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Why does the intensity of the light emitted by an incandescent light bulb fall off with the square of the distance (the inverse square law) -- but the light of

Why does the intensity of the light emitted by an incandescent light bulb fall off with the square of the distance (the inverse square law) -- but the light of a laser beam does not? ? ???

PaulHoule 3y ago
The laser beam spreads over distance, it's just the cone is really narrow so you have to get far away to notice.
datadrivenangel 3y ago
coherence. Light from a bulb is not coherent, so it interferes with itself and spreads out.
- colanderman 3y ago
  Coherence is not a necessary property of collimated (nondiverging) light (and doesn't cause it to "spread out"). You can produce noncoherent collimated light from any point source using e.g. a parabolic mirror.
- peter_d_sherman 3y ago
  OK, so if it is coherence -- then what's the generalized method to make a given EM wavelength or frequency band (light specifically, all EM wavelengths generally) coherent? That is, in Physics, how would one take an EM wavelength/set of wavelengths/frequency/set of frequencies (or even more broadly speaking, "energy") -- and make it coherent?
  - PaulHoule 3y ago
    The antenna is already coherent because it is driven by a single signal. The lightbulb isn't because individual electrons are being shaken randomly by thermal noise. The laser is coherent because it is taking advantage of stimulated emission which makes the output photons coherent with the environmental field. https://en.wikipedia.org/wiki/Coherence_(physics) https://en.wikipedia.org/wiki/Coherence_(physics) See https://www.eso.org/sci/facilities/paranal/telescopes/vlti/tuto/tutorial_spatial_interferometry.pdf https://www.eso.org/sci/facilities/paranal/telescopes/vlti/t... (I wish I could find a better writeup of the spatial interferometer, it's actually a pretty simple concept and a simple experiment but I've never seen it explained very well, even in print, when I was studying physics.)
    - peter_d_sherman 3y ago
      >"The antenna is already coherent because it is driven by a single signal. 1) Did you mean laser or antenna? 2) By single signal, did you mean single frequency? (If so, I get it. If not, please elaborate...) >"The lightbulb isn't because individual electrons are being shaken randomly by thermal noise. The laser is coherent because it is taking advantage of stimulated emission which makes the output photons coherent with the environmental field." 3) If thermal noise is the reason that a lightbulb's light cannot be made coherent -- then could you suggest a method whereby the thermal noise in the lightbulb could be removed such that the light emitted could be made coherent? 4) What do you mean exactly by "environmental field"? (A Google search for that term in the context of Physics -- seems not to yield any results -- but then again I lay no claim to being the best Google searcher out there...)
      - PaulHoule 3y ago
        (1) antenna (2) the signal of a radio transmitter is (usually) more or less a sine wave that is either modulated by varying the amplitude or the frequency. You could feed the same signal to multiple antennas. For instance in this photo https://mastodon.social/@UP8/111780254496170231 https://mastodon.social/@UP8/111780254496170231 there is a radio antenna used for emergency responder comms. Note that there are several arrays of antennas stacked on top of each other. If you feed the same signal into an array like that the radiation pattern becomes focused around the horizontal plane so that energy is not thrown into the ground and the sky. (3) It is the shaking by random vibrations that makes the black body radiation of a light bulb. If you stopped that shaking there wouldn't be any light. (4) By "environmental field" I mean the electromagnetic field inside the laser that an active molecule or atom inside the laser experiences. Note if I hooked up 50 antennas to the same oscillator that would be coherent, but if I hooked up 50 antennas to 50 different oscillators that would be incoherent. Take a look at https://en.wikipedia.org/wiki/Phased_array https://en.wikipedia.org/wiki/Phased_array there are two ways to build a phased array. A passive phased array has one transmitter and an collection of phase shifters that delay the signal to create a controlled wavefront. In the first case the emissions of all the antennas are coherent because they come from the same oscillator, in the second case the antennas are coherent because the oscillators are synchronized to a common timebase and controlled by a computer. Note the "magic" of that kind of phased array is similar to the "magic" of a hologram (they do similar things to wavefronts.) Light other than laser light has a certain amount of coherence though it is a complicated subject, see https://www.nature.com/articles/s41598-017-06215-x https://www.nature.com/articles/s41598-017-06215-x Note in Figure 10 they show that you can get enough coherence out of an LED to make a hologram. When they use a real laser the picture is really sharp but you see a speckle pattern that's caused by interference of the light with surface roughness. The LED image is blurry but doesn't have the speckle.
  - insapio 3y ago
    Absorb; and then re-emit coherently, accepting conversion losses.
sp332 3y ago
Photons from a laser are generated from amplification in a particular direction, so they point the same way from the beginning.
projektfu 3y ago
The light of a laser does diminish at the square of the distance but the cone is very, very narrow. This is because laser light is collimated. The production of laser light occurs in an optical cavity that where uncollimated light is reflected back into the cavity. I would really just be quoting wikipedia so I included the link. A parabolic antenna also collimates the energy, reducing the size of the cone that the energy is spread over. This allows things like point-to-point communication and narrow-field radio telescopy. The "power" of a transmitter is the effective radiated power so a transmitter using 1 watt might spread that out over a wide area, but with low power in each direction, or collimate it to a narrow area but with relatively high power in that one direction. https://en.wikipedia.org/wiki/Collimated_beam https://en.wikipedia.org/wiki/Collimated_beam https://en.wikipedia.org/wiki/Optical_cavity https://en.wikipedia.org/wiki/Optical_cavity https://en.wikipedia.org/wiki/Parabolic_antenna https://en.wikipedia.org/wiki/Parabolic_antenna https://en.wikipedia.org/wiki/Effective_radiated_power https://en.wikipedia.org/wiki/Effective_radiated_power
- colanderman 3y ago
  Cones are still subject to inverse square. What matters more is that the focal length is much longer (effectively infinite) with a collimated beam -- that is, the (virtual) tip of the cone is very far behind the light generating element. Inverse square only applies at distances from the source much greater than the focal length.
- mlyle 3y ago
  And just to add a little more-- Laser beams at short distances don't fall off inverse square and may even increase in intensity (decrease in spot size) with distance. But in the end, beam dispersion/the diffraction limit wins and the power density is inverse square. (I can focus a big light down to a smaller spot; but ultimately the light is going to be spreading out. This can be true for radio, too, with weird things happening close).

Why does the intensity of the light emitted by an incandescent light bulb fall off with the square of the distance (the inverse square law) -- but the light of a laser beam does not? ? ???

PaulHoule 3y ago

The laser beam spreads over distance, it's just the cone is really narrow so you have to get far away to notice.

datadrivenangel 3y ago

coherence. Light from a bulb is not coherent, so it interferes with itself and spreads out.

colanderman 3y ago
Coherence is not a necessary property of collimated (nondiverging) light (and doesn't cause it to "spread out"). You can produce noncoherent collimated light from any point source using e.g. a parabolic mirror.
peter_d_sherman 3y ago
OK, so if it is coherence -- then what's the generalized method to make a given EM wavelength or frequency band (light specifically, all EM wavelengths generally) coherent? That is, in Physics, how would one take an EM wavelength/set of wavelengths/frequency/set of frequencies (or even more broadly speaking, "energy") -- and make it coherent?
- PaulHoule 3y ago
  The antenna is already coherent because it is driven by a single signal. The lightbulb isn't because individual electrons are being shaken randomly by thermal noise. The laser is coherent because it is taking advantage of stimulated emission which makes the output photons coherent with the environmental field. https://en.wikipedia.org/wiki/Coherence_(physics) https://en.wikipedia.org/wiki/Coherence_(physics) See https://www.eso.org/sci/facilities/paranal/telescopes/vlti/tuto/tutorial_spatial_interferometry.pdf https://www.eso.org/sci/facilities/paranal/telescopes/vlti/t... (I wish I could find a better writeup of the spatial interferometer, it's actually a pretty simple concept and a simple experiment but I've never seen it explained very well, even in print, when I was studying physics.)
  - peter_d_sherman 3y ago
    >"The antenna is already coherent because it is driven by a single signal. 1) Did you mean laser or antenna? 2) By single signal, did you mean single frequency? (If so, I get it. If not, please elaborate...) >"The lightbulb isn't because individual electrons are being shaken randomly by thermal noise. The laser is coherent because it is taking advantage of stimulated emission which makes the output photons coherent with the environmental field." 3) If thermal noise is the reason that a lightbulb's light cannot be made coherent -- then could you suggest a method whereby the thermal noise in the lightbulb could be removed such that the light emitted could be made coherent? 4) What do you mean exactly by "environmental field"? (A Google search for that term in the context of Physics -- seems not to yield any results -- but then again I lay no claim to being the best Google searcher out there...)
    - PaulHoule 3y ago
      (1) antenna (2) the signal of a radio transmitter is (usually) more or less a sine wave that is either modulated by varying the amplitude or the frequency. You could feed the same signal to multiple antennas. For instance in this photo https://mastodon.social/@UP8/111780254496170231 https://mastodon.social/@UP8/111780254496170231 there is a radio antenna used for emergency responder comms. Note that there are several arrays of antennas stacked on top of each other. If you feed the same signal into an array like that the radiation pattern becomes focused around the horizontal plane so that energy is not thrown into the ground and the sky. (3) It is the shaking by random vibrations that makes the black body radiation of a light bulb. If you stopped that shaking there wouldn't be any light. (4) By "environmental field" I mean the electromagnetic field inside the laser that an active molecule or atom inside the laser experiences. Note if I hooked up 50 antennas to the same oscillator that would be coherent, but if I hooked up 50 antennas to 50 different oscillators that would be incoherent. Take a look at https://en.wikipedia.org/wiki/Phased_array https://en.wikipedia.org/wiki/Phased_array there are two ways to build a phased array. A passive phased array has one transmitter and an collection of phase shifters that delay the signal to create a controlled wavefront. In the first case the emissions of all the antennas are coherent because they come from the same oscillator, in the second case the antennas are coherent because the oscillators are synchronized to a common timebase and controlled by a computer. Note the "magic" of that kind of phased array is similar to the "magic" of a hologram (they do similar things to wavefronts.) Light other than laser light has a certain amount of coherence though it is a complicated subject, see https://www.nature.com/articles/s41598-017-06215-x https://www.nature.com/articles/s41598-017-06215-x Note in Figure 10 they show that you can get enough coherence out of an LED to make a hologram. When they use a real laser the picture is really sharp but you see a speckle pattern that's caused by interference of the light with surface roughness. The LED image is blurry but doesn't have the speckle.
- insapio 3y ago
  Absorb; and then re-emit coherently, accepting conversion losses.

sp332 3y ago

Photons from a laser are generated from amplification in a particular direction, so they point the same way from the beginning.

projektfu 3y ago

The light of a laser does diminish at the square of the distance but the cone is very, very narrow. This is because laser light is collimated. The production of laser light occurs in an optical cavity that where uncollimated light is reflected back into the cavity. I would really just be quoting wikipedia so I included the link. A parabolic antenna also collimates the energy, reducing the size of the cone that the energy is spread over. This allows things like point-to-point communication and narrow-field radio telescopy. The "power" of a transmitter is the effective radiated power so a transmitter using 1 watt might spread that out over a wide area, but with low power in each direction, or collimate it to a narrow area but with relatively high power in that one direction. https://en.wikipedia.org/wiki/Collimated_beam https://en.wikipedia.org/wiki/Collimated_beam https://en.wikipedia.org/wiki/Optical_cavity https://en.wikipedia.org/wiki/Optical_cavity https://en.wikipedia.org/wiki/Parabolic_antenna https://en.wikipedia.org/wiki/Parabolic_antenna https://en.wikipedia.org/wiki/Effective_radiated_power https://en.wikipedia.org/wiki/Effective_radiated_power

colanderman 3y ago
Cones are still subject to inverse square. What matters more is that the focal length is much longer (effectively infinite) with a collimated beam -- that is, the (virtual) tip of the cone is very far behind the light generating element. Inverse square only applies at distances from the source much greater than the focal length.
mlyle 3y ago
And just to add a little more-- Laser beams at short distances don't fall off inverse square and may even increase in intensity (decrease in spot size) with distance. But in the end, beam dispersion/the diffraction limit wins and the power density is inverse square. (I can focus a big light down to a smaller spot; but ultimately the light is going to be spreading out. This can be true for radio, too, with weird things happening close).