What does it do?
It is used to convert a single laser line into many and bends them such that they are parallel to each other.

What can one of these be used for?
It can demonstrate convergence of light in glass block analogues of lenses.

Why did I make it?
A physics teacher in the school I work in as a lab technician handed me a tear out of a kit costing £180 and asked me to make something similar. I think I was quite successful at it so I thought I share it with the world. This can demonstrate what the other kit can at the fraction of the price. See the next step for bits you would need.

Step 1: Things You Need

I had the things around, but I had a look for similar things that I used.
You will need:
A laser line leveller: the one I used cost around 5 British Pounds (GBP) on eBay.

Diffraction grating: I had used one that had 300 lines per mm. There is one on ebay that is 500 lines per mm that cost 3 GBP: http://bit.ly/11WEmLP.
The angle at which the laser beams diverge after the slit would be slightly different with the 500 lines per mm, but it should still be fine.

Flexible mirror: The one I had used was similar to this one: http://bit.ly/1asw6LB. There's 8 in the pack here but you only need one, and that would probably be cut down. This pack would cost around 3 GBP.

Retort clamp: These are the clamps used in science chemistry labs. http://bit.ly/1asxQEv, This would cost you around 10 pounds. There's many different types but this one is probably better, don't get one with 3 'fingers' and no flattish bit as this may warp the mirror in unwanted ways.

Cardboard to make a box for it. Half a metre square should be enough. Otherwise find a shoe box to adapt.

Step 2: Set Up the Equipment

Sorry that I only have pictures after I had made the box for it. Nevertheless you would still get an idea on how to arrange the bits.

The diffraction grating is placed in front of the laser, this splits the beam into many that diverge. Open the clamp wide enough for the mirror to fit in. If the mirror is too big use a pair of scissors to cut it to a smaller rectangle. The mirror is best cut to the same height of the clamp. I unscrewed the rod from the back of the clamp to save space when fitting it into a box. Tighten the screw on the clamp so that the mirror is slightly bent inward (concave). Put the clamp and mirror on the table in front of the diffracted beams and rotate it slightly so the beams go off to the side of the laser. Adjust the screw so it changes the curvature of the mirror and bring the beams parallel.

You have your parallel beams, but as there a lot of beams reflecting all over the place, it would be a good idea to encase the whole thing in a box with a lid. I made my own box but you could put it in a shoe box. You would need to cut a hole for the diffraction grating and a slit for rays to come out. The exit hole should only be from the base to 2 or 3 cm above that, and wide enough for at least 3 beams to pass through. You would need to stick the diffraction grating to the box. If you want to make your own box, arrange the bits such that it gives you parallel lines and sketch around it so you have an idea where everything will sit. I made mine in a diamond shape to keep the clamp held in place. I made a slit where I could feed the screw from the clamp through, allowing me to adjust it without lifting the lid.

I could not yet source the glass blocks needed for the demonstrations but I'll have a look around for it. Hope these instructions help you to make this, any comments and questions are welcome.