DIY double-slit with strange results

[xtempore]

I tried to make a double-slit experiment, but I got some difficult-to-explain results.

I cut a piece of opaque plastic in two and then stuck it back together with a human hair (mine) stretched in between, thus forming a double slit.

I then mounted the "double slit" 1m from a sheet of white paper and shone a red laser pointer through. The resulting diffraction pattern was very clear and I counted 13 bright bands within 2cm of the middle.

So...
slit to screen distance, L = 1m.
distance between maxima, x = 2cm/13 = 1.5mm = 0.0015m
separation of slits (diameter of hair - estimate), d = 90microns = 0.00009m

using lambda = x.d/L gives

lambda = 1.35e-7m = 135nm - HUH?

Red light is around 620-750nm. L = 1m is right, and I'm pretty sure about there being 13 bright bands in 2cm. The remaining variable is the hair width, but for the numbers to match up would require a width of around 450microns - way thicker than any human hair.

So what gives? Any ideas are welcome.

In the meantime I might try some variations on the setup to try and get some clue as to where it went wrong.

X

 

[xtempore]

I'm surprised that nobody tried to answer this. Not even a guess?

Anyway after redoing the experiment, I noticed something...

Even though bands were small at only about 1.5mm apart, I noticed that they were not consistent in their brightness but seemed to fade in and out. In fact they seemed to come in bunches, or bands of bands. Each band of bands was about 9mm wide.

Once I used this in the formula then my numbers made much better sense.

My guess is that this is an overlaid pattern of some kind. Essentially the hair is probably not sufficiently opaque and provides some sort of diffraction of its own.

 

[peter0302]

Probably just error in the accuracy of your instruments, or your estimate of the width of your own hair (everyone's hair is different).

Even though bands were small at only about 1.5mm apart, I noticed that they were not consistent in their brightness but seemed to fade in and out. In fact they seemed to come in bunches, or bands of bands. Each band of bands was about 9mm wide.

Yes, that would be consistent. There's both diffraction AND interference going on at the same time, giving you the "bands of bands"

 

[xtempore]

Probably just error in the accuracy of your instruments, or your estimate of the width of your own hair (everyone's hair is different).

Yes my "instruments" were quite inaccurate, especially in regard to the width of the hair which was purely a guesstimate.

But the point was it wasn't just out by a few percent - which would be reasonable. It gave a result SIX TIMES what was expected.

I'm sure you are right about the combination of diffraction, refraction. I wish I had a good way to capture the result though, because it was amazing how clear the banding was.

 

[peter0302]

You're lucky you were within an order of magnitude! :)

Don't you have a high resolution digital camera? Just turn off the flash, and put it on a tripod with a very long exposure.

 

[Mr_Wilson914]

For more accuracy: You could use an old glass mirror. With the aid of a $25.00 metal digital caliper, measure and cut two lines with #11 exacto blade in the painted backing material (a.k.a silvering). Shine the laser on the dull backside surface (the painted side) of the mirror for the experiment.

If you want to minimize the amount of glass the laser has to travel through after traveling through the slits, buy some thin glass used for microscope viewing. Paint one side with flat black paint mixed with graphite powder. This should give you a nice opaque surface to block the light outside of the slits.