Question:
Light, a wave or particle?
kalepensel
2011-05-01 09:49:49 UTC
I'm still new in physics, but I wanna ask, is it light a wave or particle? If it is particle, why is it when it approach us, we doesn't feel any weight? Light is moving at a very fast speed right? So, it will have a very strong momentum.

Second is, if light is a particle, why is it there is no air resistance occur on the particle? If there is, it will oppose to the friction against the air and thus, BURN!
Five answers:
2011-05-01 10:10:29 UTC
Light behaves as both a particle or a wave, but never both at the same time. It is circumstantial on what the light is doing/what kind of situation it's in. You're correct in saying that light has momentum, however not in the conventional sense. Momentum is calculated by taking the product of the mass and the velocity of an object, p = mv. However light has no mass, hence looking at it from that perspective would not suit light. This class of physics, known as newtonian physics, doesn't work with thinks on a small level, or quantum level. Hence people describe things like light and tiny (quantum) particle using quantum theory and quantum mechanics. Newtonian physics break down (don't work) on a quantum level, and the equation p = mv is an example of newtonian physics and as mentioned above, doesn't work with light, because light belongs to the quantum world, so we have to look at quantum theory to describe it.

In 1924, a physicist called Lois DeBroglie proposed that all matter has a probability wave. In simple english, all matter can behave as a wave, and all waves can behave as matter. The equation he gave for this was:

Wavelength = h/p

Where 'h' is planck's constant and p is the momentum of the object. If you re-arrange this equation for p, momentum, you get: p = h/wavelength. This is an example of an equation used in quantum mechanics and therefore you can see how light can have momentum in the quantum world despite not having mass. As long as its wavelength is known, you can know its momentum, and h (6.63 x 10^-34 if I remember correctly) is a constant so you dont need to worry about that. So you can see from DeBroglie's hypothesis and equation that it is not the speed of light which effects momentum, after all how can it as it is a constant value at all frequencies/wavelength, hence implying that all light at all frequencies has the same amount of momentum. This isn't true, and the momentum is inversely proportional to the wavelength, which makes sense. As wavelength increases, momentum decreases, because longer wavelength is a lower frequency. Energy is proportional to frequency, given by the planck equation, E = hf, again h is planck's constant, f is frequency of light and E is the energy of that light. The more energy you have, the more momentum you have, hence DeBroglie's equation makes sense logically as well as mathematically.

There is so much to wave/particle duality of light, this is just a taste of what scientists are pondering. If you want proof of matter being able to behave as waves (DeBroglie's hypothesis) read about the electron diffraction experiment, it basically showed that one electron, just one, diffracted through a narrow slit to give interference patterns, which made no sense because this implies that one electron had to be in multiple places at once. Explanation? The electron (matter) was interfering, just like a wave would. Experiments proving that waves behave as particles include the photoelectric effect. A lot of reading, but hope you understand most of this, you sound genuinely interested. And forgot to mention, a particle of light is known as a photon, so whenever you see that word, just know that it's talking about light.
busterwasmycat
2011-05-01 10:06:42 UTC
you may choose to imagine it to be either according to your needs. It has characteristics of both, particle characteristics in certain conditions and wave conditions in other conditions.



Do not make the mistake of imagining the model analogy is an actual physical truth. It may be, but that is far from a given. it is suitable for us to consider it as one or the other as needs require, meaning that we can predict behaviors of light under certain circumstances by imagining it as one or the other. Just because it behaves that way doesn't make it physically real in any sense.



I suppose the best way to think about it physically is as a discrete quantity of energy that is not localized in space but has a wave shape over an illdefined temporalspatial region.



As to friction, friction requires contact in some sense. the wavelet of light is small and diffuse relative to any physical matter and thus has no contact for the most part. Much of reality is nothing. Light can interact with mass, and then you see an energy change. you may call that friction if you so choose, but friction to me implies a partial loss of energy of something through conversion to heat, and that isn,t really what happens as a given when there is light-mass interaction.



that is, friction is generally considered to be a conversion of kinetic energy to heat energy (heat is generated from the slowing of the item subjected to friction). Kinetic energy requires mass. Nothing with mass can go the speed of light. light therefore has no mass and no kinetic energy and thus is not subject to friction in a classical sense.
OldPilot
2011-05-01 09:56:32 UTC
Both. To paraphrase the University of Coroado Physics Department: Some experimental data says with absolute certainty light is a wave. Other experiments say with equal certainty light is a particle. We can only conclude that it is both or that it is something we cannot quite visualize



We need to deal with the “Principle of Indeterminacy.” One reason why no one understands QM is that quanta behave differently depending on whether or not they are observed. An example of this is the “2 Slit Experiment. If we let a stream of quanta pass through a barrier with 2 slits then hit a screen they form an interference pattern of light and dark bands (Absolute proof that what we are looking at are waves.) BUT, when we use the photoelectric effect to detect the quanta hitting the screen, we get discrete packets of energy (Absolute proof that what we are looking at are particles). If we do NOT observe which slit, it is a wave. If we do observe where it impacts, it is a particle. Then it gets really strange. If we put a detector next to either slit so that we know which slit a given quantum went through, but leave both slits open, the pattern disappears. If we know which slit the quantum went through, we get one behavior (no pattern, it’s a particle). If we do not know, we get a different behavior (pattern, it’s a wave). We must somehow explain how a particle orders of magnitude smaller than the distance between the slits somehow passes through both slits and interferes with itself.



We need the concept of alternating electromagnetic fields (waves) to explain certain physical phenomena, like the interference pattern in the 2 slit experiment. So we keep that. We must somehow explain how a particle orders of magnitude smaller than the distance between the slits somehow passes through both slits and interferes with itself. Problem, we cannot explain this well using "quanta" (Particles, photons). Wave mechanics gives a simple easy to understand explanation.



We need the concept of quanta (particles, photons) to explain other phenomena, like the Photoelectric Effect. So we keep that too. But, this means we use 2 different, mutually exclusive systems of mechanics to explain electromagnetic radiation.



What to do? We cheat! We say that electromagnetic radiation has a dual nature and choose the system of mechanics that works best for the problem at hand. (Even Einstein did it: June 1905, Einstein completes special relativity, which adds a twist to the story: Einstein's March paper treated light as particles, but special relativity sees light as a continuous field of waves. Alice's Red Queen can accept many impossible things before breakfast, but it takes a supremely confident mind to do so. Einstein, age 26, sees light as wave and particle, picking the attribute he needs to confront each problem in turn.)

http://www.pbs.org/wgbh/nova/einstein/genius/
2011-05-01 09:52:50 UTC
Nobody knows. Seriously, it's one of the biggest questions in physics. They think it's related to quantum physics (where it can be both at the same time). It is taken in by the gravity of black holes, so must have mass, but acts as a wave nearly all of the time.



Look it up on Wikipedia, it's really interesting. Brian Cox (if you know who he is) did an episode in his series about it.
2011-05-01 09:52:40 UTC
Light is a wave

There are two types transverse and longitudial waves

Light is the transverse on which goes up and down-- and sond wves are example of longitudial waves

Light travels in straight lines and they reflect back to the surrounding thus u dont burn :D


This content was originally posted on Y! Answers, a Q&A website that shut down in 2021.
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