Maarg Manthan · Topic 8.8

Electromagnetic Wave Behaviour

Science & Technology › Physics – Mechanics, Waves and Thermodynamics · Topic 8.8

Electromagnetic waves behave in several ways when they meet matter or travel through space. They can be reflected from surfaces, refracted when they enter another medium, diffracted around obstacles, absorbed, scattered and polarised, and they can interfere with each other. Because they carry no charge and need no medium, they travel in a vacuum at the speed of light, about 3 lakh kilometres a second, and slow down in matter. Radio waves of different frequencies travel differently: ground waves follow the curve of the Earth, sky waves are reflected by the ionosphere, and space waves travel in a straight line between antennas and satellites. Electromagnetic waves also show particle behaviour, as photons, in the photoelectric effect, which Albert Einstein explained in 1905.

5 min read · UPSC Prelims · Science & Technology · Physics – Mechanics, Waves and Thermodynamics

At a Glance

Speed in vacuum About 3 x 10 to the power 8 m/s
Sky wave Reflected by the ionosphere
Space wave Line of sight
Photoelectric effect Einstein, 1905
Radar Uses reflected radio waves
Exam link UPSC Prelims, Science & Technology: Physics – Mechanics, Waves and Thermodynamics

Where Does It Fit?

This is the eighth topic of the macro on physics. After the spectrum and electromagnetism, it explains how the waves behave in propagation and why they matter for communication. Previous topic: Mechanical Wave Phenomena. Next topic: Superconductivity: History, Classification and Properties.

How Do Electromagnetic Waves Behave?

Behaviour	Explanation and example
Reflection	Waves bounce off surfaces. Radar relies on reflection of radio waves from aircraft, and metal surfaces reflect radio and microwaves strongly
Refraction	Waves bend when they change speed in a new medium, as radio waves bend slightly in the atmosphere and light bends in glass, as in Light and Optics
Diffraction	Bending around obstacles. Radio waves of long wavelength bend around hills and buildings, while short wavelength waves do not, so FM signals are blocked more easily
Interference	Overlap of waves that strengthen or cancel each other, which causes fading in signals and gives patterns in light
Absorption	Matter takes up wave energy. Water absorbs microwaves, which heats food, and the ozone layer absorbs ultraviolet
Scattering	Waves spread in many directions by particles, as in the blue sky and rain scattering radar signals
Polarisation	Waves vibrating in one plane. Shown by all transverse waves, including light and radio

How Do Radio Waves Travel for Communication?

Ground wave: follows the surface of the Earth, used by medium-wave AM radio for local reach.
Sky wave: short waves are reflected by charged layers of the upper atmosphere called the ionosphere, which lets them travel round the world and is used in shortwave radio.
Space wave: VHF, UHF and microwaves travel in straight lines, so television and mobile towers need line of sight, and the range is limited by the curve of the Earth. Tall towers increase it.
Satellite communication: uplinks and downlinks pass through the atmosphere to satellites in geostationary orbit, as in Space Technology and ISRO.
Radar: sends out a pulse of radio waves and measures the time and shift in frequency of the echo to find the distance and speed of an object, as used for aircraft, ships and weather.
Optical fibre and mobile networks: carry data on light and on microwaves, as in IT, Telecommunications and Computers.

What Else Should You Remember?

Photoelectric effect: light of sufficient frequency knocks electrons out of a metal, and the energy of each electron depends on frequency and not on the brightness, which showed the particle nature of light. Einstein received the Nobel Prize in 1921 for it.
Wave-particle duality: light and all matter show both wave and particle behaviour, a central idea of quantum mechanics, as in Theories and Laws of Physics, Including the Theory of Relativity.
Doppler effect for light: light from receding objects is shifted to red, which shows the expansion of the universe, as in Big Bang Theory and Cosmological Singularity.
Electromagnetic interference: unwanted signals disturb electronics, so aircraft ask for phones to be switched to flight mode and devices are shielded.
Faraday cage: a metal enclosure that blocks electromagnetic fields, as in the working of a lift or a car in lightning.
Mobile tower radiation: radio emissions are non-ionising, and India’s limits for exposure are stricter than international guidance, according to the Department of Telecommunications rules of 2012.
Remote sensing: satellites measure reflected and emitted waves from the ground in different bands to study crops, water and cities.
Stealth technology: aircraft are shaped and coated to scatter and absorb radar waves.
5G: uses higher frequency bands, including millimetre waves, with shorter range and more need for small cells.
Electromagnetic wave behaviour in daily life: electromagnetic wave behaviour explains why a mobile signal is weak in a lift, why a radio works near a window and why a microwave oven has a metal mesh door.

Why Does It Matter for the Exam?

Propagation questions: which wave type is used for which kind of communication.
Concept questions: photoelectric effect and wave-particle duality.
Technology link: radar, satellites, 5G and stealth.
Health link: mobile tower radiation and safe limits.
Common errors: saying that radio waves are sound waves or that they cannot pass through a vacuum.

As an analytical point, the way waves interact with air, water, metal and the ionosphere decides the design of every communication system. Higher frequencies carry more data but travel shorter distances.

Why Is This Topic Important?

Facts Propagation modes asked
Concept Photoelectric effect
Link Radar and 5G

What Came Next?

The next topic covers superconductivity: its history, classification and properties. Next topic: Superconductivity: History, Classification and Properties.

Key People and Terms

Ionosphere Charged upper layer of the atmosphere
Line of sight Straight path between antennas
Radar Radio detection and ranging
Photon Particle of light
Polarisation Vibration in one plane
Diffraction Bending around obstacles

Exam Corner

Points to Remember

Sky wave reflected by the ionosphere.
Space wave needs line of sight.
Photoelectric effect: Einstein, 1905; Nobel 1921.
Radar uses reflected radio waves.
Short wavelengths diffract less.

Do Not Confuse With

Ground wave and sky wave Follows surface versus reflected by ionosphere
Reflection and refraction Bounce versus bend
Diffraction and scattering Bending around edge versus spreading by particles

Memory Hook

Ground wave hugs the Earth, sky wave bounces off the ionosphere, space wave sees the sky; radar echoes, photons knock electrons out.

Mains Angle

Questions link wave behaviour with communication. These points add depth.

Behaviour Reflection to polarisation
Propagation Three modes
Uses Radar, satellites

A Question You May Face

An original practice question, not a past paper question.

“Explain the modes of propagation of radio waves and the behaviour of electromagnetic waves that make modern communication possible.”

How to Answer

Behaviour Reflect, refract.
Modes Ground, sky, space.
Radar Echo.
Duality Photon.

GS Relevance

Prelims: propagation modes, ionosphere, photoelectric effect, radar, mobile signals. GS Paper 3: science and technology, communication.

Frequently Asked Questions

How do radio waves travel round the world?

Short radio waves travel round the world as sky waves, which are reflected back to the Earth by the ionosphere, a layer of charged particles in the upper atmosphere. Medium waves follow the surface as ground waves for local reach, while very high frequency waves and microwaves travel in straight lines and need line of sight.

What is the photoelectric effect?

The photoelectric effect is the emission of electrons from a metal surface when light of high enough frequency falls on it. The energy of the emitted electrons depends on the frequency of the light and not its brightness, which Einstein explained in 1905 using photons, and it won him the Nobel Prize in Physics in 1921.

How does radar work?

Radar sends out pulses of radio waves and listens for the echo reflected from an object. The time delay gives the distance, and the shift in frequency, by the Doppler effect, gives the speed of the object, so it is used for aircraft, ships, weather and speed checks.

Why do mobile towers need line of sight?

Mobile networks use microwave frequencies, which travel in straight lines and are blocked or weakened by buildings and hills, since their short wavelengths diffract little. Towers are therefore placed high and close together, and higher frequency 5G needs even more closely placed small cells.

Are the waves from mobile phones harmful?

Mobile phones and towers use non-ionising radio and microwave radiation, which does not have enough energy to damage DNA, and its main effect is slight heating. Regulators set exposure limits, and India's limits for towers are stricter than the international guidelines, though research on long-term effects continues.

PYQ Practice — Statement Analysis

1 Sky waves are reflected by the ionosphere.

True

That allows long-distance shortwave radio.

2 Electromagnetic waves require a medium to travel.

False

They travel through a vacuum.

3 The photoelectric effect supports the particle nature of light.