What is Infrared: Definition and 224 Discussions

Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from the nominal red edge of the visible spectrum around 700 nanometers (frequency 430 THz), to 1 millimeter (300 GHz) (although the longer IR wavelengths are often designated rather as terahertz radiation). Black-body radiation from objects near room temperature is almost all at infrared wavelengths. As a form of electromagnetic radiation, IR propagates energy and momentum, with properties corresponding to both those of a wave and of a particle, the photon.
Infrared radiation was discovered in 1800 by astronomer Sir William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer. Slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has a critical effect on Earth's climate.
Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range.Infrared radiation is used in industrial, scientific, military, commercial, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, to detect objects such as planets, and to view highly red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect the overheating of electrical components.Military and civilian applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm (micrometers). Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, spectroscopy, and weather forecasting.

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  1. J

    What happens to the IR radiation that the Greenhouse gases don't absorb?

    I have been searching for the answer yet – as I'm here – didn't find it. The current model of climate warming says the lower parts of Earth's atmosphere don't allow the IR radiation to escape freely to the outer space because the GH gas molecules keep absorbing it virtually as soon as it's been...
  2. G

    I How is IR ink able to be seen with specialized filters?

    I stumbled upon an old thread from 2014 about invisible IR ink and have been looking into the same thing very recently. https://www.physicsforums.com/threads/does-infrared-ink-exist.744888/ So far on my research I have found an IR ink that absorbs Infrared...
  3. V

    A How is it possible to measure an IR LED source in mW/sr?

    Hello everyone, I need to make measurements of a collimated infrared light source at different angles, practically to create an intensity map, all this in mW/sr. I can't find any equipment able to measure in mW/sr, while there are plenty of instruments capable to measure in mW/cm^2. As I...
  4. L

    I Does all infrared get detected as heat?

    There are different ranges of IR. Are they all represented as heat? If not maybe Webb telescope can read those only so our own sun isn't as much of an obstacle? Maybe xray instead?
  5. J3J33J333

    I Infrared Photovoltaics: Powering Ham Radios & Cooling Earth?

    Can you use infrared solar panels to power a ham radio and turn infrared into radio waves to get the energy past the green house gases? Because infrared gets reflected by greenhouse gases and warms the Earth while radio waves don't. Would this help cool the Earth?
  6. L

    B Infrared Cameras -- Do they use a Lens to Focus?

    Can infrared cameras focus IR radiation as a visible light camera focuses visible light? I think IR doesn't pass through glass well? What are these germanium lens about? I don't think my Flir IR camera has a lens.
  7. gehrenfeld

    B Exploring Infrared Light: Is It Visible to Our Eyes?

    I am 74yo and love Physics. If my basic knowledge of physics is correct, light is a photon. My question is: 1. Is the reason we cannot see infrared light because the spectrum travels at a wave link our eyes cannot see? 2. Does the light from the sun have all the different waves combined, and we...
  8. B

    InfraRed Detector Mystery (but not Elementary....)

    Hello! I found this circuit element below in a drawer marked "IR detectors" (which I must have written on it myself, a long time ago...although I must admit don't remember doing so #OldAge). First of all, I'm trying to figure out what, exactly, it is. My initial assumption was that it looks...
  9. D

    B Infrared Detectors & The 2nd Law of Thermodynamics

    This started as an astrophotography question and morphed into a thermodynamics question & it was suggested to start a new thread in Thermodynamics. Essentially the question is how do I reconcile the behavior of particular infrared photodetectors with the 2nd Law of Thermodynamics? For example...
  10. C

    I Do my parabolic mirrors need to be really shiny to reflect infrared?

    Summary:: Do my parabolic mirrors need to be really shiny to reflect infrared? I have 2 really old CENCO parabolic reflectors. I need to use them to demonstrate infrared light radiation by placing a heat source near one mirror, reflecting its heat to a second parabolic mirror and...
  11. L

    B Heat and Infrared radiated from an object

    Dumb question. Is there a direct heat energy relationship between a heated object and infrared radiation? 1 to 1 relationship? Or can IR go up without there being much heat?
  12. F

    Metals as Infrared Reflectors and their Temperature

    Hello everyone. I have reading about IR radiation, reflectivity, and emissivity of materials. Metals, in general, seem to be excellent reflectors or IR radiation (they are like infrared mirrors. That said, why do they get so hot when they are left in the sun? A metal roof cannot be touched...
  13. didymus_alpha

    Looking to learn how infrared absorbing (700-1000nm) pigment is made

    Hello, I have a kinda neat idea for using infrared absorbing (700-1000nm) pigment powder. There are definitely a number of places where it can be purchased, but it's quite expensive for me. https://maxmax.com/phosphorsdyesandinks/infrared-phosphors-dyes-and-inks/ir-absorbing-pigment I'm...
  14. Incnis Mrsi

    I Infrared energy flux from the Moon

    I am interested in energy flux density from the Moon at Earth’s surface, specifically in infrared (albeit most of it comes in infrared anyway). Physics.SX (boyfarrell) gives an estimate 89 mW/m². I doubt it for several reasons. 400 K (as effective mean) on Moon seems to be unrealistically hot...
  15. R

    Can infrared photons also eject electrons in 'photoelectric effect'?

    Hello, I have this question whether it will be possible to generate electricity by heating up a suitable metal (conductor). We know from the photoelectric effect that high energy photons (of visible light; probably with frequency closer to that of violet) can knock off the outer electrons from...
  16. rubenhak

    Adding Zooming Capability to my Far Infrared Spectrum Sensor

    Physics noob here. I have the cheapest FLIR sensor (80x60 px sensor). It captures Longwave infrared, 8 μm to 14 μm. It has a tiny lens (probably 1mm in diameter). Trying to add a zoom ability to this sensor. I have different DSLR lens and other optical scopes. I tried holding the tiny FLIR...
  17. Z

    Animals Beyond Visible Light & Infrared

    Do any animals see in a spectrum other than visible light and infrared?
  18. S

    How Are Materials for Infrared Sensors Selected and Developed?

    Hello. I am curious to know if anyone here has interest in talking about materials used for an infrared sensor and how to grow them. I have read about carbon nanotube forests and things like that. Apparently, to achieve the highest sensitivity to infrared light, the key is in the elements used...
  19. sneakycooky

    The relationship b/w infrared, temperature, and electron excitation

    Homework Statement:: 1. Does the increase in kinetic energy in (for example) water that results from increasing its temperature result from electron excitation (i.e. increasing electron energy levels) or simply increasing their velocity or vibration amplitude/frequency? 2. If excitation is...
  20. anorlunda

    Night On Earth: Uncovering the Infrared Spectrum of Animals

    I've been watching an interesting nature program on Netflix called Night On Earth. The film was all taken at night using low light and infrared cameras. I was surprised how many animals and insects see well at night. It makes me curious. Do we know how much of the infrared spectrum...
  21. carter7gindenv

    Infrared lamp vs red heat lamp

    Hi I'm going to use a light based source of heating for a science experiment at my university and I'm a bit confused. Sometimes the names infrared (IR) lamp and heat lamp are used as two different thing and sometimes not but I can't find clear info about it. Is there a difference? thanks in...
  22. Adrian Tudini

    Infrared emission of Lithium Nitrite (LiNO2)

    Hi i am trying to find the infrared emission spectrum of Lithium Nitrite on the web but having no luck. Has the spectrum been measured? If so, can I be redirected to the resource? This for general interest and a personal project. Thanks.
  23. N

    Can I read a book with an infrared led light?

    I am not a physics guy I'm just a husband trying to read in bed at night without disturbing my wife. Does infrared light reading glasses exist?Scientificly, can they be made?
  24. darksat

    Infrared energy absorption of H20 molecules as a percentage of energy

    I eventually managed to find that a CO2 molecule can absorb about 8% of infrared energy passing through it that's radiated from Earth but its actually been incredibly hard to find a comparable figure for H2O molecules. Has anyone got a percentage figure for H2O in the form of atmospheric water...
  25. C

    Does plain glass reflect infrared radiation?

    Glasshouses warm because supposedly heat (infrared) is trapped inside. However I am looking for a description of the process. Is it by glass reflecting infrared back into the glasshouse, or is it by glass absorbing infrared, heating up itself and then warming the glass house interior by...
  26. M

    Irradiance from this infrared light source after a diffuser

    I need to calculate irradiance in a system shown on the image. There is an infrared LED on the left and a diffuser in front of it. I need to calculate irradiance at a certain distance after the diffuser. I think I first have to calculate irradiance at the diffuser. Then multiply it by the...
  27. hagopbul

    A Can multiple layers of thin films increase infrared light absorption?

    hello : i have a small question , can we use multi layer thin films to increase absorption of infrared spectrum , on certain surface , ? taking in mind that we use the same kind of thin film for each layer . is that possible ? best hagop
  28. J

    Why does PPG (photoplethysmography) Use Infrared and Green Both Light

    Hi Everyone, I'm doing research in Implanted Devices in Human Body (Sensors, Hearing Aid, Pacemaker) for my college course. I've started with FitBit that uses the PPG Principle. But I'm struggling to understand why the FitBit uses both Green and IR LED whereas the sensor at the receiving end is...
  29. Lukeblackhill

    Heat during the Day: Answers to Your Questions

    Hi! My question is: I understand that at noon-day it’s hotter because of the angle with which sun rays enter our atmosphere. But at the same time, I was wondering that the band of radiation responsible for heating things is infrared, and that at noon basically all bands of lower frequency than...
  30. Ken Gallock

    A QED: redshifting light and infrared divergence

    I am looking for some resources describing the following content: A light with wavelength ##\lambda## is propagating in flat spacetime. The light redshifts as its wavelength gets larger and larger. In quantum field theory, this causes an infrared divergence of the field. What I want to know...
  31. R

    Relative eye safety of infrared and ultraviolet lasers

    My question is about the relative eye safety of infrared (##\lambda \gtrsim 1400 \ nm##) and ultraviolet (##\lambda \lesssim 300\ nm##) lasers. Both of these wavelengths are highly absorbed by the pre-retinal water content in the cornea, so they don't really penetrate into the retina. So one...
  32. S

    Optical filter that blocks infrared and allows UV?

    Hello. I want to build camera for ultraviolet photography and reflectography. I plan to use debayer'ed backlit CMOS camera with or without Wood's ZWB3 glass UV filter. But I also need to find infrared cut filter because abudant IR radiation will ruin photos. Problem is that there are plenty of...
  33. G

    Problems with Raman Spectroscopy

    Hello, Recently I had a problem in the analysis of a raw material called Propranolol HCl, when performing Raman identification for the specific batch the obtained spectrum does not generate signal and, consequently, identity of the material. By testing other techniques like infrared and NMR, I...
  34. G

    Question about infrared radiation

    In Serway and Jewett's 'Physics for Scientists and Engineers', the authors state that "People all over the world have skin that is dark in the infrared, with emissivity about 0.900.' Similarly, in Louis A Bloomfield's 'How Things Work: Physics of Everyday Life', he states that 'Your skin is...
  35. B

    I What is the Average Angle of Infrared Emission from a Flat Diamond Surface?

    does anyone know what is the average angle of infrared emission is by a flat diamond surface or tha directional distrabution
  36. A

    Infrared thermometry remote sensing

    hi all, This isn't simply physics but is very much related. I'm trying to make an infrared thermometer or some means of remote temperature measurements for temperatures from 0 degree C to around 400 degree C I started with looking up low cost silicon photo diodes / photo transistors e.g. on...
  37. J

    How to calculate color temperature?

    I am looking for calculating color temperature in the IR/Microwave scale frequencies, for room temperature, 273K, 263K, 253K, all google is showing is about visible light and about display systems.
  38. S

    I Allowed infrared absorption in simple London crystals

    Which infrared photons can London force bound liquids and crystals of simple particles (atoms, and homonuclear diatomic molecules) legally absorb? For comparison: diamond is bound by strong covalent bonds. The restoring forces permit vibrations about the C-C bonds at about 1000/cm. But since a...
  39. A

    A fascinating fault with my automatic soap dispenser

    I have a Dettol automatic soap dispenser (see fig. 1) which uses a little propellor-type pump to let out a small amount of soap when you put your hand in front of the passive IR sensor. It's been working fine until just before, when I turned the lights off in the kitchen (night time, so...
  40. F

    Infrared Cameras Spectral Range and Derived Temperature

    Hello Forum, I am wondering why infrared cameras can only detect radiation between 1micron to 15micron. This clearly depends on the sensor's material. Based on Wien's law, this corresponds to temperatures from -79C to 89C. Are infrared cameras only detecting within this spectral range because...
  41. D

    Where can I get some hot mirror (near) infrared mirror) film

    I searched for "hot mirror film" on alibaba and ebay with no luck. Only seller I found was selling such films meant for car windows. For that reason it was too thick and a bit expensive because too big. I got one but yeah, it's too thick. I figured I would find few Alibaba sellers for glasses...
  42. T

    What can I do with an infrared camera?

    I just inherited an infrared camera (https://www.flir.com/products/c2/). Other than taking entertaining thermographs (cold ice cube in a warm hand, etc.), can you recommend some clever demonstrations I could show to my fellow calc-based physics students? The best I came up with is taking a...
  43. K

    Is Infrared Light a kind of light or just heat?

    Hi guys I know we feel infrared light as heat but my question is that is infrared a kind of light or just heat ? Thank you
  44. H

    I ABC model for GaAs infrared LED

    Hello, I have found references giving the equation for an ABC model for GaN-based LEDs (see: file://icnas3.cc.ic.ac.uk/lsm16/materials-10-01233.pdf), but I am not able to find any for simple GaAs infrared LEDs (LED information: https://uk.rs-online.com/web/p/ir-leds/8108247/#). Does similar...
  45. C

    Is it possible to see objects at 600m distance in infrared

    Hi I am wondering if it is possible to see the full distance of a 600m beam from a 500lm torch with an infrared filter and infrared goggles?
  46. M

    Quantum Jump that produces an infrared line?

    1. A certain type of atom has only four energy levels, as shown in the diagram. The "spectral lines" produces by this element are all visible, except for one infrared line. The quantum jump that produces the infrared line is: (see attached file) A) state 4 to 3.B) state 4 to 1.C) state 2 to 1...
  47. hilbert2

    Infrared spectrum of n-heptane

    I have a work assignment where I'm supposed to calculate the transmittance of blackbody radiation through liquid n-heptane, and I have data of the heptane's absorption coefficient on the wavelength interval from 2.6 to 15 micrometers (it has significant peaks at about 3.5 ##\mu##m, 7.0 ##\mu##m...
  48. M

    Infrared thermometer working principle

    So, as far as I understand IR thermometer works by measuring light irradiance coming from an object (in the IR spectrum) and then calculating the object temperature using Stefan's law. Since the irradiance falls like 1/r2 with distance, I am wondering how it takes distance into consideration...
  49. D

    A Infrared Divergences in QED Revisited

    What do you think of the following paper about QED? https://journals.aps.org/prd/pdf/10.1103/PhysRevD.96.085002 Infrared divergences in QED revisited Daniel Kapec, Malcolm Perry, Ana-Maria Raclariu, and Andrew Strominger Phys. Rev. D 96, 085002 (2017) – Published 10 October 2017 It has been...