Laser Question

[Rootje]

Suppose i take three equal laserbeams (equal phase, wavelength, etc.).

I then put two of these laserbeams next to each other, seperated a distance D less then their common wavelength, and directed in the exact same direction. Next I put the third laserbeam some distance L opposite the other two, and direct it so that the first two laserbeams will meet the third at some point P. Because the beams are in exact phase, there will be a certain L for which the beams will be in exact anti-phase in the region where they meet.

 

In this situation, will the first two beams then act as a sort of wall for the third, and will the portion of the third beam shining through D act as a point source and spread light in other directions then the direction original beam was going in?

[swansont]

You'll get constructive interference where the phases are equal, and the amplitudes will add to a larger value. If the phase is different by pi radians, youll get destructive interference and the amplitudes will add to a smaller value (due to the sign difference).

[Rootje]

Yes, that's the idea. Will the interference cause the first to beams to act as a solid barrier?

[swansont]

A few problems:

 

-The beam doesn't just stop at the place where you have labeled "point source." You will get interference over the whole overlap region.

 

-The side-by-side beams will cause a more complicated interference relationship, because the wavefronts will be gaussian rather than plane waves.

 

-You can't just have them stop in the transverse direction as you've drawn. You won't have a constant intensity over some finite range, and then zero outside of that.

 

In any event, an area of destructive interference isn't intrinsically a barrier. It's just a region of low intensity. To use it as a barrier you'd have to create a scenario where an intensity gradient exerted a force on a particle, which can be done.

[Rootje]

Now put a piece of transparent material in front of each laser, which is variably semi-transparent so that the gaussian intensity distribution becomes a homogeneus one, and the shape and size of the beam cross-section are modified to create perfect conditions. What if you then incline all beams so that they all come "out of the screen" a bit, with a very tiny interference region as a result?

[swansont]

Now put a piece of transparent material in front of each laser, which is variably semi-transparent so that the gaussian intensity distribution becomes a homogeneus one, and the shape and size of the beam cross-section are modified to create perfect conditions. What if you then incline all beams so that they all come "out of the screen" a bit, with a very tiny interference region as a result?

 

It's not that easy to get the profle you want.

 

Once you tip the laser you change the interference, and won't get 100% cancellation.