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The following is a very simple physics experiment that illustrates interference fringes.
We need an artificial star for our interferometer. It could be almost any bright source viewed from a distance, however a desk lamp serves the purpose well if it is masked to look like a point source. Take a desk lamp, cover its output with aluminium foil, and punch a small hole in the foil with a ballpoint pen, or whatever is handy. The hole could be a millimeter or two wide. When you turn on the lamp and view it from several meters away you should see a very bright point source. [The lamp may get very hot, so make sure it doesn't melt under the foil!]
Step 1: Single Aperture and Airy Disk
The interferometer is just as simple to build as the artificial star. Take a piece of aluminium foil, perhaps two inches square, and with a needle make one very small hole in it such that the needle just breaks through the foil. This is not yet an interferometer, but pause and hold the aluminium foil up to your eye and look through it at the artificial star. What you should see is the diffraction pattern of the hole, an Airy disk with some nice rings around it. Try looking around at any other bright objects in the room. Go outside with your piece of aluminium and look through it. You should note the following:
Step 2: Interferometer, Airy Disk, and Fringes
Now let's build the interferometer. Take the same piece of aluminium foil and put another hole as close to the first hole as you can manage. The separation of the two holes should be only about a third of a millimeter, so you need a very small, sharp needle to make both holes.
As before, stand back from the desk lamp, hold the aluminium foil up to one eye, and look at the artificial star through the holes. Presto! You should see fringes. If you don't see fringes you probably have made the holes too far apart or the holes are too large. If you do see fringes, you can experiment with placing the holes slightly further apart to get a finer fringe spacing, or trying a combination of three holes, etc...
Optical Aperture Synthesis Imaging
As before, your field of view looking through the interferometer is limited by the distance you place the holes away from your eye, however the available resolution is set by the fringe spacing. A source whose angular extent is much smaller than the fringe spacing will produce high contrast fringes, whereas a larger source will produce fringes with reduced contrast, or no noticeable fringes at all. A large source can be thought of as being composed of numerous unresolved sources, each of which sets up its own fringe pattern in your field of view. Because each of the unresolved sources are at different locations in the sky, they set up fringe patterns that are slightly displaced from each other. Where these fringe patterns partially overlap their relative displacement reduces the average fringe contrast. Therefore, by observing the fringe contrast you can tell how large the source is relative to the fringe spacing.
If you were to make an artificial star that had two closely spaced holes in it (ie. a binary star), and if you rotated your aluminium foil interferometer in front of your eye, you will see the fringe contrast change with the angle of rotation. This is because the binary is more extended along the line joining the two stars. You can therefore deduce the angle of the binary even if you couldn't otherwise resolve it.
The following paper by Michelson has a nice explanation of the fringes that would be seen through a telescope and illustrations that should be familiar to anyone who builds their own aluminium-foil interferometer.
A. A. Michelson, Measurement of Jupiter's satellites by interference, Nature 45, 160-161 (1891).
Rudolf Danner at JPL has been using the same approach to illustrate fringes for popular science talks relating to the Space Interferometry Mission. His version is more professional and is made by photolithography and mounted (with illustrations) onto a business-card. The holes are 0.1 mm in diameter and spaced 0.3 mm apart.
Last Updated 18 January 1999