Electromagnetic spectrum

• 1. Introduction

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  The electromagnetic spectrum stretches from radio waves whose wavelengths run thousands of kilometers to gamma rays smaller than the nucleus of an atom. It is a single continuous range of radiation, all of it traveling at the speed of light, yet split into bands with names as different as their behavior: radio, microwave, infrared, visible light, ultraviolet, X-rays, and gamma rays. The waves in each band are produced differently, interact with matter differently, and serve us in entirely different ways. Some pass through your body. Some cook your dinner. Some can rip electrons clean out of atoms. The story of how this range was mapped runs through prisms, thermometers, evacuated tubes, and a coincidence in a number that startled the physicist who found it. Why does one family of waves include both the signal in your phone and the radiation that treats cancer? What makes the boundary between an X-ray and a gamma ray a question of where it came from rather than what it is? And how did scientists fill in a spectrum whose ends nobody could see?

• 2. A Spectrum Revealed

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  In 1672, Isaac Newton submitted his first paper to the Royal Society, describing how white light splits into a range of colours through a prism, and he gave that range a name: spectrum. Newton showed the colours were intrinsic to light and could be recombined to make white light again. Around 1801, Thomas Young ran a series of interference experiments that helped win acceptance for the wave theory of light. The edges of Newton's rainbow turned out to be just the beginning.

  William Herschel, in 1800, moved a thermometer through light split by a prism to measure the temperature of different colours. He found the highest temperature lay beyond red, where no colour appeared at all. Herschel proposed that invisible "calorific rays" caused the change, and these became known as infrared radiation. The next year, Johann Ritter worked the opposite end of the spectrum and noticed "chemical rays" that triggered certain chemical reactions. They behaved like violet light but sat beyond it, and were later renamed ultraviolet.

  Wilhelm Röntgen, in 1895, saw a new radiation during an experiment with an evacuated tube under high voltage. He named it x-rays, and found it could pass through parts of the human body while denser matter such as bone reflected or stopped it. Uses for this radiography appeared quickly. Five years later, Paul Villard studied the radioactive emissions of radium and found a radiation far more penetrating than the known alpha and beta particles, the last great gap in the spectrum waiting to be named.

• 3. The Number That Startled Maxwell

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  The study of electromagnetism began in 1820, when Hans Christian Ørsted discovered that electric currents produce magnetic fields, a result known as Oersted's law. Light was first tied to electromagnetism in 1845, when Michael Faraday noticed that the polarization of light moving through a transparent material responded to a magnetic field, the effect that now bears his name. The two threads were drawing together.

  During the 1860s, James Clerk Maxwell wrote four partial differential equations for the electromagnetic field. Two of them predicted that waves could exist in that field. When Maxwell analyzed how fast those theoretical waves must travel, he found a speed close to the known speed of light. This startling coincidence led him to infer that light itself is a type of electromagnetic wave. His equations predicted an infinite range of frequencies, all moving at light speed, the first hint that an entire spectrum existed.

  Maxwell had hypothesized waves at very low frequencies, perhaps made by oscillating charges. In 1886, Heinrich Hertz built an apparatus to generate and detect what we now call radio waves. By measuring their wavelength and multiplying by frequency, Hertz inferred they traveled at the speed of light. He showed they could be reflected and refracted like light, even focusing them with a lens made of tree resin, and later produced microwaves the same way, opening the door to the wireless telegraph and the radio.

• 4. Wave Or Particle

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  In 1903, Ernest Rutherford named gamma rays, recognizing they were fundamentally different from charged alpha and beta particles. In 1910, William Henry Bragg demonstrated that gamma rays are electromagnetic radiation rather than particles. Four years later, Rutherford and Edward Andrade measured their wavelengths and found gamma rays resembled X-rays but with shorter wavelengths, settling their place at the high-energy edge of the spectrum.

  Max Planck, in 1901, discovered that light is absorbed only in discrete "quanta," now called photons, which implied light has a particle nature. Albert Einstein made this explicit in 1905, though Planck himself and many contemporaries never accepted it. The modern position is that electromagnetic radiation has both a wave and a particle nature, the wave-particle duality. The contradictions it raises are still debated by scientists and philosophers today.

• 5. Measuring The Range

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  Frequencies observed in astronomy reach as low as the local plasma frequency of the ionized interstellar medium, around 1 kHz, while radiation at the opposite end can be indefinitely long in wavelength. Photon energy is directly proportional to frequency, so gamma ray photons carry around a billion electron volts while radio wave photons hold roughly a femtoelectronvolt. Three properties describe any electromagnetic wave: frequency, wavelength, and photon energy, linked through the speed of light and the Planck constant.

  Spectroscopy separates waves of different frequencies so radiation intensity can be measured against frequency or wavelength, and it is used to study how electromagnetic waves interact with matter. A common laboratory spectroscope reaches far past the visible range of 400 nm to 700 nm, detecting wavelengths from 2 nm to 2500 nm. From such devices, detailed information about objects, gases, and even stars can be drawn.

  Many hydrogen atoms emit a radio wave photon with a wavelength of 21.12 cm, a signal widely used in astrophysics. Frequencies of 30 Hz and below matter in the study of certain stellar nebulae. The bands themselves have no sharp boundaries; they fade into each other like the colours of a rainbow, so red light resembles infrared and must excite chemical bonds to power photosynthesis and human vision.

• 6. Why The Names Differ

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  Electromagnetic radiation interacts with matter so differently across the spectrum that history applied separate names, as though these were separate kinds of radiation. In the radio region, the main interaction is the collective oscillation of charge carriers in bulk material, the kind of motion electrons make in an antenna. Through microwave and far infrared, molecular rotation joins in, and in the visible range, electrons in molecules jump between energy levels, including the pigment molecules in the human retina.

  Ultraviolet excites and ejects valence electrons through the photoelectric effect, while X-rays reach the deep core electrons of an atom and add Compton scattering for low atomic numbers. Gamma rays energetically eject core electrons in heavy elements and can excite or even break apart atomic nuclei. At the highest energies, a single gamma ray photon can create particle-antiparticle pairs, and a shower of high-energy particles and antiparticles upon striking matter.

  The line between an X-ray and a gamma ray is drawn by source, not by energy. Photons from nuclear decay or other nuclear processes are gamma rays, while X-rays come from electronic transitions involving deep inner atomic electrons. In astrophysics a simpler rule applies: energies below 100 keV are X-rays, and higher energies are gamma rays. Where radiation is observed can even differ from where it was emitted, through the Doppler shift, gravitational redshift, or the expansion of the universe.

• 7. Waves That Carry Words

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  Radio waves are emitted and received by antennas, conductors such as metal rod resonators. A transmitter generates an alternating current applied to an antenna, where oscillating electrons radiate electric and magnetic fields away as radio waves. Earth's atmosphere is mainly transparent to them, except for charged layers in the ionosphere that reflect certain frequencies. To carry information, a radio frequency current is modulated by varying its amplitude, frequency, or phase, then recovered by demodulation at the receiver. These waves underpin radio broadcasting, television, mobile phones, communication satellites, wireless networking, GPS, radar, and remote control. Their use is strictly regulated by governments and coordinated by the International Telecommunication Union, which allocates frequencies among users.

  Microwaves run from about 10 centimeters to one millimeter, in the SHF and EHF bands, produced by klystron and magnetron tubes and by solid state devices such as Gunn and IMPATT diodes. Unlike infrared and visible light, which are absorbed mainly at surfaces, microwaves penetrate materials and deposit energy below the surface, the effect that heats food in microwave ovens and powers medical diathermy. Copper cables lose too much power at these frequencies, so metal pipes called waveguides carry them instead.

  Terahertz radiation, also called sub-millimeter radiation or T-rays, fills the region from about 100 GHz to 30 terahertz between microwaves and far infrared. Long left unstudied for lack of sources, this terahertz gap now sees applications in imaging and communications. It is strongly absorbed by atmospheric gases, which makes it useless for long-distance communication but keeps scientists looking toward uses such as disabling enemy electronic equipment.

• 8. Light And Beyond

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  The sun emits its peak power in the visible region, though integrated across all wavelengths it gives off slightly more infrared than visible light. Visible light is the part of the spectrum the human eye is most sensitive to, detected between 380 nm and 760 nm, and it is absorbed and emitted by electrons in atoms and molecules shifting between energy levels. That same action drives human vision and plant photosynthesis. Passing white light through a prism splits it into the colours seen in the visible spectrum, and infrared sits just beyond the red edge of a rainbow while ultraviolet lies past the violet.

  Ultraviolet ranges from 399 nm to 10 nm, divided into UVA, UVB, and UVC, and it is the lowest energy range able to ionize atoms. Middle range UV cannot ionize but breaks chemical bonds; it causes sunburn and is the main cause of skin cancer, and at shorter wavelengths it produces thymine dimers that make it a potent mutagen. The sun emits about 10% of its power as UV, but the ozone layer absorbs strongly in the 200-315 nm range, leaving less than 3% of sunlight at sea level in UV, mostly the gentler UV-A.

  X-rays come next, ionizing and able to interact through the Compton effect, with hard X-rays passing through substances thinner than a few meters of water. They serve diagnostic imaging in medicine and probe high-energy physics, and around neutron stars and black holes their accretion disks emit X-rays. After hard X-rays come gamma rays, the most energetic photons, with no defined lower limit to their wavelength. They sterilize foods and seeds, treat cancer, and drive nuclear-medicine imaging such as PET scans. To see astronomical X-rays or gamma rays, telescopes must rise above an atmosphere whose depth, equal to about 10 meters of water, blocks almost all of them from reaching the ground.