The Electromagnetic Spectrum

1. The Electromagnetic Spectrum EG5503 (GIS & Earth Observation)

2. Lecture Topics • What is the Electromagnetic Spectrum? • The concept of wavelength • Properties of EMR waves • EMR and the Sun-Atmosphere system • How does remote sensing exploit EMR

3. What is the Electromagnetic Spectrum? • The term radiation covers a wide variety of natural phenomena • All radiation involves the exchange of energy • The energy associated with electromagnetic radiation is called radiant energy • Radiant energy may exist in the absence of matter

4. What is the Electromagnetic Spectrum? • All types of EMR are transmitted, or propagated, as waves • In common with all waves, the two most fundamental properties of electromagnetic waves are length and frequency • The longer the wavelength the lower the frequency and vice versa

6. The concept of wavelength • Wavelength is usually measured in metres (the SI unit of length), micrometres (1µm=10-6m) and nanometres (1nm=10-9m) • The SI unit of frequency is hertz (cycles per second) • The electromagnetic spectrum may be defined as the entire range of radiation wavelengths

7. Electromagnetic spectrum with enhanced detail for visible region of the spectrum Note the large range of wavelengths encompassed in the spectrum - it is over twenty orders of magnitude!

8. Properties of EMR radiation • Transfer energy from place to place • Can be emitted and absorbed by matter • Do not need a material medium to travel through • Travel at 3 X 108 metres per second in a vacuum • Can be polarised (made to vibrate in a plane) • Can be reflected and refracted • Can be diffracted (e.g. using a prism) • Carry no electric charge

9. EMR and the Sun-atmosphere system • In order to understand how we can measure the physical environment with remote sensing, we must first understand solar radiation • The amount of energy received by a surface perpendicular to the Sun’s rays at the Earth’s outer atmosphere is called the solar constant (about 1370 J m-2 s-1 average)

11. EMR and the Sun-atmosphere system • About 50% of incoming solar radiation is lost by the atmosphere: scattered (30%) and absorbed (20%) • Scattering involves the absorption and re-emission of energy by particles • Absorption (unlike scattering) involves energy exchange

12. EMR and the Sun-atmosphere system • Wavelengths less than and greater than 0.8µm (800nm) are often referred to as shortwave and longwave radiation respectively • The shortwave solar radiation consists of ultraviolet and visible • The terrestrial longwave component is known as infrared

13. EMR and the Sun-atmosphere system • Just under 50% of the radiation reaching the Earth’s surface is in the visible range • Components of visible light are referred to as colours • Each colour behaves differently and white light can be separated out by use of a prism • Colour separation occurs because of differential refraction

14. EMR and the Sun-atmosphere system • The human eye cannot see infrared radiation • Infrared radiation is absorbed by water vapour and carbon dioxide in the troposphere • The atmosphere’s relative transparency to incoming solar (SW) radiation, and ability to absorb/re-emit outgoing infrared (LW) radiation is the natural greenhouse effect

15. Remote Sensing and EMR • Remote sensing exploits the different characteristics of the electromagnetic spectrum • Satellites use channels - a channel corresponds to a specific waveband, or portion of the electromagnetic spectrum • The European geostationary weather satellite METEOSAT for example has 3 channels

16. CHANNEL SPECTRAL RANGE USE Visible 0.45 to 1µm Daytime imaging Infrared 10.5 to 12.5 µm Temperature estimation and Imaging Water Vapour 5.7 to 7.1 µm Tropospheric humidity estimation

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