In physics, terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency, T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz; 10¹² Hz). Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 µm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy.