic Waves

Electromagnetic waves are energy transported through space in the form of periodic disturbances of electric and magnetic fields. All electromagnetic waves travel through space at the same speed, c = 2.99792458 x 108 m/s, commonly known as the speed of light. An electromagnetic wave is characterized by a frequency and a wavelength. These two quantities are related to the speed of light by the equation,

speed of light = frequency x wavelength

The frequency (and hence, the wavelength) of an electromagnetic wave depends on its source. There is a wide range of frequency encountered in our physical world, ranging from the low frequency of the electric waves generated by the power transmission lines to the very high frequency of the gamma rays originating from the atomic nuclei. This wide frequency range of electromagnetic waves constitute the Electromagnetic Spectrum.

## The Electromagnetic Spectrum

The electromagnetic spectrum can be divided into several wavelength (frequency) regions, among which only a narrow band from about 400 to 700 nm is visible to the human eyes. Note that there is no sharp boundary between these regions. The boundaries shown in the above figures are approximate and there are overlaps between two adjacent regions.

Wavelength units: 1 mm = 1000 µm; 1 µm = 1000 nm.

• Radio Waves: 10 cm to 10 km wavelength.

• Microwaves: 1 mm to 1 m wavelength. The microwaves are further divided into different frequency (wavelength) bands: (1 GHz = 109 Hz)

• P band: 0.3 - 1 GHz (30 - 100 cm)

• L band: 1 - 2 GHz (15 - 30 cm)

• S band: 2 - 4 GHz (7.5 - 15 cm)

• C band: 4 - 8 GHz (3.8 - 7.5 cm)

• X band: 8 - 12.5 GHz (2.4 - 3.8 cm)

• Ku band: 12.5 - 18 GHz (1.7 - 2.4 cm)

• K band: 18 - 26.5 GHz (1.1 - 1.7 cm)

• Ka band: 26.5 - 40 GHz (0.75 - 1.1 cm)

• Infrared: 0.7 to 300 µm wavelength. This region is further divided into the following bands:

• Near Infrared (NIR): 0.7 to 1.5 µm.

• Short Wavelength Infrared (SWIR): 1.5 to 3 µm.

• Mid Wavelength Infrared (MWIR): 3 to 8 µm.

• Long Wanelength Infrared (LWIR): 8 to 15 µm.

• Far Infrared (FIR): longer than 15 µm.

The NIR and SWIR are also known as the Reflected Infrared, referring to the main infrared component of the solar radiation reflected from the earth's surface. The MWIR and LWIR are the Thermal Infrared.

• Visible Light: This narrow band of electromagnetic radiation extends from about 400 nm (violet) to about 700 nm (red). The various colour components of the visible spectrum fall roughly within the following wavelength regions:

• Red: 610 - 700 nm

• Orange: 590 - 610 nm

• Yellow: 570 - 590 nm

• Green: 500 - 570 nm

• Blue: 450 - 500 nm

• Indigo: 430 - 450 nm

• Violet: 400 - 430 nm

• Ultraviolet: 3 to 400 nm

• X-Rays and Gamma Rays

## Photons

According to quantum physics, the energy of an electromagnetic wave is quantized, i.e. it can only exist in discrete amount. The basic unit of energy for an electromagnetic wave is called a photon. The energy E of a photon is proportional to the wave frequency f,

E = h f

where the constant of proportionality h is the Planck's Constant,

h = 6.626 x 10-34 J s.

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