Double slit results that I can't explain, help

In summary, the conversation is about a university student seeking help with their lab assignment involving a laser, slits, and a CCD camera. They are getting unexpected results and are unsure why. After discussing the setup and measurements, it is determined that the slit size is significantly larger than the wavelength of the light source being used, which explains the lack of diffraction and interference patterns. The student expresses gratitude for the quick response and learns about the differences between Fraunhofer and Fresnel diffraction.
  • #1
Yakult
6
0
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! :smile:

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! :smile:

Will
 

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  • #2
What is the wavelength of your light source? Compare that with the size of the slit.

Zz.
 
  • #3
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 :smile:
 
  • #4
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.
 
  • #5
Oh man, I feel like such a dumbass. I can't believe we didn't realize that :redface: 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 :tongue:

Thanks for your help :smile:
 
  • #6
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.
 

1. What is the double slit experiment?

The double slit experiment is a classic experiment in quantum physics that demonstrates the wave-particle duality of light. It involves shining a single beam of light through two parallel slits and observing the resulting interference pattern on a screen.

2. Why do we see an interference pattern in the double slit experiment?

The interference pattern is caused by the wave-like nature of light. As the light passes through the two slits, it diffracts and creates two sets of waves that overlap and interfere with each other. This creates areas of constructive and destructive interference, resulting in the observed pattern on the screen.

3. How does changing the distance between the slits affect the interference pattern?

Changing the distance between the slits alters the wavelength of the light passing through them, which changes the interference pattern. A smaller distance between the slits creates a wider spacing of the interference pattern, while a larger distance creates a narrower spacing.

4. Why do we see a different pattern when only one slit is open?

When only one slit is open, there is no interference between waves, and the light behaves more like a particle, resulting in a single-slit diffraction pattern. This is because the waves from the single slit spread out in all directions, while the waves from two slits are confined to a smaller area.

5. What happens if we shoot particles other than light through the double slit?

The double slit experiment has been done with various particles, such as electrons and even large molecules. The results are similar, showing wave-like behavior and interference patterns. This demonstrates that the wave-particle duality is not limited to light, but is a fundamental property of all matter.

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