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Electromagnetic spectrum
The continuous range of all electromagnetic wavelengths and frequencies, divided into regions such as radio, microwaves, infrared, visible, ultraviolet, X‑rays and gamma rays.
Electromagnetic Wave
A transverse wave made of oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel.
Ionizing radiation
Ionizing radiation is radiation with enough energy to remove electrons from atoms.
Ionosphere
A region of Earth’s upper atmosphere containing charged particles that can reflect some radio waves, extending communication range.
Ultrasound
Sound with frequency above the upper limit of human hearing (about $20\ \text{kHz}$).
Vacuum
A region with very low gas particle density and pressure, but not necessarily zero particles.