EMR SPECTRUM

Distribution of the continuum of radiant energy can be plotted as a function of wavelength (or frequency) and is known as the electromagnetic radiation (EMR) spectrum. EMR spectrum is divided into regions or intervals of different wavelengths and such regions are denoted by  different names. However, there is no strict dividing line between one spectral region and its adjacent one.

The EM spectrum ranges from gamma rays with very short wavelengths to radio waves with very long wavelengths. The EM spectrum is shown in a logarithmic scale in order to portray shorter wavelengths.

The visible region (human eye is sensitive to this region) occupies a very small region in the range between 0.4 and 0.7 μm. The approximate range of color “blue” is 0.4 – 0.5 μm, “green” is 0.5-0.6 μm and “red” is 0.6-0.7 μm. Ultraviolet (UV) region adjoins the blue end of the visible region and infrared (IR) region adjoins the red end.

The infrared (IR) region, spanning between 0.7 and 100 μm, has four subintervals of special interest for remote sensing:

(1) Reflected IR (0.7 - 3.0 μm)

(2) Film responsive subset, the photographic IR (0.7 - 0.9 μm)

(3)  Thermal bands at (3 - 5 μm) 

(4) (8 - 14 μm)

Longer wavelength intervals beyond this region are referred in units ranging from 0.1 to 100 cm. The microwave region spreads across 0.1 to 100 cm, which includes all the intervals used by radar systems. The radar systems generate their own active radiation and direct it towards the targets of interest. 

Energy in the gamma rays, X-rays and most of the UV rays are absorbed by the Earth’s atmosphere and hence not used in remote sensing. Most of the remote sensing systems operate in visible, infrared (IR) and microwave regions of the spectrum. Some systems use the long wave portion of the UV spectrum also. 

APPLICATIONS
 Different regions of the electromagnetic spectrum have diverse applications across various fields, including:

Communication: Radio waves and microwaves are used for wireless communication, broadcasting, and satellite communication.

Remote Sensing: Infrared, visible, and microwave radiation are used for remote sensing applications, such as weather forecasting, environmental monitoring, and agriculture.

Medical Imaging: X-rays and gamma rays are used for medical imaging techniques such as X-ray radiography, computed tomography (CT), and positron emission tomography (PET).

Astronomy: Different regions of the electromagnetic spectrum are used for astronomical observations, including radio astronomy, optical astronomy, and gamma-ray astronomy.