EM Waves - Quick Review Notes

Electromagnetic (EM) waves are transverse oscillations of electric (E) and magnetic (B) fields that carry energy through space.
Travel in vacuum at $c = 3 \times 10^{8} \ \mathrm{m/s}$ .
Do not require a medium to propagate; can travel in empty space.
Also called Hertzian waves (historical name).

EM waves arise from changing electric and magnetic fields; disturbances produced by accelerated charges.
Key characteristics:
- Travel through empty space at the speed of light $c$ (approx $3 \times 10^{8} \ \mathrm{m/s}$ ).
- Transverse in nature.
- They have electric and magnetic field components.
The changing E field induces a changing B field, and vice versa.

Demonstrated that EM waves can be produced and detected at a distance.
EM waves can be focused, reflected, and refracted like light.
Light itself is a form of EM wave; the speed equals the speed of light as predicted by Maxwell.

Frequency $f$ : number of waves passing a fixed point per unit time; unit: Hz.
Wavelength $\lambda$ : distance between identical points (e.g., adjacent crests).
Amplitude: maximum displacement of particles in the medium or field strength.
Energy carried by a wave increases with higher frequency and higher amplitude; longer wavelength carries less energy.

In vacuum: $c = f \lambda$ ; equivalently, $\lambda = \dfrac{c}{f}$ .
Example: For frequency $f = 4 \times 10^{14} \ \mathrm{Hz}$ ,
$\lambda = \dfrac{3 \times 10^{8}}{4 \times 10^{14}} = 7.5 \times 10^{-7} \ \mathrm{m}$ .

EM spectrum from long to short wavelength: Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma.
Visible light wavelengths: approximately $0.38 \mu\mathrm{m} \le \lambda \le 0.78 \mu\mathrm{m}$ ; colors: ROYGBIV (shortest to longest: Violet to Red).
Rough solar distribution (distribution varies): Infrared ≈ 51%, Visible ≈ 47%, Ultraviolet ≈ 2% of total solar energy.
Ultraviolet bands: UVA, UVB, UVC; UVC largely absorbed by the ozone layer.

Radio waves: longest wavelength, lowest frequency; wavelength range roughly $1 \text{ cm} \text{ to } 1 \text{ km}$ ; frequency range roughly $30 \text{ kHz to } 300 \text{ GHz}$ .
Key bands and applications vary by frequency (ELF, VLF, LF, MF, HF, VHF, UHF, etc.).

Ground waves: low-frequency propagation along the Earth's surface.
Sky waves: medium/short waves reflected by the ionosphere.
Space waves: very high and ultra-high frequencies; direct line-of-sight propagation.

Transmitting system: microphone, oscillator, antenna, amplifiers, etc.; convert sound to electrical signals and then to EM waves.
Receiving system: antenna, demodulator, amplifiers, speaker; convert RF/AF signals back to sound.

Shorter wavelengths than IR; can penetrate the ionosphere.
Uses: satellite communications, radar (R&D), mobile networks, Wi-Fi, cooking (heating of water molecules).
Discovered conceptually via EM theory largely attributed to Maxwell.

Located between microwaves and visible light; subdivided into near, mid, and far IR; generally invisible but felt as heat.
Absorbed and re-emitted by objects; a large portion of the sun’s energy is emitted as IR.
Common applications: heat detection (heat leaks), remote controls, night-vision, infrared photography, heating appliances.

Only EM radiation visible to the unaided eye; also called light.
Sun and flames are common sources; white light is a combination of all visible wavelengths.
Applications: vision, photosynthesis, photography, spectral analysis.
When white light passes through a prism, it splits into constituent colors: ROYGBIV; boundaries between colors are continuous.

Wavelengths shorter than visible violet; UVA, UVB, UVC bands.
UVC largely absorbed by the ozone layer; UVA and UVB reach Earth's surface and can affect skin.
Natural source: mainly the Sun.
Uses: Vitamin D production (UVB), water sterilization, forensic applications, counterfeit detection, and fluorescent materials (UVA).
Health effects: excessive exposure can cause skin aging, DNA damage, skin cancer risk.

High-energy, ionizing radiation; energies often quantified in eV.
Uses: medical radiography, radiotherapy, astronomy; can penetrate soft tissues but are absorbed by dense materials like bone, making bones appear white on X-ray images.
Safety: ionizing radiation poses potential cancer risk; exposure levels are managed in medical imaging.

Highest energy and frequency; shortest wavelength.
Sources: extreme environments (black holes, neutron stars, supernovae) and radioactive decay (e.g., cobalt-60).
Uses: sterilization of medical instruments, cancer diagnosis and treatment, astronomy research.
Hazards: highly damaging to healthy cells; strong ionizing radiation.

Background radiation is present on Earth at all times; major sources include minerals in soil/rock, cosmic rays, air, water, food, and the human body.
Banana example: bananas contain potassium-40 and emit about $0.1 \ \mu\mathrm{Sv}$ per banana, a very small dose.
Natural radiation levels vary by location and over time; some components are from natural materials, others from cosmic rays and human activities.

Non-ionizing EM radiation: Radio, Microwave, Infrared, Visible, and (some) Ultraviolet.
Ionizing EM radiation (primarily X-rays and Gamma rays) has enough energy to remove electrons from atoms.
Nuclear radiation includes particles and EM radiation emitted from the nucleus; EM radiation spans the entire spectrum.
Overall, EM radiation interacts with matter through-energy transfer, while nuclear radiation involves nucleus-originated particles and photons.

Wave speed relation: $c = f \lambda$
Wavelength from frequency: $\lambda = \dfrac{c}{f}$
Example: if $f = 4 \times 10^{14} \, \mathrm{Hz}$ , then $\lambda = \dfrac{3 \times 10^{8}}{4 \times 10^{14}} = 7.5 \times 10^{-7} \, \mathrm{m}$ .