Light

[Kyrisch]

I've always wondered how light can be a wave and a particle simultaneously. I asked my uncle, who knows a lot about physics, and he couldn't answer my question directly. He said it is sometimes useful to show light as a particle and other times it is more convenient to represent light as a wave, depending on the topic and what one is trying to prove. However, this does not answer my question.

[Daecon]

Photons are tricky buggers. I just think of them as "a particle of wave-ness".

 

Although that depends on what you regard as the true nature of wavality.

 

Is the pattern a wave can take pre-determined by the structure of the space in which it passes, or does the wave make up the pattern on it's own as it goes along...?

[Kyrisch]

I'm confused also because for something to be a wave, doesn't it need a material in which to travel? An ether? And since light crosses the vast distances of space with no ether, it can't exactly be a true wave, can it?

[ydoaPs]

no. the wave is the alternating electric and magnetic fields perpendicular to the direction in which the wave is travelling.

[danny8522003]

If you think of energy as having mass, then a wave which is energy must have mass. For this reason light can behave as both a wave and a particle.

 

Electrons, which have a much larger mass than photons, can also exhibit wave behaviour. This leads me to think that the Universe is made of 'stuff' that can behave just as easily as a particle or a wave.

[ydoaPs]

photons don't have mass

[elfstone]

photons don't have mass

 

They don't have rest mass, but their energy is equivalent to some mass according to e=mc^2, right? I think the wave/particle duality is related to the mass/energy duality somehow.

[ydoaPs]

no. that isn't the whole equation. [math]E^2=(mc^2)^2+(pc)^2[/math]

[danny8522003]

Lets not start all this again. Im talking about relativistic mass not 'real' mass.

[Jonsy123]

The definition of a wave is: "a disturbance in a medium that carry energy" (certainly work well for waves at the sea, in a rope, and sound waves).

 

Now, in Electromagnetic waves, is the magnetic field suppose to be the medium in which the magnetic wave is oscillating through ?, the same for the electric field, is it the medium in which the electric wave is oscillating through ?. If so, it means that unless the universe is somehow filled with a magnetic and electric medium, then the medium for the electromagnetic wave, is concentrated only near the wave, and moving together with the wave itself - the medium is moving with the disturbance, at its same speed and direction, and by this, allowing it to happen. that's weird (it reminds me of "ice man" in the spiderman cartoon, where he "paves his way in ice", as he moves).

 

Btw, I don't see it any easier to understand how an electromagnetic wave is moving through water or air, than moving through vacuum. Water and air are not the right medium for electromagnetic radiation (but since speed of light through water is slower than through vacuum, I may be wrong, and water can be a medium for electromagnetic waves, or on the other hand, it might not be a right medium, but can cause some interference...).

 

Anyway, I guess that thanks to einstein and his interpertation of the photoelectric effect, we can just say that when in vacuum (or whatever), electromagnetic waves can be seen as photon particles, so there is no need for a propagation medium at all.

[5614]

Light is not a wave and a particle... it is something that exhibits properties of both a wave and a particle.

 

It cannot be 2 things at once, it is a 3rd thing that exhibits properties of the previous 2 (wave/particle).

[Jonsy123]

Right. Still, it is particle-like enough to propagate in the medium-less vacuum in space, and wave-like enough to refract as it enters the atmosphere.

[Severian]

People keep talking about wave particle duality, but imho that is the wrong way to think of it.

 

Despite the whole subject being called 'particle physics', we really need to get away from the idea that the fundamantal objects are point particles. We should think of them as fields instead.

 

A field is a basically a set of numbers, one (or somethimes more) for each point in space. So a wave on the ocean can be thought of as a field, since the size of the wave varies from place to place, and changes in this magnitude make the wave appear to move. Light is just such a field (it is actually an electromagnetic field), and this is why it has wave like properties.

 

The difficult thing to get your head round with Quantum Mechanics is that you cannot make a measurement of a field's properties without changing the field irreversibly. When I make a measurement the field changes in all of space instantaneously (this looks like it violates special relativity, but in fact it does not). If I try to measure the position of a field (that is, the value at the various space time points) the field 'collapses' into all being very very close to one point - the shape of the field basically becomes a very sharp spike at one point in space. This is what we think of as a particle, but really it is still a field - it just has a zero value for most of space, but is not even pointlike because we cannot measure position infinitely well (there will always be some error in our measurment). Left on its own, this very localised field will spread out in a wave like way and eventually will be spread out over a large volume of space again. But if we keep measuring its position (ie watch it) it will keep collapsing to a localised version and will look and behave like what we traditionally call a particle.

 

In other words, there really is no particle-wave duality. The fundamental objects are always 'waves', and it is just that when they are very densly concentrated in one place they look like particles.

[Jonsy123]

Isn't there any problem understanding the photoelectric effect this way ?.

[Severian]

No - the field is still quantized.

[Jonsy123]

So I don't understand, why couldn't Einstein interpret the photoelectric effect using wave theory, and instead needed to "invent" the particle nature of light ?. Am I missing something here ?.

[mezarashi]

Einstein did so, because without the use of the particle nature of light, the photoelectric effect could not be explained. I'm sure you know about it and the reasons why that is so. Remember, at that time, everyone believed strongly that light was truly and only a wave following the great success of Maxwell's electromagnetic wave equations. Einstein connected Max Planck's quantized nature of light and its particle nature.

 

Severian's model is constructed with the knowledge that light can be like both. In anycase, I like Severian's model for visualizing the concept of light. It's probably the best understanding we have of light. It's just the nature of light is possibly beyond our human comprehension. It's like the famous example of showing a coin to someone who lives in a two-dimensional world. He can either see heads OR tails, but not both at the same time. It would totally confuse the poor fellow.

[Jonsy123]

If he understands that he might be living in a 2D world, there's still hope for him...

[5614]

The basics of why light can be viewed as a particle can be seen here:

http://www.colorado.edu/physics/2000/quantumzone/photoelectric.html

The reason it can be viewed as a wave would be Young's Double Slit experiment which I'm sure there's a lot written about on the internet if you so wished to search for it.

[Jonsy123]

Taken from the explanation:

the details of the photoelectric effect come out differently depending on whether light consists of particles or waves. If it's waves, the energy contained in one of those waves should depend only on its amplitude--that is, on the intensity of the light. Other factors, like the frequency, should make no difference. So, for example, red light and ultraviolet light of the same intensity should knock out the same number of electrons, and the maximum kinetic energy of both sets of electrons should also be the same. Decrease the intensity, and you should get fewer electrons, flying out more slowly; if the light is too faint, you shouldn't get any electrons at all, no matter what frequency you're using.

 

I don't understand why a *wave* can't hold more energy as the frequency goes higher ?. True, I also can't explain why it should (because it oscillates more frequently ?), but is there a scientific explanation that inhibits a dependence of a wave's energy, at its frequency ?.

[ydoaPs]

[math]E=hf[/math]

or

[math]E={\hbar}{\omega}[/math]

[Jonsy123]

yourdadonapogos, so what is your point ?. According to the explanation I quoted in my last post, Einstein said that E=hf proves light can NOT be just a wave, because E=hf does not work for waves, because for waves, E is proportional only to the amplitude (and frequency is irrelevant).

[ydoaPs]

...is there a scientific explanation that inhibits a dependence of a wave's energy, at its frequency ?.

i was responding to that

 

edit: iirc, [math]\frac{1}{2}m_{electron}v^2_{electron}=hf_{photon}+W[/math]

[5614]

because E=hf does not work for waves

That is precisely the point, e=hf shows that light must be a particle... then Young's double split shows it must be a wave.... hence wave-particle duality.

[Jonsy123]

Sorry for the confusion.