Electromagnetic Radiation (EMR)
EMR refers to the energy that travels through space as oscillating electric and magnetic fields. It is the fundamental mechanism by which energy moves through the Electromagnetic spectrum.
What is the Electromagnetic Radiation?
Electromagnetic Radiation (EMR) or Electromagnetic energy is the energy propagated in the form of an advancing interaction between electric and magnetic fields. It travels with the velocity of light. Visible light, ultraviolet, and infrared rays, heat, radio waves, X-rays all are different forms of electromagnetic energy.
EMR ranges from gamma rays with very short wavelength to long radio waves. The shortest wavelengths can also be modeled as particles (photons). The interaction of EMR with matter forms the basis for Remote Sensing.
| UV | Visible(µm) | 0.8-0.9 (µm) | 0.9-1.3 (µm) | 1.3-14 (µm) | Microwaves | ||
| 0.4-0.5 | 0.5-0.6 | 0.6-0.7 | |||||
| Blue | Green | Red | Near IR | Mid IR | Far IR | ||
Electric and Magnetic fields that are perpendicular to each other and propagation to the direction of travel. Although it is a wave, it also can be detected as discrete particles of light, called photons.
EMR Properties
- EMR radiated by atomic particles at the source (Sun)
- Propagates through the vacuum of space at the speed of light
- Interacts with the Earth’s atmosphere
- Interacts with the Earth’s surface
- Various optical systems and detectors
EMR Waves
EMR covers a wide range of wave frequencies and wavelengths, collectively known as the Electromagnetic spectrum. This spectrum includes (from longest wavelength to shortest):
| Waves | λ (m) | v (hertz[Hz]) | E (electron-volt [eV]) |
|---|---|---|---|
| Radio waves | 1 | 3☓108 | 1.24☓10-6 |
| Microwaves | 1☓10-3 | 3☓1011 | 1.24☓10-3 |
| Infrared waves | 0.75☓10-6 | 4☓1014 | 1.65 |
| Visible Light | 0.4☓10-6 | 7.5☓1014 | 3.1 |
| Ultraviolet | 1.2☓10-8 | 2.4☓1016 | 1☓102 |
| X-rays | 1.4☓10-11 | 3☓1019 | 1.2☓105 |
| Gamma rays |
Atmospheric Effects on Electromagnetic Radiation
Electromagnetic radiation (EMR) is significantly influenced by the Earth’s atmosphere. These atmospheric effects depend largely on the wavelength of the radiation and the chemical composition and physical structure of the atmosphere. The main atmospheric interactions that affect EMR are described below:
Absorption
Absorption occurs when atmospheric gases absorb specific wavelengths of electromagnetic radiation and convert them into heat energy. This process selectively eliminates certain parts of the EMR spectrum before the radiation reaches the Earth’s surface.
Major atmospheric absorbers include:
- Ozone (O₃), located in the stratosphere, is highly effective at absorbing ultraviolet (UV) radiation, particularly at wavelengths shorter than 300 nm. This absorption protects living organisms from harmful UV exposure.
- Carbon dioxide (CO₂) found in the lower atmosphere and absorbs energy primarily in mid and far infrared region of the electromagnetic spectrum with maximum absorption occuring from 13 to 17.5 μm.
- Water vapor alsopresents the in the lower atmosphere where major absorption take place between 5.5 to 7 μm.
Effect: Due to absorption, only specific wavelengths—such as visible light (approximately 400–700 nm) and certain radio frequencies—are able to reach the Earth’s surface. This selective transmission is critical for life and for technologies like remote sensing and communication.
Scattering
Scattering refers to the redirection of EMR as it interacts with particles and molecules in the atmosphere. This interaction alters both the direction and intensity of the radiation.
Types of scattering:
- Rayleigh Scattering: Caused by small particles or gas molecules, it is more effective at shorter wavelengths (e.g., blue and violet light). This phenomenon explains why the sky appears blue during the day and reddish at sunrise and sunset.
- Mie Scattering: Occurs when larger particles such as dust, smoke, and pollution scatter all visible wavelengths more uniformly. This results in white glare or hazy skies, particularly in polluted urban areas.
- Non-selective Scattering: Caused by particles much larger than the wavelength of light, such as water droplets in clouds. This scattering affects all wavelengths almost equally, causing clouds to appear white.
Effect: Scattering influences visibility, image clarity, and color perception. It is especially important in remote sensing, astronomical observations, and weather forecasting.
Reflection
Reflection involves the bouncing back of electromagnetic waves from surfaces or particles in the atmosphere.
- Clouds and aerosols reflect a portion of incoming solar radiation back into space, which can reduce the amount of sunlight that reaches the Earth’s surface.
- Albedo refers to the reflective property of a surface. Surfaces with high albedo (e.g., snow, ice, and deserts) reflect more radiation, while surfaces with low albedo (e.g., forests and oceans) absorb more energy.
Effect: Reflection reduces the total solar energy available at the ground level and contributes to the local and global energy balance.
Refraction
Refraction is the bending of electromagnetic waves as they pass through layers of the atmosphere with varying density or composition. As EMR moves from space into the denser layers of the Earth’s atmosphere, its path is slightly altered.
Effect: Refraction causes apparent shifts in the position of celestial objects (e.g., stars appear slightly higher in the sky than they are). It also affects the accuracy of satellite communication and astronomical measurements.
Delay and Dispersion (in radio waves)
Radio waves—especially those utilized in communication and navigation—are affected by the ionosphere, a region of the upper atmosphere containing charged particles (ions and electrons). This ionized layer influences the propagation, speed, and direction of radio signals.
- Radio waves traveling through the ionosphere experience time delays due to the varying density of charged particles.
- Different radio frequencies travel at slightly different speeds, causing the signal to spread out over time.
Effect: These phenomena affect the accuracy of GPS, radio signal quality, and timing systems used in various technologies.
Atmospheric Window
Atmospheric windows are specific ranges of wavelengths that can pass through the atmosphere with minimal absorption or scattering.
Major atmospheric windows:
- Visible Window (approx. 400–700 nm): Allows most of the Sun’s radiation to reach the Earth’s surface, making visible light crucial for life and observation.
- Radio Window (approx. 1 mm to 30 m): Permits the transmission of radio waves, which is essential for radio astronomy, television, and satellite communication.
- Infrared and Ultraviolet Windows: These windows are partially open, allowing only certain infrared and UV wavelengths to reach the surface. Most are blocked by water vapor, carbon dioxide, and ozone.
Effect: Atmospheric windows determine which wavelengths can be observed or measured from the satellite.
