This document defines key terms related to electromagnetic radiation and waves, including wavelength, frequency, period, and speed of light. It describes the electromagnetic spectrum and visible light spectrum. Key points covered include: - Wavelength, frequency, and period are properties that characterize waves. - Electromagnetic radiation propagates through space as waves at the constant speed of light in a vacuum. - The electromagnetic spectrum orders all electromagnetic radiations from radio waves to gamma rays by their wavelength or frequency. - Visible light is a small portion of the electromagnetic spectrum that is visible to the human eye, ranging from red to violet light.