Harry's Interferometer

To make this table-top model you need to be able to saw some small pieces of wood, drill some holes, cut some pieces of rubber and assemble everything using either wood screws or bolts. The rest of the parts are held together with the putty used to attach posters to walls and similar. In the UK the most widely known brand of this putty is called Blu-Tack but elsewhere it may be called Fun-Tak, Sticky Stuff or Poster-Tack or something like that. Any store selling basic stationery supplies should have it. I’ll refer to it as ‘putty’ from now on. It's entirely possible to put the whole interferometer together by holding the parts to the base with blobs of putty alone and without bothering to construct the ‘mounts’ for the mirrors etc, but the putty is slightly elastic and also tends to creep away from where it's first set. I tried it both ways and found that it's better to spend a bit more time cutting, drilling and assembling at the outset as it will save more time at the end and be much less strain on your patience when you come to operate the interferometer! The whole thing with mounts can be built in an hour or so once the materials have been collected and the saw, drills and other bits and pieces are all to hand.

Parts needed

1. Laser pointer. This should be the kind that's the size of a thick pencil. I've had most success with red laser pointers - see note A below for more details and for notes on safe use of laser pointers.
2. Concave (diverging) lens with a focal length of about 30mm - see note B below.
3. Two small plane (flat) mirrors – the kind used for shaving or putting on makeup – but a couple of broken bits of glass mirror will do just fine. About 25mm (1 inch) across is plenty big enough. (Be careful to avoid sharp edges when handling broken glass.)
4. A piece of clear, flat glass to split the beam from the laser pointer into two beams (a 'beam-splitter') - see note C below.
5. A solid piece of wood about 50cm (18 inches) square for the base. This needs to be nice and rigid, and since you're going to be putting screws into it to hold things in place it should be at least 18mm (three-quarters of an inch) thick - see note D below.
6. A piece of softwood measuring around 50mm x 19mm (2 inches x three-quarters of an inch) and about 60cm (24 inches) long. The surface of this timber should be smooth - in the UK we call timber supplied like this "cut and planed", but local terms may vary. You're going to use this timber to construct mounts for the various components and to fix them to the base, so it's important that the edges are square (at 90 degrees to each other). You're going to need to cut 10 pieces from this timber, each 50mm (2 inches) long.
7. A soft piece of rubber which can be cut into pieces approximately 30mm (1.25 inches) square. Ideally this should about 5mm (a quarter of an inch) thick and should not be too 'squishy' - a neoprene mousemat is perfect for our purposes.
8. Screws for assembly - and possibly some bolts and washers with wingnuts (sometimes these are called butterfly nuts) if you want to build the 'Rolls Royce' version of the interferometer. Read through the instructions below, look at the diagrams and then decide how fancy you want to make things. The exact length of the screws or bolts that you'll need will depend upon the dimensions of the wood you have available.

Construction

Start by constructing the three mounts that you'll need for the two mirrors and the beam-splitter. Each mount is made up of two blocks of the same size - for our purposes here I've made these 50mm x 50mm (2 inches) square using timber that is 19mm (three-quarters of an inch) think. You'll need six of these pieces for the mounts. Each mount uses a block that will be fixed to the base board (Block A) and a similar-sized block (Block B) that will carry the mirrors (2) and the beam-splitter. The two different types of block are shown in the diagram below - notice that the holes you'll need to drill are slightly different for Block A and Block B, and will also depend on whether you're going to hold the blocks together with screws or bolts. There's more information about this choice below. Notice also that each mount has a layer of rubber sandwiched between the two blocks.

If you're going to hold the blocks together with wood screws, the holes in each Block A (including the long hole that's going to hold these blocks to the base board) should have a diameter that's big enough to allow the screw to pass through without too much friction. In this case, the holes in each Block B should be narrower (carpenters refer to these as 'pilot holes') and just enough to allow the screw threads to 'bite'. And of course, you're going to want to use screws that are long enough to go through Block A and into Block B without coming out the other side. For the dimensions shown in the diagram, screws which are 35mm long (around one and a half inches) would be pretty much perfect. Note that the holes in Block B are offset by about 2mm to allow this block to sit slightly above the baseboard - this allows the block to move when you turn the screws to adjust the alignment of the mirror. In order to make it easier to turn the screws to adjust the angle of each block to the laser pointer beam falling on it, it will be a good idea to put a little lubricant into these pilot holes by smearing a little petroleum jelly on each screw as you assemble the mount.

Here's what the assembled mounting blocks, with the mirrors held on by putty, should look like:

(If you choose to use bolts to hold the blocks together, then the holes in each block should have the correct diameter to allow the bolts to pass freely through. It's a good idea to use washers to prevent the head of each bolt and each nut sinking into the softwood mount. As long as the bolts are long enough to go through the two blocks with the rubber layer between them, the length of these is not super-critical.)

Now you're going to make the mount for the laser pointer. This is very similar to the other mounts, and is shown in the diagram below. You'll need to use screws to hold this together as the bottom block has to be screwed to the base board first - the two rubber pads and the laser pointer are then placed on top and held in position by screwing the top block in place. Make sure when you do this that the laser pointer switch is easily accessible so that you can hold it in the 'on' position using the clothes peg.

OK, now it's time to start putting our interferometer together! Start by using a straight edge to draw a pencil line parallel to one of the long sides of your base board, about 50mm (two inches) from the edge. This is going to be the 'y-arm' of our interferometer. Now draw another line at right-angles to the first line and about 20cm (9 inches) from the left hand edge of the base. This will be the ‘x-arm’ of the interferometer. Finally, draw a third line which passes through the intersection of the first two lines at exactly 45 degrees - this is going to act as a guide for mounting the beam splitter. This is all shown in the diagram below. (In case your school geometry is a bit rusty, you can construct a 45 degree angle using a pair of compasses - or just draw a square and join the corners.)

Now let's mount the laser pointer. Screw the bottom block through the hole in its middle to the base near to the left hand end of the y-arm such that its centre-line lies along the y-arm. Put two pieces of rubber, one above the other, onto this piece of wood between the guide holes. Now lay the laser pointer on the rubber so that it points down the y-arm, making sure that the ‘on’-button is forward of the mount and sticks out to the side. Now screw the top block of the laser mount down onto the first part so that the laser pointer is held lightly in place on the mouse-mat and as parallel to the base as you can make it and pointing along the y-arm.

Before going any further we need to get the laser pointer aligned so that the beam is parallel to the base board. To do this we'll use two more of our 50mm x 50mm blocks. Take one of these and draw a pencil line down the middle at right-angles to the edge. Rest this piece on the base with the vertical line touching the pencil line marking the y-arm near the laser pointer; the pencil line should be on the side toward the laser pointer leaving just enough room to mark the face of the block with a pencil. Switch on the laser pointer (keep it switched on using the clothes peg), adjusting it if necessary so that the beam hits the vertical pencil line on the block. Use a pencil to mark the position where the beam hits the vertical line. Now screw down the screws on the laser mount so that the mouse-mat is much more fully compressed. Make another pencil mark where the beam hits the vertical line. These two pencil marks indicate the range of adjustment of the laser beam – the ‘adjustment range’.

Switch off the laser and remove the alignment block with the pencil marks. Clamp it to the other alignment block with the pencil marks visible and so that the edges are exactly lined up. Now drill a vertical hole about 5mm (3/16th inch) in diameter straight down through both blocks. The hole should be centred on the vertical line and in the middle of the adjustment range. It will then be at exactly the same height above the square edge in both blocks. Draw a vertical line on the second block which goes straight down through the centre of the new hole. (If any of this goes wrong you can always do it again starting with new vertical lines slightly offset from the first. I found I had drilled the holes a bit too high so added a third layer of mouse-mat to the laser mount.)

Now put one of the alignment blocks back in front of the laser with the vertical line touching the y-arm line and switch on the laser pointer. Adjust the laser pointer by pushing it sideways while using the screws on the laser-mount for vertical adjustments until the beam shines straight through the hole. Place the second alignment block near the far end of the y-arm line and continue to adjust the laser pointer manually and using the laser mount screws until the beam shines through the second hole too. Moving the second alignment block to the far end of the y-arm line should reveal a bright and undiminished beam, while there should be as little illumination as possible on the side of the two holes through which the beam is shining. The laser pointer beam now passes straight down the y-arm parallel to the base.

Mount one of the pieces of mirror on one of the mirror mounts. Remove the right hand alignment block and replace it with the mounted mirror, screwing it down lightly to the base so that the centre of the mirror is roughly on the y-arm pencil line. Rotate the mirror mount until the beam is reflected directly back along the y-arm – the vertical screw should allow the mount to be turned but not too freely. Now adjust the four screws on the mirror mount until the beam shines directly back through the hole in the alignment block near the laser pointer. It should be possible to see where the beam falls on this alignment block, and you should adjust the screws on the mirror mount until the beam goes back straight back through this hole too.

Now mount the glass to the beam-splitter mount with some putty and screw the mount down to the base board. Place the second alignment block with the vertical line close to the end of the x-arm, with the vertical line touching the x-arm line. Adjust the beam splitter horizontally and vertically until the beam reflected from the beam-splitter passes through the hole in the alignment block on the x-arm. The beam splitter is now at 45 degrees and vertical. Mount the other piece of mirror on the remaining mirror mount and screw this down at the far end of the x-arm. Block off the y-arm mirror with the remaining alignment block and, as before, adjust the x-arm mirror until the reflected beam passes back through the hole in the alignment block close to the laser pointer.

If the lens you obtain is like mine, a simple disk of plastic about 15mm in diameter, it will make things easier if you drill a slightly bigger hole in a piece of thick cardboard or wood about 30mm square and mount the lens inside it using the putty. When you then use more putty to attach the lens-mount to the base, you will see where the beams are hitting the mount and be able to move things around until both beams go through the lens. If you use the lens by itself it is harder to line it up. The positioning of the lens, by the way, is not critical so long both beams go through it; you may need to make some small adjustments to the mirrors etc to make sure this happens.

Mount the screen (a piece of white paper will do) beyond the lens at a distance that gives a good size image on the paper. You may need to wait until dark to get big enough and bright enough images.

The likelihood is that you will see two bright fuzzy patterns on the screen that are close together. Make final adjustments to one mirror mount or the other or the laser pointer until the two patterns coincide. Keep making tiny adjustments until you suddenly see an unmistakable and, possibly, jittery pattern of bright and dark stripes ('fringes'). It won’t be too neat a pattern because we're not using optical glass - but it should be striking.

You might find the pattern jumping around or even blurred if the floor or table are not firm. Play around until the fringes appear. Gently touch the base near to one of the arms and the fringes should move in response to the miniscule distortion. Touch the base near to the other arm and the fringes should move in the other direction. By moving things around very gently you should also be able to change the width of the fringes, even if not hugely.

Now get a mug of hot coffee or tea and place it on the base near to the y-arm mirror but without blocking the light (as indicated by the dashed circles in the interferometer diagram above). The pattern should move steadily to one side as the base expands from the heat of the hot drink because the y-arm is lengthening or bending. When the cup is taken away, the pattern should move slowly back as the base cools. Try the same thing with the cup close to the x-arm - the fringes should move in the opposite direction.

This model shows just how responsive interferometers are to minute relative changes in the arms - even though we have we have used a tiny interferometer made from rough materials. The idea is that the demonstration will help your imagination grasp how much more can be achieved with the enormous and hugely complex LIGO interferometers.

Image Gallery

A final reflection

What is a sociologist doing creating a model interferometer and explaining its wonders (if I have it right – remember I am a sociologist not a physicist)? Well, I’m doing what any good anthropologist or ethnographer or participatory sociologist should do – trying to get inside the world of the natives. On the other hand, this interferometer is very far from that world in most ways and not too much should be read into what has been accomplished. This is not us practising interferometry as it is understood by real physicists - this is like driving a family car to get a sense of Formula One – but it’s useful if you’ve never driven any kind of car. As for how this demonstration of the way a bit of the real world works squares with my position in sociology as a ‘methodological relativist’, an attempt to explain it can found in Gravity’s Shadow, on pages 756-58.

Notes

1. I’ve made the model with red (most successful) and green (slightly more difficult) laser pointers, but when I tried blue I could not get any interference fringes. I don’t know why the colours worked differently; it could be something to do with the design of the particular pointers rather than anything fundamental. Laser pointers usually have a press switch which needs to be kept depressed in order to provide continuous operation, which is what you're going to need here. So you're going to need something like a clothes-peg (clothespin if you're in the US) that fits round the laser pointer in order to keep the switch depressed. This is quite easy to do with laser pointers that are pencil-like in shape.
   Be sure to read up on the safety notes on buying and using laser pointers which relate to where you live - be aware that some laser pointers that are available online do not conform to these regulations and we advise against purchasing or using them. If you're going to use this interferometer in teaching or demonstrating you should carry out a risk assessment in accordance with your institution's policies before doing so.
   • UK - Laser radiation: safety advice (HSA)
   • US - Illuminating Facts About Laser Pointers (FDA)
   • EU - Stricter rules for laser pointers (BfS)
2. I used a plastic concave lens with a focal length of 30mm which gave me decent sized fringes at a distance of about 60cm (24 inches) from the lens.
   
3. In theory the beam-splitter should be made of semi-silvered glass (which allows some of the light straight through while reflecting the rest) but plain glass works well enough for our purposes. A piece of glass that's about 150mm x 100mm (6 inches x 4 inches) taken from a small picture frame (from IKEA or a similar home furnishing store) is ideal. This is far larger than necessary but that’s not a problem.
   
4. For the model I built to test out these instructions I used a shelf from a small IKEA wardrobe. It was veneered chipboard about 50cm x 60cm x 18mm.