Glossary of Key Terms

Term	Definition	Formula/Value
Electromagnetic wave	Self-sustaining transverse oscillation of coupled E and B fields, propagating at speed c	c = $3 \times 10^{8}$ m/s
Displacement current	Fictitious current arising from a time-varying electric flux; not actual charge flow	Id = ε_{0}(dΦ_E/dt); unit: Ampere
Electric flux (Φ_E)	Product of electric field and area through which it passes (for uniform E ⊥ surface)	Φ_E = EA; unit: V·m = N· $m^{2}$ /C
Permittivity of free space (ε_{0})	Measure of how well vacuum supports an electric field	ε_{0} = $8.85 \times 10^{-12}$ $C^{2}$ /(N· $m^{2}$ )
Permeability of free space (μ_{0})	Measure of how well vacuum supports a magnetic field	μ_{0} = 4π×10^{-7} T·m/A = H/m
Intensity (I)	Power per unit area of EM wave	I = (1/2)ε_{0}c $E_{0}^{2}$ ; unit: W/ $m^{2}$
Radiation pressure	Force per unit area exerted by EM radiation on a surface	P = I/c (absorption); 2I/c (reflection)
Transverse wave	Wave where oscillation direction is perpendicular to propagation direction	EM waves: E ⊥ B ⊥ propagation
In phase	Two oscillations that reach peaks and zeros at the same instant	E and B in an EM wave are in phase
Dispersion	Phenomenon where waves of different frequencies travel at different speeds in a medium	Occurs in glass, water; not in vacuum
Bremsstrahlung	X-rays produced by rapid deceleration of electrons	Source of X-rays in Coolidge tube
Gamma rays	Highest-energy EM radiation from nuclear transitions	f: 10^{18}–10^{24} Hz; λ: 10^{-16}–10^{-10} m
VIBGYOR	Visible spectrum colours from highest to lowest frequency	Violet (400 nm) → Red (700 nm)
Magnetron/Klystron	Vacuum tube devices that produce microwaves	Source of microwave ovens and RADAR
Radiation momentum	Momentum carried by EM waves	p = U/c (abs.); 2U/c (refl.)