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Step 5
So, there you have it.
A way of measuring nanometric disturbances.
Every time that a bright band is swapped for a dark band, the path lengths of the two rays in the interferometer will have been changed by exactly 1/4 of a wavelength. For red light that's a shade over 100nm.
Now the question is how to engineer a way of generating controlled disturbances at that scale - and for that we'll need a feedback loop and a transducer! But that's another project - enjoy!
A way of measuring nanometric disturbances.
Every time that a bright band is swapped for a dark band, the path lengths of the two rays in the interferometer will have been changed by exactly 1/4 of a wavelength. For red light that's a shade over 100nm.
Now the question is how to engineer a way of generating controlled disturbances at that scale - and for that we'll need a feedback loop and a transducer! But that's another project - enjoy!
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As partial inspiration I did indeed borrow the idea used in Greg Bear's Eon of a portable space-time flatness meter (a 'pi-ometer' if you will).
More accurately, the fringe pattern depends on the fractional number of wavelengths that the beams have in their paths.
Say that one beam travels 1cm. The number of wavelengths in that distance;
= 0.01 m / 650 x 10^-9 m
= 15384.6
That's how many full 'cycles' the electric field in the light beam makes in travelling that distance.
Say that the other beam travels 1.1cm, the number of wavelengths in that beam is;
0.011m / 650 x 10^-9m
=16923.08
The difference between these two 'numbers of wavelengths' is 1600 or so, but what dictates the type of interference is the fractional wavelength difference. In this case that difference is just over half a wavelength.
The interference pattern for a difference of 0.5 wavelenghths is exactly the same as one for a path difference of 1.5 wavelengths, or 2001.5 wavelengths.
Imagine, you've got one 'wave' and you slide another of the same frequency along side it. They'll match shen you slide the 2nd wave a distance of n wavelengths - the interference depends only on the fractional mismatch.
Hope that I've made it clearer!
I have read, and tried/completed many Instructables. This one goes to the top of my list. Why? Because it is one of the best Instructables I have ever read. Easy as that. 5 out of 5 dude!
Glad that you liked it.
As for applications?
Well, ever since I had acces to an AFM at ESTEC I have toyed with the idea of manipulating stuff at small scales. Clearly, before one uses a sub-micron lathe, one needs to know where things are - and interferometry could allow a very tiny lathe or mill to be constructed.
As a side note, there are other paths to that goal for the DIY experimenter, even a CD drive head (slide assembly and optical platform) would allow for sub-micron positioning, but interferometry gives a useful factor of 5 or so finer resolving power at little cost.
Dunno.
Still thinking. :D
Every word of this instructable was interesting, clear and informative. I really, really doubt I'll ever use this new knowledge, but it's cool to think this stuff is possible.
I especially loved the cheese references.
Glad that you found it informative and entertaining - I now spot that the movie I had taken of the fringes moving had not uploaded correctly - and I'm trying to fix that now.