The study of the properties of visible light, i.e., the portion of the electromagnetic spectrum with wavelengths between 360 and 780 nm (1 nm = m/s).
When an object is held in circular motion about a massive body, like a planet or a sun, due to the force of gravity, that object is said to be in orbit. Objects in orbit are in perpetual free fall, and so are therefore weightless.
A back-and-forth movement about an equilibrium position. Springs, pendulums, and other oscillators experience harmonic motion.