How to estimate how many wavelengths wide a laser is?

Smythe

I've conducted two laser experiments. One with single slit, and one with a double slit.

I've produced two graphs which look something like these;

http://www.google.co.uk/imgres?q=laser+beam+graph+slit&hl=en&safe=vss&biw=1280&bih=874&gbv=2&tbm=isch&tbnid=MqHewN7COLoLGM:&imgrefurl=http://www.doitpoms.ac.uk/tlplib/diffraction/printall.php&docid=W3rOVdf3jiXN_M&imgurl=http://www.doitpoms.ac.uk/tlplib/diffraction/images/diagram2.gif&w=604&h=437&ei=8vJxT9qaC6ap0AWJ8eX4Dw&zoom=1

http://www.chegg.com/homework-help/questions-and-answers/graph-laser-pointer-650-nm-shined-narrow-slits-separated-small-distance-slits-3-meters-awa-q1840625

Would anyone know how to go about roughly estimating how many wavelengths wide the laser beam used is?

Possibly from the graphs? (or maybe from other data)

Thank you.

krd

Smythe wrote: »

Would anyone know how to go about roughly estimating how many wavelengths wide the laser beam used is?

Do you mean the width of the slit?

If you look at the diagram just before the one you posted - you'll see an explanation of what λL/w means.

If it's width of the slit - divide w by the wavelength. But that sounds like a silly question. If you know the wavelength, the distance from the slit to the screen, then you should be able calculate w (the width of the slit)

I can't really remember any of this stuff - what I do know it's not rocket science. There are a few equations that explain the patterns, the wavelength etc.

Smythe

krd wrote: »

Do you mean the width of the slit?

Thanks krd. Earlier in the experiment I've calculated slit width.

To quote from my question sheet it asks to roughly estimate how many wavelengths wide the laser beam used in the experiment is.

krd

Smythe wrote: »

To quote from my question sheet it asks to roughly estimate how many wavelengths wide the laser beam used in the experiment is.

That's a weird question. Practically, it's unimportant how wide the laser beam is, until it hits the slit. Then it's a new wave front - I'm assuming the laser beam is the slit width.

It's been a long time since I did these experiments or the calculations for them.

I'm not sure I understand what they're asking.

Delphi91

Winging it a bit here, but if the beam were narrower than the slit, then it should pass through, with no diffraction - correct?

Now if that is true, then imagine gradually narrowing the width of the slit until diffraction does occur. When this happens, then surely the width of the slit is the same as the width of the beam?

If you've worked out the width of the slit, then just divide it by the wavelenght emitted by the laser and that should give you then number of wavelengths wide that the beam is.

krd

Delphi91 wrote: »

If you've worked out the width of the slit, then just divide it by the wavelenght emitted by the laser and that should give you then number of wavelengths wide that the beam is.

That's kind of what I said. On the Cambridge Uni site most of the equations are there. From my memory of doing the experiment the width of the beam hitting the filter didn't really matter - you just focussed the laser so you got more intensity.

I haven't done the equations in years - but when I did, once you work them out yourself. Use a compass to draw some circles, and make your triangles, it all makes sense.