# What do they mean by Light being a wave?

1. Jan 4, 2010

### Dav333

In regards to the wave particle duality.

When they say it behaves like a wave, do they mean the up/down wavelength of it?

Or is it always traveling in that up/down motion? If so then please help explain what they mean by wave.

thanks.

2. Jan 4, 2010

### sophiecentaur

When you are thinking of light as a wave: An Elecromagnetic wave carries energy. The Electric and magnetic fields which constitute the wave are at right angles to the direction that the wave is travelling - just as the 'up and down' are at right angles to the direction a sea wave travels. The 'disturbance' is said to be Transverse. There is no actual transverse 'motion' in the case of an EM wave, the transverse fields just change strength at the frequency of the wave. Don't think in terms of a 'wiggle'.
Remember, Light is just the same as Radio waves and all the others. It's just the frequency of oscillation that is different.

3. Jan 4, 2010

### I_am_no1

Do light behave to be more like waves? Or photons?

4. Jan 4, 2010

### diazona

It doesn't make sense to say it's more like one or the other. Sometimes light acts like a wave, and sometimes it acts like a particle.

5. Jan 4, 2010

### sophiecentaur

If you shine it through two narrow slits (or even look at a lamp through the gap between two of your fingers) then it will produce interference fringes - so it behaves as a wave.
If you shine it on a piece of potassium (in a vacuum) then it will cause individual electrons to be knocked off - so it behaves as a particle.

6. Jan 4, 2010

### arunma

Understanding the wave nature of light is, I think, pretty easy. It's simple to show from Faraday's and Ampere's Laws that electric and magnetic fields obey the same wave equation that works for waves on a string. The electric and magnetic field vectors are the things doing the waving. So basically you've got an electric field whose magnitude oscillates according to some wave function (usually a trig function, but it can be anything of the form $f(x-vt)$. And you've got a magnetic field with the same wave function (up to the amplitude), but pointing in a perpendicular direction.

I think it's much harder to understand why light is a particle. I didn't understand that until very recently.

7. Jan 4, 2010

### HallsofIvy

No, it is not "up/down" like a wave on water or in a string. It is a wave in the electro-magnetic field. If you were to carefully measure an electron in the path of a light wave you would find it was pushed one way or another by the electric field force being strong in one direction, the dropping back to the negative direction, the positive again, etc. (That is, in fact, what the nerve cells in your eye measure.)

8. Jan 4, 2010

### jobyts

What causes the electric field to change the direction?

In case of water wave, the original disturbance made in the water (say throwing a stone) pushed the neighboring water molecules away. The pushed water molecules took the least resistance path, which is upwards in the air. But due to gravity, the water molecules had to come down with some force, and pushes all the neighboring molecules. Thus a water wave works. So without gravity and a lighter adjacent medium (air, in this case), a water wave will not happen.

In case of EM wave, what pulls the field back to the propagation line of the EM wave. Why can't the field expand forever?

9. Jan 4, 2010

### Staff: Mentor

There are certain things that water waves or sound waves or waves on a string do:

Diffraction
Reflection
Transmission
Refraction
Interference

When we say that something behaves like a wave we are simply saying that it does one or more of those things. EM fields in the visible range do all of these things, so we say that light is a wave.

10. Jan 4, 2010

### sophiecentaur

Actually, I have much more of a problem with the concept of light (or EM waves in general) consisting of actual particles. This suggests a view of 'little bullets' which brings in the question of "how big are they?" The trivial answer is "about a wavelength in size" but that implies a very undefined frequency (relating the frequency and time domains). Whilst there is much evidence to suggest the Quantisation of EM energy, the Localisation of the energy is open to a lot of doubt. I don't think it is even necessary to think in those terms - apart at a very trivial level. After all, it doesn't have to be a particle just because they told us that at School.
It is a very large can of worms, though!

11. Jan 4, 2010

### f95toli

No, it does not suggest "bullets". The problem here is that a "proper" description of light requires the use of quantum electrodynamics (QED) which is a pretty complicated topic so it it is difficult to explain (Feynmann's book is probably the best attempt I've seen).
However, when QED is used these "inconsistencies" disappear, ALL phenomena can be described using a single mathematical framework (you don't even need "advanced" QED for that, any book on quantum optics will describe this is some detail).

Also, there is no reason to expect particles to have "size" as such, e.g electrons are -as far as we know- point particles (meaning they don't have a size, or "zero size" if you want) but most people have no problem thinking about them as particles.
The same thing is true for photons, it is tricky to describe what a photon is in words (the most accurate description would probably be something "a localized excitation of the vacuum") but its "size" is not really an issue once the proper math is used.

12. Jan 5, 2010

### sophiecentaur

"but most people have no problem thinking about them as particles. "
Precisely. People 'know' what a particle is - and that is something inherently small and localised. Get your average person to draw what they think of as a particle and they will put a small dot on a piece of paper. To use the word "particle" to describe a quantum of EM energy does no service to the understanding of what is going on at all. It assumes a certain spatial distribution which doesn't have to be involved at all. The notion of the particular nature of EM was introduced (historically) with Light in mind. There may be not so much of a problem where such short wavelengths are involved but what about a 1500m radio wave? Are we still talking "particles"? Why? Apart from the inertia of the language.

13. Jan 5, 2010

### I_am_no1

But how can light be wave and particle at a time? Either of the proposition must be wrong, must not be?

14. Jan 5, 2010

### DocZaius

What is wrong with light exhibiting particle-like behavior in some cases and wave-like behavior in other cases? Why would that be logically impossible?

15. Jan 5, 2010

### f95toli

Why not? It is important to realize that the wavelength isn't directly related so the "size" of a photon. Imagine you had a single photon source capable of emitting 1500m wavelength photons (which doesn't exist, but there are single photon sources for microwave frequencies; i.e. a few GHz), if you measure the time it takes for such a photon to travel from the emitter to a the detector is will just be given by the the distance divided by c; just what you would expect for a particle. Hence, it is not like the photon is a both the emitter and detector "at once" even if the distance between them is much smaller than the wavelength.

It IS -however- interesting to note that the classical formulas work even when for example designing resonators intended to be used with single photons. A nice example is that if you quantize a waveguide the maximum coupling strength between the excitations (the photons) occurs where you would (classically) expect the field to have anti-nodes. However, an alternative (and equally valid) method is to assume that the photons are just bouncing back and forth between the ends of the resonator; the result will be the same.

From a practical point of view this means that I can use normal EM tools for designing my MW circuits, even though they are intended to work in regimes where QED is needed to describe the "real" physics.

16. Jan 5, 2010

### sophiecentaur

I know the statistical approach will give you the identical answer to the wave approach but, bearing in mind that the particular idea has been around since long before they had an idea of what they really meant (or thought they meant). My problem is that, unlike other particles (the electron, for instance, which you quoted, has a stated radius - more applicable when it's not in a bound state) a photon's physical extent never comes into consideration. So why say it is a particle? It seems to be so unlike the more familiar particles, which really behave like particles under many conditions and have a measurable size that it just adds confusion to insist it is the same animal.
I realise that there are problems associated with what happens during diffraction - e.g. "which hole does it go through?" and these need not arise if you don't insist it has to do either.

17. Jan 5, 2010

### rcgldr

On a somewhat related note, what is the range of frequencies of photons that can be captured by or emitted from electrons? I would assume this is the closest reaction where light acts as a bunch of particles?

18. Jan 5, 2010

### f95toli

Because we don't care about size, all "particle" means is that it has properties that it can be described by equations that assumes that is is particle-like. There are a LOT of particles in physics that aren't "real" in the sense that they are not small "balls". A good example would be phonons (quantized lattice vibrations) or holes in semiconductors, but all forms of quasiparticles or -even more generally- collective excitations are often described as particles.
Remember that we also talk about waves when e.g describing how traffic flows through a city, but all that means is that the differential equations uses are classified as wave-equations; it doesn't mean that the cars are part of a liquid.

Also, "classical" particles simply do not exist in the real world; EVERYTHING has both particle- and wave-like properties; the fact that we rarely observe the wave-like properties of macroscopic objects does not change that fact that e.g a tennis ball or you or me also have waves in some sense.

19. Jan 5, 2010

### sophiecentaur

f95toli
What you say makes a lot of sense and, of course, there's the matter of the Momentum of photons, which can be easily demonstrated. That is certainly 'object-like', if not 'particle-like'.
I still have the problem of trying to draw a mental picture which adequately reconciles the size, implied by the word 'particle'. A battleship has a de Broglie wavelength but would never qualify to be described as a particle.
I find the idea of quantisation of energy and momentum absolutely fine but the spatial thing is just a step too far for me.
I have read so much rubbish talked about the possible dimensions of photons, clearly resulting from well established indoctrination at the more elementary level (as opposed to your informed comments :-) ) (That's not a double chin btw) that I react strongly against the use of the word. The many half arsed explanations I have read, based on a quasi mechanical 'feeling' for photons as particles, couldn't exist for a moment if the word 'particle' had not been, as I see it, mis-appropriated. In any case, what's wrong with the term "quantum"?

20. Jan 5, 2010

### f95toli

Well, words very often have different meaning in science. Just think of the word "theory"...

I agree that the use of words like "particle" and "wave" can lead to a lot of confusion, I would personally like to stop using them in this context (not only for photons, but also for electrons etc).
But we are stuck with them for historical reasons and there is not much we can do about it.

But once, more particles do NOT neccesarily have a size in physics. Photons are one example but an even better example is the abovementioned phonon which is obivously just a mathematical abtraction since it is just a quantized lattice vibration (and comes from treating the vibrating ions in a solid using QM); but we still refer to them as particles.