# If light is a wave then how can it be formed into a laser?

1. Feb 9, 2010

### chopficaro

i know light is also a particle, but if a laser can hit an open slit and spread out i cant see how it couldn't be more spread out when it came out of the laser in the first place

2. Feb 9, 2010

### Staff: Mentor

diffraction is a manifestation of wavelike properties, not particle-like properties.

3. Feb 9, 2010

### ZapperZ

Staff Emeritus
The light coming out of the laser are "plane waves". You only can start seeing the diffraction effects when it passes through a slit that has a width comparable to the wavelength. If you let it pass through something large, you still can't detect the diffraction pattern that closely.

While this is certainly true and has been used as one of those "wave-like" property, it should be clarified that one can get such diffraction pattern simply by using photons and applying the HUP. I've described this a few times already. Furthermore, if one looks the Marcella treatment of the diffraction/interference phenomena, one doesn't have to invoke wave mechanics to get the same wave-like results.

Zz.

4. Feb 9, 2010

### Naty1

Would you have the same issue/concern with everyday sunlight??

5. Feb 10, 2010

### IttyBittyBit

The beam from even the most 'perfect' laser still spreads out a little, at an amount that depends upon wavelength, and the thickness of the beam. It's just that you can't see the spread with ordinary distances since the wavelength of light is so small. It's the same thing with corners; ordinary sunlight bends around corners but the amount is too little to be noticed in everyday experience. When you 'thin' the beam size, the amount of spread increases to be noticeable.

Without resorting to the mathematics, you can think of a large beam as being emitted from a large group of point sources with the 'spread-out' of each source canceling the effects of the other sources, except in the direction right in 'front' of the beam. When you thin the beam, in effect making the number of point sources small, the canceling effect stops and you see the diffraction.

I should also point out that none of this has anything to do with quantum effects or the wave/particle duality. You can see the same effect with any wave. If you're interested, I recommend reading up more on wave dynamics.

6. Feb 10, 2010

### Frame Dragger

Ignoring interpretation, light is not a wave or a particle, but a duality of both. You can isolate one property or another, but there is no discrete defintion that makes it a particle at one moment, and a wave at another.

7. Feb 11, 2010

### chopficaro

ty i understand a little better

8. Feb 15, 2010

### Tao-Fu

Nice use of Huygens's principle.

9. Feb 15, 2010

### Tao-Fu

This isn't quite right. It would be better to say that the light coming out of a laser can be described in terms of an infinite set of plane waves traveling in slightly different directions. The relationship between the plane waves is such that the interference between them produces a bright spot along a given axis and a Gaussian intensity distribution in the transverse plane normal to this axis. The wavefronts of the resulting beam are curved everywhere except near the waist/focus of the beam.

You can also think of the Gaussian beam (which approximates the output of many lasers pretty well) as the result of diffracting a single plane wave through a Gaussian aperture.

The non-zero width of the beam at its focus is a manifestation of diffraction and no additional slit is needed to observe this.

Given that Maxwell's equations (i.e. the wave equation when dealing with propagation through vacuum) are correct quantum mechanical field equations for light, it seems ill-founded to presume a particle-like object and derive wave-like behavior from this. But the word "photon" can lead to lots of misunderstanding, so I won't assume that this is actually what you meant. A photon seems to most closely correspond to an excitation in the electromagnetic field. It may have a certain energy, in the case of a monochromatic field, or an approximately certain location, in the case of localized pulses of light (wavepackets, not really particles), but not both.

10. Feb 15, 2010

### ZapperZ

Staff Emeritus
I'm not sure what you are objecting to here. I was responding to the notion that wave-like properties are strictly a manifestation of 'physical waves', which really isn't true for the case of light (and other quantum "particles"). One can skip using the classical wave description, and invoke purely quantum mechanical formalism, to get the identical diffraction/interference effects. This means that one can adopt a single, consistent description of light, without having to switch gears between "particle description" and "wave description", which is what we've been teaching students in school.

You may want to read the Marcella paper first (T.V. Marcella Eur. J. Phys. v.23, p.615 (2002)) if you want to figure out what my point here is. Are you saying that this is wrongly done, or should not be pointed out?

Zz.

11. Feb 15, 2010

### Tao-Fu

Sorry. It looked like you were saying that diffraction can be gotten at by simply positing a quantum particle and invoking the Heisenberg uncertainty principle. I guess I got your point a bit muddled. See what harm the use of the word "photon" can do?

I will have to check that paper later. My current connection is too slow to handle VPN.

12. Feb 16, 2010

### ZapperZ

Staff Emeritus
To a certain extent, it can! However, it cannot arrive at the quantitative aspect of diffraction. The diffraction phenomenon, however, can be used to illustrate the HUP.

Zz.

13. Feb 16, 2010

### Frame Dragger

I believe you, but I need to read up on this... a lot. Any good links or books?