Electromagnetic waves and communication systems
Accelerating charges generate changing electric and magnetic fields that propagate through space, carrying energy and information without requiring a material medium.
Wave structure and spectrum
In vacuum, electric field E and magnetic field B oscillate perpendicular to each other and to the propagation direction. Their speed is c=1/√(μ0ε0), and c=fλ. Frequency is set by the source and remains unchanged across a boundary; wavelength changes when wave speed changes in matter. The spectrum is one continuous family from radio through microwave, infrared, visible, ultraviolet, X-ray and gamma radiation. Photon energy hf increases with frequency.
Antennas and propagation
An alternating current drives charges in a transmitting antenna and radiates a wave. A receiving antenna converts part of the arriving field into a voltage. Efficient antenna dimensions are commonly related to wavelength, such as a half-wave dipole. In free space, power spreads over increasing area, so ideal point-source intensity follows I=P/(4πr²). Reflection, diffraction, absorption, polarization mismatch and obstacles modify a real link.