Radiation

Radiation is the emission or transmission of energy through space or a material medium. This energy can travel in the form of waves or as physical particles. In physics, radiation is a broad category that includes many different phenomena. For example, acoustic radiation includes sound and seismic waves. Gravitational radiation consists of ripples in spacetime. Most people encounter electromagnetic radiation every day. This includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma radiation.

Scientists categorize radiation based on its energy levels. The most important distinction is between ionizing and non-ionizing radiation. Ionizing radiation carries more than 10 electron volts (eV) of energy. This energy is high enough to ionize atoms and molecules. Ionization is the process where an electron is stripped or knocked out of an atom's electron shell. This leaves the atom with a net positive charge and can break chemical bonds. Non-ionizing radiation has lower energy. It cannot knock electrons away from atoms. However, it can still disrupt the bonds between atoms that form molecules. For instance, long-wavelength solar ultraviolet light can cause sunburn by breaking molecular bonds.

Particle radiation consists of subatomic particles accelerated to relativistic speeds by nuclear reactions. These particles have non-zero rest energy and often carry an electrical charge. Common types include alpha radiation, beta radiation, protons, and neutrons. Alpha particles are helium-4 nuclei, consisting of two protons and two neutrons. Beta radiation involves electrons or positrons. Because these particles have mass and charge, they interact strongly with matter. This interaction often allows them to knock electrons out of atoms. However, their electrical charge often limits how deeply they can penetrate materials compared to certain waves.

Electromagnetic radiation behaves differently because it consists of photons. Photons are particles of light that have no mass and no electric charge. This lack of mass allows some electromagnetic waves to penetrate much further through matter than particle radiation. Gamma radiation is a high-energy form of electromagnetic radiation. It is often emitted from an unstable nucleus to rid it of excess energy. Because gamma rays are photons, they can pass through many materials that would stop alpha or beta particles. X-rays are another high-energy electromagnetic wave. They have wavelengths shorter than about 10⁻⁹ meters. When an X-ray photon hits an atom, it may boost an electron to a higher orbital or knock it away entirely.

Radiation is a natural part of our environment. Most rocks and soil contain small concentrations of radioactive materials. We also receive cosmic rays from space. These are produced when primary cosmic rays interact with Earth's atmosphere, creating secondary particles like muons, mesons, and positrons. The Earth's atmosphere acts as a shield. It absorbs all X-rays and gamma rays from the sun. The ozone layer also absorbs about 98% of dangerous non-ionizing UV-C and UV-B radiation. Some ionizing ultraviolet light, known as vacuum ultraviolet, is absorbed by air and ozone before it reaches the ground.

Because radiation is invisible, humans cannot detect it with their senses. We must use instruments like Geiger counters to find it. In some cases, radiation can create visible light through processes called Cherenkov radiation or radio-luminescence. We can also see the paths of particles in a cloud chamber.

Understanding how radiation spreads is also vital. The intensity of radiation from a point source follows an inverse-square law. This means that as radiation expands through space, its intensity decreases in relation to the square of the distance from the source.

While radiation has many uses in medicine and research, it poses significant biological risks. Ionizing radiation can damage living cells and DNA. This increases the risk of cancer. High doses can lead to Acute Radiation Syndrome (ARS), which causes skin burns, hair loss, and organ failure. Calculating these risks is difficult. Scientists use population data from events like the atomic bombings of Hiroshima and Nagasaki or the Chernobyl disaster to make estimates. The probability of harm depends on the absorbed dose, the type of radiation, and the sensitivity of the specific tissue being irradiated.