Double slit results that I can't explain, help

[Yakult]

Hi there!

I'm currently sat in the lab working on one of my uni assignments (I'm in my second year at Exeter, UK), and I'm getting results that I just don't understand, so thought that someone here might know what's going on!

We're using a laser (through two lenses: I think one's an attenuator, but I can't remember what the other one is) that's being shone through different slits, and then onto a CCD camera, and the resulting patterns are recorded on the computer. We got good results when it went through the single slits (by good I mean nice central maxima), but when we moved onto double slits everything went strange.

We're getting results that look like the attached image below (sorry for the crudeness of the diagram, but you get the idea)

We get these regardless of the slit seperation, and for slit widths greater than 0.25mm!

Surely we should get a central maxima and then loads of smaller maxima, and not a central minimum? Our demonstrator mentioned something called "aperture diffraction" as a possible reason, but when pressed didn't really seem to know what that was.

Any ideas?

Thankyouuu!

Will

 

[ZapperZ]

What is the wavelength of your light source? Compare that with the size of the slit.

Zz.

 

[Yakult]

The wavelength's 6.328nm, the slit width's 0.5mm and the slit separation is 0.2 mm but that should still give a central maximum, surely :/ Sorry that this was in the wrong place, I wasn't really sure where to put it!

Thankyou for such a quick response

 

[ZapperZ]

For any diffraction effect to be noticeable, the slit size must be comparable, or less, than the wavelength of the light being used. Here, your slit is significantly larger than the wavelength. Of course you see no diffraction, and thus, no interference.

Zz.

 

[Yakult]

Oh man, I feel like such a dumbass. I can't believe we didn't realize that I think we just assumed that they'd be looking for us to be getting a double slit diffraction pattern with the slits they provided... Just goes to show that physicists should assume nothing

Thanks for your help

 

[jtbell]

The diffraction patterns that one normally studies at the introductory level are produced in situations that allow simplifying assumptions that produce what we call Fraunhofer diffraction. If those assumptions aren't valid, we have instead something called Fresnel diffraction which can produce quite different patterns, including ones with minima at the center!

See for example this Fresnel diffraction applet whose initial setup for a single slit in fact has a mininum at the center.