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This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually
by aphextron 7y ago
This is a bit hand-wavey in its' description of light reflecting. One of the most mind blowing bits of physics I've ever learned is that photons do not actually "bounce" off of a surface like little balls. They are always absorbed and re-emitted. The actual photons that hit an object are not the same ones that eventually enter your eyes. The atoms in an object are stimulated by photons hitting them to then emit a new photon via the photo-electric effect, which we perceive as light "bouncing".
- aryankashyap 7y agoCould you please elaborate on how it's hand-wavey. Happy to take constructive criticism and improve :)
- deleted 7y ago[deleted]
- ignoramous 7y agoThis might interest you: https://www.quora.com/What-are-the-physics-behind-colour/answer/Joshua-Engel https://www.quora.com/What-are-the-physics-behind-colour/ans...
- lonelappde 7y ago> The actual photons that hit an object are not the same ones that eventually enter your eyes. For anything opaque, you should expect this, because otherwise how could you see something's color if you only saw the same photons as the light source it reflected?
- anamexis 7y agoBecause the object only reflects photons of certain wavelengths.
- tigershark 7y agoThat doesn’t look right at all to me. Why the photon is then re-emitted in exactly the same direction that it would have if it was reflected rather than a random direction?
- mac01021 7y agoI think the two slit experiments indicate that a photon does exactly have a direction of travel?
- lonelappde 7y agoOnly after it arrives, so the situation's a lot more nuanced than that.
- whatshisface 7y agoIt has enough direction of travel for laser pointers to work.
- tyre 7y agoOne constant in quantum electrodynamics is that it doesn't look right to me, but it seems to work! Richard Feynman's "QED" is a great read on this.
- cameldrv 7y agoYou can also watch the original lecture. This is one of my very favorite explanations of anything: http://www.vega.org.uk/video/programme/46 http://www.vega.org.uk/video/programme/46 This explanation starts at about 30 minutes, but I highly recommend watching from the beginning.
- yesenadam 7y agoThanks! I hadn't seen these. There are 4 Feynman lectures on this page[0], the Douglas Robb Memorial Lectures, of which you linked to the second. http://www.vega.org.uk/video/subseries/8 http://www.vega.org.uk/video/subseries/8 1. A gentle lead-in to the subject, Feynman starts by discussing photons and their properties. 2. What are reflection and transmission, and how do they work? 3. Feynman diagrams and the intricacies of particle interaction. 4. What does it mean, and where is it all leading? The site has many other science videos too: http://www.vega.org.uk/video/series/5 http://www.vega.org.uk/video/series/5
- thaumasiotes 7y agoThe little balls model perfectly explains why light reflects off a mirror at the same angle at which it arrived. How does the absorb-and-reemit model handle this?
- silvestrov 7y ago"The Science Asylum" explains it: https://www.youtube.com/watch?v=cep6eECGtw4 https://www.youtube.com/watch?v=cep6eECGtw4
- lonelappde 7y agoThat's almost exactly the same as Feynman's QED lecture from the 1980s(?), but with MSPaint animation instead of a chalkboard.
- sieabahlpark 7y agoExcept light is a wave and not a particle so reflection is explained when you consider that.
- lonelappde 7y agoThat's half right.
- reubenmorais 7y agoLight is not a wave. Photons are particles. Light as a wave is a convenient abstraction that explains a lot of light's behaviors, but not all of them. I recommend Feynman's QED lectures (or the book), where he explains this.
- hutzlibu 7y agoThis debate is very old. Newton and Huygens fought long over it .. and eventually it was solved by: it is both or rather neither. It is a quant. https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality#Classical_particle_and_wave_theories_of_light https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality#...
- fennecfoxen 7y agoA "quant" is someone who studies quantitative finance, typically found in investment fields. The word for this wave-packet with particle properties is "quantum". It is a singular noun; its plural is "quanta".
- hutzlibu 7y agoThx. Not a english native speaker. In german it would be just "quant".
- melon_madness 7y agoI don't think this is quite the photoelectric effect. The atoms absorb photons and re-emit photons--they aren't emitting electrons.
- CrazyStat 7y agoHow does absorption and reemission preserve phase information? What about total internal reflection? Why does the light travel through the entire body of a glass prism without issue and then suddenly get absorbed and reemitted at the surface? Glass (and water) are transparent specifically because they don't absorb photons in the visible spectrum, so how is it that they absorb and reemit?
- maskedinvader 7y agoIANAP but from my limited understanding (mostly reading Richard Feynman) light beam = stream of photons, they don't travel through entire body of glass prism without issue, they get absorbed and re emmitted along the way too (Which is what explains the different speed of light in different mediums)
- CrazyStat 7y agoI'm not convinced but even if we accept that explanation, why do all the atoms along the way emit the light in the direction it was travelling and then the ones at the surface suddenly emit it in a different direction?
- lonelappde 7y agoThey don't! That's why the thickness of th glass of affects it's reflectivity, and not the simple way you might guess. Read Feynman's QED book or watch the YouTube vidoes for a lay intro.
- georgeburdell 7y agoI can recommend Hecht’s Optics textbook for this. I found it at my local library. I have had a number of electromagnetics courses over my life and none explained the unifying scattering principles of reflection, transmission, and refraction concept so clearly as that book
- dhimes 7y ago
- whatshisface 7y agoHaving thought about it, I'm not sure if I agree. Light can reflect off of transparent crystals, which do not have the right electron energy levels for absorption (that's why they're transparent.) It's possible that the Feynman diagram of a reflection might involve the photon "going away and another one replacing it" (I don't know what the diagram looks like), but Feynman diagrams cannot be interpreted as sequences of events. Instead, they describe an instantaneous flow of amplitude between different quantum states. I would only go along with the absorption and re-emission interpretation if there was always a time delay between the two steps (which there cannot be if the mirror does not have any energy level deltas suitable for storing the energy). The other side of the debate would be that if the mirror is moving towards or away from the light, the reflection will be Doppler-shifted to a higher or lower frequency. Does this mean that the reflected photons are not the same photons as the incident photons, or does it mean that the same photons have had their energy changed? I think there is no meaningful distinction because every two particles with the same name in quantum mechanics are identical anyway. There's no telling which are which. If I showed you a photon, then took it back and showed you another, you would never be able to tell whether I had opened the same box twice or if I had taken the old one out and captured a new one from my desk lamp.
- chemeng 7y agoThey are not the same photons. They are emitted by the electron changing energy states after excitation of the incoming photon. But your intuition that there is some quantum weirdness is correct. Quantum Electrodynamics is the theory you're looking for. It unifies quantum mechanics and special relatively in the context of light/matter interaction. Given you seem to be familiar with Feynman. You're in for a treat! His book on QED is meant to be an accessible explanation.[1] [1]https://en.m.wikipedia.org/wiki/QED:_The_Strange_Theory_of_Light_and_Matter https://en.m.wikipedia.org/wiki/QED:_The_Strange_Theory_of_L...
- whatshisface 7y agoHydrogen atoms can't store photons that do not have the same wavelength as any of its spectral lines. Nonetheless, hydrogen gas has an index of refraction that is a continuous function of frequency, and as a result it can reflect light of any wavelength. Therefore, hydrogen does not store any energy for any length of time in the course of typical reflection. Even if the Feynman diagram for reflection looks like a combination of absorption and re-emission, if the absorption and emission process can't occur at separate times, then it is not right to say that one is followed by the other.
- elcomet 7y agoI don't think it's hand wavey. It's just a different (simpler) model. To describe this phenomena (wet things are darker), you don't need to go into details about photons, or quantum mechanics or whatever. Geometrical optics is really sufficient here, it explains it perfectly, and is much more intuitive than other more complex models.
- aryankashyap 7y agoThanks!
- 8bitsrule 7y ago>Everything in this essay comes from this source. https://fermatslibrary.com/s/why-some-things-are-darker-when-wet https://fermatslibrary.com/s/why-some-things-are-darker-when... It's less hand-wavey. Also, the reflectivity of water is very low at low angles of incidence, rising to 100% at normal angles. Less light reaches the wetted material. https://en.wikipedia.org/wiki/File:Water_reflectivity.jpg https://en.wikipedia.org/wiki/File:Water_reflectivity.jpg
- dan-robertson 7y agoIt doesn’t really make sense to talk about “the same” photon because photons don’t really have identity. Even in a vacuum you can send a photon in one end and see one on the other and not really know that the one going in was “the same” as the one going out. It can turn into other particles on its way and then recombine into a photon and there is no way to know this based on the observation that a photon came out of the end of your vacuum.
- zelphirkalt 7y agoThat's one of the things, that annoys me with questions about physics, which me might not know well enough yet: When someone asks about whether something "is", people answer with what we can in general know at our current level of understanding and then make it seem as though that is the same, as answering the question, what really "is". > Even in a vacuum you can send a photon in one end and see one on the other and not really know that the one going in was “the same” as the one going out. I don't really care, if _I or current physicists_ cannot _distinguish_ them, when I ask, whether they _are_ the same. I would rather have an answer like as follows: "We do not know this yet. We do not have the means of telling, whether it is really the same photon or not. However, even if they were not the same, it would make no difference (according to our current understanding of physics!), as their effect on the surroundings would still be the same, because ..."
- keithnz 7y agono, what dan said is better, it might annoy you, but physics is such that wording is important. Even then, nearly all wording is losing information and in some cases changes it from the reality. You are asking a question which you want an answer but the question itself has a troubling fit with reality as you try to relate to things in terms that seem familiar.
- StuffedParrot 7y agoNo, physics is such that semantics are important. What’s the observable difference between absorption/emission and reflection? If the latter doesn’t happen at all the distinction is meaningless.
- mrlala 7y agoDoes that mean like, all the time? For example.. if I'm looking at a star, and thinking it's the 'same' photon that came off of that star billions of years ago... that might be true in the vacuum of space, but since it's passed through the medium of the atmosphere, does that mean it's constantly been absorbed/new photons being emitted?
- empath75 7y agoIt’s not even true in the vacuum of space. It might have been created and annihilated billions of times on the way.
- marcosdumay 7y ago> The atoms in an object are stimulated by photons hitting them to then emit a new photon via the photo-electric effect, which we perceive as light "bouncing". That's how electrons and photons interact. It doesn't make sense to say that they are different photons or the same ones, since you can't observe the interaction.
- amluto 7y ago> The actual photons that hit an object are not the same ones that eventually enter your eyes. This part is philosophy, not physics. Photons are “indistinguishable particles”. You cannot tell two photons apart. This has a rigorous meaning: if you swap two photons, the state of the universe is unchanged. (This is in contrast to fermions — if you swap two fermions, the state of the universe has its phase shifted 180 degrees.) If photons we’re distinguishable, then swapping two photons would swap them, and the resulting state would be different. The matters from a statistical perspective. Look up Bose-Einstein statistics if you’re interested. As for whether photons are absorbed and re-emitted or merely reflected, this surely depends on the surface on question.
- sjg007 7y agoWhat surfaces reflect?
- amluto 7y agoMetals and, generally, homogenous materials (dielectrics, conductors, etc). Light reflects off of a conductor or a dielectric due to a relatively straightforward solution to Maxwell’s equations. In contrast, anything phosphorescent most definitely absorbs and re-emits, sometimes hours later. Imagine glow-in-the-dark pigment.
- sjg007 7y agoThanks, yes makes sense. But do metals actually reflect or do they just absorb and just release spontaneously?
- amluto 7y agoThis probably depends on your perspective. If you think of a metal as a homogenous linear medium (which is a good approximation for long wavelengths but is not exact), then light is a wave, photos are an unnecessary detail, and the wave mostly reflects and is absorbed as heat. If you look the metal as a whole bunch of atoms and a sea of mobile, discrete electrons, then the incoming photons excite the sea of electrons and, when the interaction settles down, photons are leaving. If you insist on modeling the electrons with QED, congratulations, you’ve made your life unnecessarily complicated unless the photons have such high energies that electrons are created or end up with relativistic velocities. If you believe in string theory, maybe the photons were only an approximation in the first place. Physics is all about choosing an appropriate model of the world that captures the detail you need without being more complicated than needed. Is a person on a playground swing a pendulum, or is it a bag of molecules sitting on a flexible piece of rubber suspended by a bunch of rusty metal links from a flexible steel bar that is, in turn, supported by many geological layers in the Earth’s crust?
- thelittleone 7y agoIf I recall a high school physics class correctly the colour of the photons that are emitted from the object (the colour we see) are the ones the object does not absorb. If so, is the object the colour we see or is it the colour it absorbs.
- ddingus 7y agoColor may not actually be a basic physical property. Purple is an artifact of our minds, for example. Does not exist as a distinct wavelength of light. The object absorbs, does not absorb, emits, whatever wavelengths of light. To us, those have colors, where they are in the visible spectrum. Take us out of the equation, and?
- socceroos 7y agoDumb question: why do these ejected photons mirror the incoming photon's trajectory if they're a new particle?