Thermal radiation

• 1. Ancient Observations Of Heat

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  Burning glasses appear in historical records dating back to about 700 BC. One of the first accurate mentions of these devices appears in Aristophanes's comedy The Clouds, written in 423 BC. Ancient Greeks understood that mirrors could concentrate heat rays to start fires or damage objects. Archimedes is purported to have developed mirrors to burn attacking Roman ships during the Siege of Syracuse around 213, 212 BC. No sources from that specific time confirm this story however. Catoptrics is a book attributed to Euclid on how to focus light to produce heat. This text might have been written as late as 300 AD instead of earlier periods.

  During the Renaissance, Santorio Santorio created one of the earliest thermoscopes. He published his results on heating effects from the Sun and attempts to measure heat from the Moon in 1612. Giambattista della Porta reported feeling heat on his face emitted by a remote candle facilitated by a concave metallic mirror in 1589. Della Porta also reported cooling felt from a solid ice block. His experiment was replicated many times with increasing accuracy. Astronomers Giovanni Antonio Magini and Christopher Heydon replicated it in 1603. Rudolf II, Holy Roman Emperor performed the experiment himself in 1611. The Accademia del Cimento updated della Porta's experiment in 1660 using a thermometer invented by Ferdinand II, Grand Duke of Tuscany.

• 2. Caloric Fluid And Early Debates

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  Antoine Lavoisier considered radiation of heat concerned the condition of the surface rather than the material itself. He described a poor radiator as having molecules lying in a plane closely bound together creating a surface layer of caloric fluid. This fluid insulated the release of the rest within the substance. A good radiator possessed a rough surface where only a small proportion of molecules held caloric within a given plane. Count Rumford later cited this explanation as insufficient to explain the radiation of cold. This became a point of contention for the theory as a whole.

  Augustin-Jean Fresnel responded to a view he extracted from a French translation of Isaac Newton's Optics in his first memoir. He refuted the idea that particles of light traversed space uninhibited by the caloric medium filling it. Fresnel argued that a body under illumination would increase indefinitely in heat if such a view were true. Marc-Auguste Pictet conducted a famous experiment in 1790 reporting that a thermometer detected lower temperature when mirrors focused frigoric rays from a cold object. Pierre Prevost introduced the concept of radiative equilibrium in 1791 stating all objects both radiate and absorb heat simultaneously.

• 3. Electromagnetic Waves And Quantum Shifts

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  At the end of the 19th century scientists showed transmission of light or radiant heat allowed propagation of electromagnetic waves. Television and radio broadcasting waves are types of electromagnetic waves with specific wavelengths. All electromagnetic waves travel at the same speed so shorter wavelengths associate with high frequencies. Bodies generate and receive electromagnetic waves at the expense of heat exchange. Gustav Kirchhoff published a mathematical description of thermal equilibrium known as Kirchhoff's law of thermal radiation in 1860.

  By 1884 Josef Stefan inferred emissive power of a perfect blackbody using John Tyndall's experimental measurements. Ludwig Boltzmann derived this relation from fundamental statistical principles creating what is now called the Stefan-Boltzmann law. Max Planck offered the microscopic theory of radiation first in 1900 calling it quantum theory. He noted energy emitted by a radiator not continuous but in form of quanta. The energy E of an electromagnetic wave in vacuum found by expression E equals hf where h is Planck constant and f its frequency. Bodies at higher temperatures emit radiation at higher frequencies with increasing energy per quantum.

• 4. Blackbody Laws And Mathematical Models

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  The spectral intensity of a blackbody determined by Max Planck given by Planck's law per unit wavelength. This formula mathematically follows calculation of spectral distribution of energy in quantized electromagnetic field complete thermal equilibrium with radiating object. Planck's law shows radiative energy increases with temperature explaining why peak emission spectrum shifts to shorter wavelengths at higher temperatures. Energy emitted at shorter wavelengths increases more rapidly with temperature relative to longer wavelengths.

  Total emissive power of a blackbody calculated via Stefan-Boltzmann law yielding remarkably elegant equation involving Steffan-Boltzmann constant. Wien's displacement law gives wavelength for which emission intensity highest as inversely proportional to absolute temperature T. Photosphere of sun at approximately 6000 K emits radiation principally human-visible portion electromagnetic spectrum. Earth surface approximates behavior of black body at 300 K with spectral peak at fmax. At lower frequencies atmosphere largely opaque so radiation from Earth surface absorbed or scattered by atmospheric gases. About 10% escapes into space while most re-emitted by atmospheric gases creating planetary greenhouse effect.

• 5. Solar Power And Engineering Uses

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  Sunlight represents incandescence white hot surface Sun with peak wavelength about 550 nm harvestable generate heat electricity. Thermal radiation concentrates on tiny spot via reflecting mirrors allowing concentrating solar power systems to function. Fresnel lenses also used concentrate radiant energy either method quickly vaporize water steam. Sunlight reflected from mirrors heats PS10 Solar Power Plant heating water to high temperatures during day. Selective surfaces tuned maximize amount energy absorb sun's radiation minimize amount lose own thermal radiation.

  Incandescent light bulb creates light heating filament temperature emitting significant visible thermal radiation. Tungsten filament typical temperature 3000 K small fraction emitted radiation visible majority infrared light. Infrared light does not help person see transfers heat environment making incandescent lights relatively inefficient source. No materials able withstand higher temperatures appropriate use lamps currently exist. More efficient sources fluorescent lamps LEDs do not function incandescence. Low-emissivity windows houses technology complicated requiring low emissivity thermal wavelengths transparent visible light.

• 6. Human Comfort And Safety Protocols

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  Humans lose considerable energy due to infrared thermal radiation additional conduction air aided concurrent convection drafts. Human skin emissivity very close 1.0 continuous radiates approximately 1000 W having roughly 2m2 surface area temperature about 307 K. Indoors surrounded surfaces 296 K receive back about 900 W wall ceiling surroundings resulting net loss 100 W. Estimates highly dependent extrinsic variables wearing clothes. Lighter colors whites metallic substances absorb less illuminating light heat up less color makes little difference heat transfer everyday temperatures surroundings.

  Thermal radiation phenomenon burns skin ignites flammable materials time damage exposure function rate delivery heat. Radiative heat flux effects given specific values kilowatts per square meter. Maximum flux measured post-flashover compartment reaches 170 kW/m2. Thermal Protective Performance test personal protective equipment requires 80 kW/m2. Fiberboard ignites five seconds at 52 kW/m2. Wood ignites given time 29 kW/m2. Typical beginning flashover floor level residential room occurs 20 kW/m2. Human skin sudden pain second-degree burn blisters after 5 seconds at 16 kW/m2.

• 7. Spacecraft Navigation And Nanostructures

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  Shiny metal surfaces low emissivities visible wavelengths far infrared reduce heat transfer directions example multi-layer insulation used insulate spacecraft. Any electromagnetic radiation including thermal radiation conveys momentum energy inducing very small forces radiating absorbing objects. Normally forces negligible must taken account considering spacecraft navigation Pioneer anomaly motion craft slightly deviated expected gravity eventually tracked asymmetric thermal radiation spacecraft. Orbits asteroids perturbed since asteroid absorbs solar radiation side facing Sun re-emits energy different angle rotation carries warm surface Sun's view YORP effect.

  Nanostructures spectrally selective thermal emittance properties offer numerous technological applications energy generation efficiency daytime radiative cooling photovoltaic cells buildings. Applications require high emittance frequency range corresponding atmospheric transparency window 8 to 13 micron wavelength range. Selective emitter radiating strongly range exposed clear sky enabling use outer space very low temperature heat sink. Near-field radiative heat transfer Planck law accurate distances scale wavelength radiated electromangetic wave smaller quantum tunneling EM waves significant impact rate radiation.