Electromagnetic radiation

Also called: EMR · Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"

Meaning

Electromagnetic radiation (EMR) is energy that travels as waves at the speed of light, which is 300,000 km per second. In remote sensing it carries information from an object on Earth to a sensor (the instrument that records energy, such as a camera or scanner on a satellite).

It matters because every satellite image is a record of EMR. Objects absorb, transmit (let it pass through), reflect or emit (give off) this energy in different ways, and that is how we tell forests, water, soil and cities apart from space. The Sun is the main source of the EMR that is reflected back to sensors.

Explanation

How EMR works: wavelength and frequency

• Wavelength is the distance from one wave crest to the next.
• Frequency is the number of waves that pass a point in one second.
• Wavelength and frequency go in opposite directions:
• All EMR moves at the same speed.
• So short waves have high frequency.

• Long waves have low frequency.

• Unit: remote sensing bands are measured in micrometres (µm). 1 µm is one-millionth of a metre.

The electromagnetic spectrum: types of EMR

• Electromagnetic spectrum: the full range of EMR, from short, high-frequency waves to long, low-frequency waves.
• Order, shortest to longest wavelength: gamma rays → X-rays → ultraviolet → visible → infrared → microwaves → radio waves.
• Remote sensing mainly uses three regions: visible, infrared and microwave. X-rays are not used.

Region	Approx. wavelength	What the sensor records	Typical use
Visible (blue, green, red)	0.4–0.7 µm [4]	Reflected sunlight	True-colour images, water and land-use mapping
Near infrared (NIR)	0.7–1.1 µm	Reflected sunlight	Vegetation health, separating land from water
Thermal infrared	~8–14 µm	Heat given off by the surface	Surface temperature, fires, urban heat islands
Microwave	mm to cm range	Energy sent out and bounced back (radar), or given off naturally	Imaging in all weather, day or night

• Visible light (about 0.4–0.7 µm) is the only part the human eye can see [4].
• Infrared (IR) (about 0.7–100 µm) lies just beyond visible red [4]. It is divided into near infrared, middle infrared and thermal infrared.
• Spectral band: one chosen wavelength range in which a sensor records energy. Key NCERT bands are green 0.5–0.6 µm, red 0.6–0.7 µm and NIR 0.7–1.1 µm. A multispectral sensor records several bands at once.

What happens to EMR on its way to the sensor

• Cause: energy reflected or emitted by Earth must pass back up through the atmosphere to reach the sensor.
• Absorption:
• Carbon dioxide, hydrogen and water vapour absorb middle infrared energy.

• Gases such as water vapour, carbon dioxide and ozone absorb certain wavelengths. These ranges are called absorption bands [5].

• Scattering: dust and other particles scatter blue light, sending it off in all directions.

• Result: that energy never reaches the sensor, so those wavelengths tell us little about the ground.
• Atmospheric windows: wavelength ranges where the atmosphere is transparent, so energy passes through easily [4][5].
• Sensors are tuned to these windows so they can "see" the surface [5].

• Examples: the visible range, short-wave infrared windows at about 1.4–1.8 µm and 2–2.5 µm [5], the thermal window (about 8–14 µm), and the microwave region [4].

• Microwave and cloud: most microwave energy passes through the atmosphere without being blocked, so radar waves reach the surface [4]. This is why radar satellites can image through cloud.

How objects interact with EMR: spectral signatures

• When EMR hits an object, the object can absorb, transmit, reflect or emit it. How much of each happens depends on the wavelength and on the object.
• Reflectance: the share of incoming energy that a surface reflects.

• Absorptance: the share of incoming energy that a surface absorbs.

• Spectral signature: an object's pattern of reflectance across different wavelengths. It is unique, like a fingerprint (NCERT Fig. 6.4). Grasslands, pinewoods, red sand and silty water can each be told apart by their reflectance curves [4].

• Healthy vegetation:
• It absorbs red light and uses it for photosynthesis.
• It reflects NIR strongly, because a healthy leaf has many internal layers of cells and air spaces that bounce NIR back out.

• A stressed leaf loses this structure, so its NIR reflectance drops. Satellites can see this before the leaf looks brown.

• Clear water absorbs red and NIR, so it looks dark. Turbid (muddy) water reflects more blue and green, so it looks light.

• Reflected vs emitted EMR:
• Visible and NIR sensors record reflected sunlight, so they need daylight.
• Thermal IR sensors record heat emitted by the surface, so they also work at night.

Where it occurs

• Sambhar Lake, Rajasthan (India's largest inland salt lake, NCERT Fig. 6.5):
• Its clear, deep parts look dark, because water absorbs red and NIR.

• Its shallow, muddy or salty parts look light, because they reflect more blue and green.

• India's monsoon months: clouds cover the land, so optical sensors fail. Microwave (radar) EMR passes through cloud and keeps monitoring going [4].

• NADAMS (National Agricultural Drought Assessment and Monitoring System), India:
• It was started at the end of 1986. The nodal agency was the National Remote Sensing Agency (now NRSC) under the Department of Space, supported by IMD and state agriculture departments [3].
• It became operational in 1990. It reports agricultural drought at state, district and sub-district level [3].
• It uses red and NIR EMR through NDVI (Normalised Difference Vegetation Index) = (NIR − Red) ÷ (NIR + Red), along with NDWI, SASI, and ground data on rainfall and crop-sown area. NRSC makes fortnightly NDVI products [3].

• Sensors used: Resourcesat AWiFS, Oceansat-2 OCM, NOAA AVHRR (USA) and Terra MODIS [3].

• TRISHNA, a joint ISRO–CNES (France's space agency) mission, uses thermal infrared EMR [2]:

• Payloads: a 4-channel thermal infrared sensor from CNES and a 7-band VNIR-SWIR sensor from ISRO [2].
• Orbit: sun-synchronous, at 761 km, crossing the equator at 12:30 pm local time [2].
• Resolution: 57 m over land and coasts, 1 km over oceans and polar regions. Planned life: 5 years [2].

• Uses: water stress in crops, surface temperature, water quality in coastal and inland waters, urban heat islands, volcanic hot spots and glacier changes [2].

• Worldwide: multispectral sensors that record red, green, blue and infrared EMR help resource managers watch fragile lands, wetlands and forests [4].

Don't confuse with

• Electromagnetic spectrum vs EMR: EMR is the energy itself. The spectrum is the full ordered range of that energy, from gamma rays to radio waves.
• Reflected EMR vs emitted EMR (thermal): visible and NIR sensors record sunlight bounced back, so they need daylight. Thermal infrared sensors record heat given off by the surface, so they also work at night.
• Absorption band vs atmospheric window: in an absorption band, gases block EMR and it never reaches the sensor [5]. In an atmospheric window, the air is transparent and EMR passes through [4][5].
• Wavelength vs frequency: they go in opposite directions. Gamma rays have the shortest wavelength but the highest frequency. Radio waves have the longest wavelength but the lowest frequency.

Prelims Hooks

• Spectrum order (short → long wavelength): gamma → X-ray → UV → visible → infrared → microwave → radio. Frequency runs the other way.
• Trap: remote sensing uses visible, infrared and microwave. X-rays are NOT used (NCERT Class 11, Ch 6, exercise 1(ii)).
• Band ranges: visible 0.4–0.7 µm [4], green 0.5–0.6 µm, red 0.6–0.7 µm, NIR 0.7–1.1 µm, thermal IR ~8–14 µm.
• Healthy vegetation absorbs red and reflects NIR strongly. Clear water looks dark. Turbid water looks light. Example: Sambhar Lake, Rajasthan.
• Middle infrared is absorbed by CO₂, hydrogen and water vapour. Blue light is scattered by dust. Microwave (radar) passes through cloud, so it works in all weather [4].
• TRISHNA: ISRO + CNES (France), thermal infrared, 761 km sun-synchronous orbit, 57 m resolution over land [2].

Mains Points

• EMR-based indices support farm and drought management (GS-III):
• Red and NIR readings from Resourcesat AWiFS and MODIS give NDVI.
• NDVI is read together with IMD rainfall to make fortnightly drought reports down to sub-district level under NADAMS [3].

• These reports guide drought declarations, contingency crop plans and crop insurance checks.

• Thermal infrared for water security and climate adaptation:

• TRISHNA will measure crop water stress and evapotranspiration (water lost from soil and plants) at 57 m, and map urban heat islands [2].

• This helps irrigation planning in water-scarce states and heat action plans for cities.

• Matching the sensor to the atmosphere and the weather:

• Absorption bands and monsoon cloud (June–September) limit optical sensors.
• Placing sensors in atmospheric windows [5] and adding microwave radar [4] keeps monitoring going for floods, cyclones and kharif crops.
• Limit: different objects can have similar signatures (for example turbid water and wet soil), so EMR data must be checked against ground truth (field data). This is why NADAMS combines satellite indices with rainfall and sown-area data [3].

Related concepts

• Electromagnetic spectrum
• Spectral band
• Near infrared
• Atmospheric absorption and scattering
• Thermal infrared remote sensing
• Reflectance
• Absorptance
• Spectral signature
• NDVI

Read more

• Electromagnetic energy, the spectrum and spectral signatures