Electromagnetic spectrum (EMS) represents the continuum of electromagnetic radiation (EMR) arranged on the basis of wavelengths or frequency.
Electromagnetic spectrum ranges from shorter wavelengths (gamma rays to x rays) to the longer wavelengths (microwave and radio waves).
Most common remote sensing systems operate in one or several of the visible, infrared and microwave portions of the electromagnetic spectrum. Within the infrared portion of the spectrum it should be noted that only thermal infrared energy is directly related to the sensation of heat; not the near and mid infrared
Before discussing about EMS with reference to remote sensing it is important to understand it fully. Different radiations which constitute the EMS are as follows:
These are the longest wavelength (lowest frequency) radiations of the EMS. The wavelength of radio waves is more than 100 cm. These passes through Earth's atmosphere easily. Radio signals are used in radios, televisions, aircrafts, ship etc. These are also emitted by stars.
Their wavelength ranges between 1mm & 1m. RADAR (Radio Detection And Ranging) is the most common device used in Microwave Remote Sensing. Other applications are in cooking food (microwave oven), in broadcasting transmissions etc.
When we feel hot it is because of infrared (IR) radiations. For common understanding we can call them as 'Heat'. The wavelength of IR is longer than visible light and shorter than microwaves approximately ranges between 1micron to 100 microns. These are very useful radiations for remote sensing. Thermal Imaging Systems detect objects by recording their temperature (infrared emissions).
As their name itself suggests these are the EMRs which are visible to our eyes in different colours. These ranges between 700nm to 400nm. Most of the remote sensing systems and camera records images in this range.
Ultraviolet (UV) Radiations
These radiations have wavelength shorter violet colour of visible light and longer than X-rays. UV radiations can be divided into near UV (400–200nm), far UV (200–10) and extreme UV 1–31 nm.
These are very short length electromagnetic radiations wavelength in the approximate range from 0.01 to 10 nanometers. In EMS these falls between UV radiations and Gamma-rays. Mostly used in medical sciences.
Gamma-rays are the electromagnetic radiations with shortest wavelength in the range of the range of 10^-11 m to 10^-14 m. Their very high energy can cause serious damage to living cells.