Electromagnetic spectrum

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

Meaning

The electromagnetic spectrum is the full range of electromagnetic radiation (EMR), arranged from short-wave, high-frequency gamma rays to long-wave, low-frequency radio waves. Remote sensing does not use all of it. It uses mainly three parts: visible, infrared and microwave. Each part shows a different side of the Earth's surface, such as land use, crop health, heat or ground hidden under clouds.

Explanation

How EMR works: wavelength and frequency

• Electromagnetic radiation (EMR): energy that travels as waves at the speed of light, which is 300,000 km per second.
• EMR carries information to the sensor.
• Energy leaves an object, either bounced off it or given off by it.
• It travels up to a sensor (an instrument that records energy, such as a camera or scanner on a satellite).

• The sensor records it, and we read the object from that record.

• Wavelength: the distance from one wave crest to the next.

• Frequency: 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, and long waves have low frequency.

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

The parts of the spectrum

• Order, shortest to longest wavelength: gamma rays → X-rays → ultraviolet → visible → infrared → microwaves → radio waves.
• Visible light: about 0.4–0.7 µm (400–700 nm). This is the only part the human eye can see [4].

• Green: 0.5–0.6 µm. Red: 0.6–0.7 µm.

• Infrared (IR): about 0.7–100 µm, just beyond visible red [4]. It has three parts:

• Near infrared (NIR), 0.7–1.1 µm: records reflected sunlight. Healthy leaves reflect it strongly, and clear water absorbs it.
• Middle infrared: largely absorbed by gases in the air.

• Thermal infrared, about 8–14 µm: records heat given off (emitted) by the surface, not reflected sunlight. So it works at night too.

• Microwave (mm to cm range): most microwave energy passes through the atmosphere without being blocked, so radar waves reach the surface [4]. Radar satellites can therefore image through cloud, by day or night.

• Spectral band: one chosen wavelength range in which a sensor records energy. A multispectral sensor records several bands at once, for example red, green, blue and infrared [4].

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	Radar energy sent out and bounced back, or energy given off naturally	Imaging in all weather, day or night

The atmosphere filters the spectrum

• Cause: energy from the ground 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: energy at those wavelengths never reaches the sensor, so they tell us little about the ground.
• Atmospheric windows: wavelength ranges where the atmosphere is transparent and lets energy pass easily [4][5]. Sensors are tuned to these windows so they can "see" the surface [5]. The windows are:
• the visible range;
• two short-wave infrared windows at about 1.4–1.8 µm and 2–2.5 µm [5];
• the thermal window (about 8–14 µm);
• the microwave region, where radiation passes almost freely [4].

Why the spectrum lets us tell objects apart

• When energy hits an object, the object can absorb, transmit (let it pass), 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.
• Spectral signature: an object's pattern of reflectance across 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 curves [4].

• Sand reflects more than green vegetation at some wavelengths and less at others [4].

• Healthy vegetation: absorbs red for photosynthesis and reflects NIR strongly, because of its many-layered leaf structure.

• NIR light bounces many times among the cell layers and air spaces inside a leaf, then comes back out.

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

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

• NDVI (Normalised Difference Vegetation Index): NDVI = (NIR − Red) ÷ (NIR + Red), with a range from −1 to +1.
• Dense, healthy vegetation gives a high value.
• Bare soil gives a value near 0.
• Water and clouds give negative values.

Where it occurs

• Sambhar Lake, Rajasthan (India's largest inland salt lake, NCERT Fig. 6.5): its deep, clear parts look dark because water absorbs red and NIR. Its shallow, muddy or salty parts look light because they reflect more blue and green.
• NADAMS (National Agricultural Drought Assessment and Monitoring System), all of India:
• It uses the red and NIR bands (NDVI) to track agricultural drought.
• Started at the end of 1986 and operational in 1990. Run by the National Remote Sensing Agency (now NRSC) under the Department of Space, with support from IMD and state agriculture departments [3].
• It reports at state, district and sub-district level, using fortnightly NDVI products [3].
• Sensors used: Resourcesat AWiFS, Oceansat-2 OCM, NOAA AVHRR and Terra MODIS [3].

• It now adds NDWI (Normalised Difference Wetness Index, which shows moisture) and SASI (Shortwave Angle Slope Index), which use the short-wave infrared part of the spectrum [3].

• TRISHNA (Thermal Infra-Red Imaging Satellite for High-resolution Natural Resource Assessment): a joint ISRO–CNES (France's space agency) mission that uses the thermal infrared region [2].

• Payloads: a 4-channel thermal infrared sensor from CNES, and a 7-band VNIR-SWIR (visible, near infrared and short-wave infrared) 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, and 1 km over oceans and polar regions. Planned life: 5 years [2].

• Uses: crop water stress, surface temperature, water quality, urban heat islands, volcanic hot spots and glacier change [2].

• Microwave radar during the Indian monsoon (June–September): clouds block visible and infrared sensors, but radar still reaches the ground [4]. This keeps flood, cyclone and kharif crop monitoring going.

• Worldwide: resource managers use multispectral data to monitor fragile lands, wetlands and forests [4].

Don't confuse with

• Spectral band vs electromagnetic spectrum: the spectrum is the whole range of EMR. A band is one narrow slice of it that a sensor records, for example red at 0.6–0.7 µm.
• Reflected NIR vs thermal infrared: NIR (0.7–1.1 µm) records sunlight bounced back, so it needs daylight. Thermal IR (~8–14 µm) records heat given off by the surface, so it works at night too.
• Atmospheric window vs absorption band: a window is a wavelength range the atmosphere lets through, such as visible, thermal IR and microwave. An absorption band is a range that gases block, such as middle infrared, which CO₂ and water vapour absorb [5].
• Wavelength vs frequency: they run 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

• Order (short → long wavelength): gamma → X-ray → UV → visible → infrared → microwave → radio. Frequency runs the opposite 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; infrared overall 0.7–100 µm [4].
• Healthy vegetation absorbs red and strongly reflects NIR. Clear water absorbs red and NIR, so it looks dark.
• Middle infrared is absorbed by CO₂, hydrogen and water vapour. Blue 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

• Using the spectrum for farm and drought management (GS-III):
• Red and NIR bands give NDVI. NADAMS reads NDVI from Resourcesat AWiFS and MODIS together with IMD rainfall [3].
• It produces fortnightly drought reports down to sub-district level [3].

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

• Thermal infrared for water security and climate adaptation:

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

• This supports irrigation scheduling in water-scarce states and heat action plans for cities.

• Matching sensors to the atmosphere and the weather:

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

Related concepts

• Electromagnetic radiation
• 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