 The electromagnetic spectrum is the range of frequencies, the spectrum of electromagnetic radiation and their respective waves lengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below 1 Hz to above 1025 Hz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus. This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names, beginning at the low frequency long wavelength end of the spectrum. These are radio waves, microwaves, terahertz waves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high frequency. Short wavelength end. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the plank length. Gamma rays, X-rays, and high ultraviolet are classified as ionizing radiation as their photons have enough energy to ionize atoms, causing chemical reactions. Exposure to these rays can be a health hazard, causing radiation sickness, DNA damage and cancer. Radiation of visible light wavelengths and lower are called non-ionizing radiation as they cannot cause these effects. In most of the frequency bands above, the technique called spectroscopy can be used to physically separate waves of different frequencies, producing the spectrum showing the constituent frequencies. Spectroscopy is used to study the interactions of electromagnetic waves with matter. Other technological uses are described under electromagnetic radiation.