The electromagnetic spectrum and its uses

The first principle

Light, radio, X-rays and gamma rays are the same thing: waves of electric and magnetic fields that travel through empty space at about 300,000 km per second. They differ only in wavelength, and wavelength decides everything else, because the energy of each packet of light, the photon, rises as the wavelength shrinks. That single fact explains why radio waves pass through walls but do no harm, why ultraviolet burns skin, and why X-rays can both show a broken bone and damage the DNA in it.

The core

One spectrum, ordered by wavelength

• Speed, wavelength and frequency are tied together. In vacuum every electromagnetic wave travels at the speed of light, c, and c equals frequency multiplied by wavelength, so a shorter wavelength means a higher frequency. A 2.4 gigahertz (GHz) Wi-Fi signal has a wavelength of about 12.5 cm, while green light, at about 550 nanometres, oscillates about 545 trillion times a second.
• Photon energy rises with frequency. Max Planck and Albert Einstein showed that light comes in packets of energy E = hf, where h is Planck's constant and f the frequency. A photon of visible light carries about 1.6 to 3.3 electronvolts, enough to drive photosynthesis or trigger the eye, while an X-ray photon carries thousands of electronvolts.
• Ionising and non ionising radiation are separated by that energy. Above roughly 10 electronvolts, from the far ultraviolet upwards, a single photon can knock an electron out of an atom and break chemical bonds, including those in deoxyribonucleic acid (DNA), the molecule of heredity. Radio waves, microwaves, infrared and visible light cannot do this however strong they are; their main effect on the body is heating, which is why exposure limits for them are set to prevent heating.
• Each band has its own source. Radio waves come from currents oscillating in an antenna, infrared from warm bodies, visible and ultraviolet light from electrons jumping between atomic levels, X-rays from fast electrons that are stopped or that fill inner shells, and gamma rays from changes inside the nucleus.
• The atmosphere is transparent only through windows. Visible light, near infrared and a wide range of radio waves reach the ground, while ozone absorbs the shorter, more harmful ultraviolet and the air absorbs X-rays and gamma rays. This is why optical and radio telescopes work on the ground, while X-ray astronomy such as India's AstroSat and XPoSat needs satellites, and why the ozone layer protects life.

Lasers, radar and LiDAR

• A laser makes light by stimulated emission. An outside source of energy, the pump, lifts more atoms into an excited state than remain in the lower state, a condition called population inversion. A passing photon then stimulates an excited atom to emit an identical photon, in the same direction and in step, and mirrors at both ends bounce the light back and forth so that it builds up. The result is light of one colour (monochromatic), in step (coherent) and in a narrow beam (directional), so lasers cut metal, read barcodes, carry data in optical fibres and reshape eyes.
• Radar (radio detection and ranging) sends out pulses of radio or microwave energy and times the echo, so the delay gives the distance and the change in frequency of the echo, the Doppler shift, gives the speed. Radio waves pass through cloud, smoke and darkness, which is why radar guides aircraft, tracks storms and watches borders.
• Synthetic aperture radar (SAR) puts a radar on a moving satellite or aircraft and combines the echoes received along its path, so a small antenna behaves like a very long one and produces sharp images day or night, through cloud. Longer wavelengths such as the L band pass through forest canopy and dry soil, while shorter ones such as the X band show finer surface detail.
• LiDAR (light detection and ranging) does the same with laser pulses. Because light has a much shorter wavelength than radio, it maps the ground, buildings and trees to within centimetres, which is why it is used for terrain models, forest surveys, archaeology and the sensors of self driving cars, though it is blocked by cloud and fog.

Dimensions

• Health. Non ionising radiation heats, while ionising radiation can break DNA, so X-rays and computed tomography (CT) scans are used when the medical gain outweighs a small added cancer risk, and sunlight's ultraviolet is the main cause of skin cancer. The World Health Organization (WHO) finds no established health harm from the radio waves of phones and towers at permitted levels, though its cancer agency lists them as possibly carcinogenic (Group 2B, 2011) on limited evidence.
• Economic. Radio spectrum is a scarce natural resource, because two users on the same frequency in the same place interfere, so its sale and use drive mobile telephony, broadcasting and satellite services, and auctions are a large source of non tax revenue.
• Security. Radar, infrared seekers, laser guided weapons, electronic warfare and laser weapons against drones all work in this spectrum, and the drones note covers the military side.
• Environmental. Infrared and microwave satellites measure sea temperature, soil moisture, forests and greenhouse gases, and a thinner ozone layer lets more ultraviolet reach the ground.
• Legal. In Centre for Public Interest Litigation v Union of India (2012), the 2G case, the Supreme Court cancelled 122 telecom licences and held that spectrum, a scarce natural resource held in trust for the people, must be allotted fairly, preferably by auction; its opinion on a Presidential reference later that year clarified that auction is not required for every natural resource.

In practice

In medicine

• X-ray imaging and CT pass X-rays through the body; bone absorbs them strongly and soft tissue weakly, and a CT scanner rotates the source to build cross sections. Magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves that make hydrogen nuclei in water and fat give off signals, so it shows soft tissue such as brain and ligaments without ionising radiation. Positron emission tomography (PET) detects the pairs of gamma rays given off when positrons from an injected tracer meet electrons in the body, and so shows activity such as the uptake of sugar by a tumour.
• Radiation treats as well as shows. Gamma rays from cobalt-60 and high energy X-rays from linear accelerators destroy cancer cells, ultraviolet C lamps disinfect water and air, and lasers reshape the cornea and seal blood vessels.

In India's programmes

• Earth observation. The NASA–ISRO Synthetic Aperture Radar (NISAR), built by the United States National Aeronautics and Space Administration (NASA) and the Indian Space Research Organisation (ISRO) and launched by the Geosynchronous Satellite Launch Vehicle (GSLV) on 30 July 2025, is the first Earth observation satellite to fly synthetic aperture radars in two bands, L and S, and maps changes in ice, forests, crops and the ground. The Radar Imaging Satellite (RISAT) series gives India its own all weather imaging, and the geospatial technology note covers how such data are used.
• Weather. The India Meteorological Department (IMD) runs a network of Doppler weather radars that track storms and cyclones, and Mission Mausam, approved in September 2024 at Rs 2,000 crore for two years, is expanding radars, satellites and supercomputers for forecasting.
• Astronomy and physics. The Giant Metrewave Radio Telescope (GMRT) near Pune observes the radio sky, India has approved about Rs 1,250 crore to take part in building the Square Kilometre Array, the largest radio telescope being built, and LIGO-India, India's node of the Laser Interferometer Gravitational-Wave Observatory, approved in April 2023 at about Rs 2,600 crore at Hingoli in Maharashtra, will use laser interferometry to detect gravitational waves.

Spectrum as a public resource

• Who manages it. The International Telecommunication Union (ITU) allocates bands to services worldwide through its World Radiocommunication Conferences, the Department of Telecommunications (DoT) sets India's National Frequency Allocation Plan and assigns frequencies through its Wireless Planning and Coordination Wing, and the Telecom Regulatory Authority of India (TRAI) recommends prices and terms.
• How it is given out. The Telecommunications Act 2023 keeps auction as the normal route but lists uses, including several satellite services, for which spectrum is assigned administratively; the telecom note covers the details.
• Exposure rules. Since 1 September 2012 India has limited radio frequency exposure from mobile towers to one tenth of the levels recommended by the International Commission on Non-Ionizing Radiation Protection (ICNIRP), and the DoT's Tarang Sanchar portal lets the public check tower compliance.

Issues and way forward

• Fear of towers against evidence. Worry about tower radiation delays network rollout though Indian limits are ten times stricter than international guidance; published measurements and plain explanation of heating versus ionisation work better than dismissal.
• Overuse of medical imaging. CT scans give far larger doses than plain X-rays, and repeated scans in children add up, so referral guidelines, dose recording and the principle of as low as reasonably achievable (ALARA) should be followed in every hospital.
• Spectrum pricing and sensors. High reserve prices have at times left spectrum unsold while operators carried heavy debt, so pricing must balance revenue against affordable coverage, while detectors and radar parts, still largely imported, need Indian makers.

Common confusions

• Ionising and non ionising. The divide depends on the energy of each photon, not on the strength of the source, so a powerful radio transmitter is still non ionising while a faint X-ray beam is ionising.
• Radar, LiDAR and sonar. Radar uses radio waves and LiDAR uses laser light, both electromagnetic, while sonar uses sound waves in water, which are vibrations of matter and not electromagnetic at all; medical ultrasound is likewise sound.
• Microwave and microwave oven. Microwaves are a band of radio waves used for radar, mobile and Wi-Fi; an oven uses about 2.45 GHz to make water molecules turn back and forth and heat food, and it makes nothing radioactive.
• Laser light and ordinary light. A bulb gives many colours, out of step, in all directions; a laser gives one colour, in step, in a narrow beam, which is why it can be focused to a tiny spot.
• X-rays and gamma rays. They overlap in energy and are named by origin: X-rays come from electrons, while gamma rays come from the nucleus or from matter meeting antimatter.

For the exam

• Keyword bank: wavelength and frequency, photon energy (E = hf), ionising radiation, atmospheric window, population inversion, stimulated emission, coherent light, Doppler shift, synthetic aperture radar, LiDAR, MRI, spectrum as a natural resource, administrative allocation, ICNIRP limits, ALARA.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: India limits radio frequency exposure from mobile towers to one tenth of the international ICNIRP guideline, a rule in force since September 2012 (Department of Telecommunications).
  • Introduction with a source: In the 2G case of 2012 the Supreme Court described spectrum as a scarce natural resource that the state holds in trust for the people and should allot by a fair and transparent process.
  • Conclusion: From the radar that tracks a cyclone to the laser in an optical fibre and the X-ray in a district hospital, the spectrum is shared infrastructure, and its value depends on fair allocation, safe use and Indian made sensors.
  • Conclusion: The line between ionising and non ionising radiation is the key to both public health policy and public trust, so regulation should follow that physics rather than fear.