Everyone knows that you can see rainbow on the back of a CD disc. This phenomenon is called interference and it's happening because the white light reflects off small ridges in the disc. The small ridges are produced by red laser that burns into CD layer. The burned areas represents digital 0 and the unburned areas represents digital 1, and that's how all data are written on CD case.

The ridges in the CD disc are very small and the surface of the CD behaves like a thin film. Thin-film interference occurs when incident light waves, reflected by the upper and lower boundaries of a thin film, interfere with one another to form a new wave. The path length varies with the angle of incident light and create differences for the internally and externally reflected rays at different points on the CD surface.

The Rainbow contest inspired me to design and build a small compact CD rainbow projector. Something that can be placed anywhere in your house and project rainbow anywhere you wish. In this Instructables I will show you step by step how to make such a projector.

Supplies

- Mini CD disc (don't use DVD or Blu-ray)

- 6 x M4x20 bolt

- 4 x M4 wingnut

- 2x M4 hex nut

- 2 x M3x15 bolt

- 2 x M3 hex nut

- Mini torch (keying torch)

- PLA

- Superglue

Tools:

- FDM 3D printer

- Screwdriver

Step 1: CAD Design

I have started to draw my idea of rainbow projector in Fusion 360, which is my favourite CAD software for quite few years now. First version was made for regular size CD, but the whole device was a little bit too large compared with what I imagined.

I have realized then that I had a mini CDs, which I haven't used for ages. I found one and, after few adjustments to the device, I had a ready prototype. The device have ball head, which you can rotate at any angle, and then fix in desire position. It also have mini torch that is mounted on arms. It can be adjusted too, so you can shine the light at various positions.

After initial test print I had to go back to Fusion 360 to make minor corrections. It took about 5-6 hours to design the model and about 3-4 hours to print all necessary parts.

Step 2: Slicing and 3D Printing

I recommend to use Cura for slicing model, but if you have your favourite slicing software that works well, there is no reason not to use Cura.

I have printed model in PLA, with brim (optional) and 20% infill was more than enough. It takes just over 4 hours to print main body with 0.2mm layer resolution which give really good finish. If you wish to save a little bit of time, you could use 0.26mm layer resolution and 15% infill,

Step 3: Assembly Part 1

First step is to use superglue and glue two CD holder parts together (ball head). After that, you can use sand paper to smooth the surface a little bit. This will ensure smooth movement once the ball head is inside the base.

Now you need to use soldering iron and sink couple of M4 hex nuts inside the 3D printed parts. You need to embed M4 hex nut in 2 different parts, first one is 'CD_holder_part_1 and CD_holder_part_2 and the second one is 'projector_base_part_1'.

Place your hex nut over the hole in the plastic part and then use hot soldering iron to heat up the nut. After few seconds you should see nut slowly sinking into 3D printed part. You may need to push it slightly to make sure it's fully embedded inside your plastic parts (see picture above).

Lastly, using same technique you have to heat sink M4 bolt into M4_bolt_cover 3D printed part. In this step use pliers to hold the bolt and than heat it up and push inside the printed plastic part (see picture above).

Step 4: Assembly Part 2

- Put two base parts together (blue parts) and secure them with M3 bolts and nuts on both sides. Remember to put glued ball head inside (dark green part).

- Screw the M4 bold with 3D printed cover and lock ball head in place for now

- Assemble the torch arms (yellow and orange parts) and use M4 bolt and wingnuts. Thanks to wing nuts you will be able to quickly change the position of the torch.

- Install the torch holder (dark red part) at the end of the arms

- Place mini CD on the ball head (dark green part) and add the 3D printed washer (grey part) and then secure it with M4 bolt.

Step 5: Setting Up Your Device and Test

- Insert the mini torch into torch holder (dark red part) and secure it with M4 bolt with wingnut

- Adjust the arms (yellow and orange parts) so the torch will shine directly on the CD disc (see picture above)

- Switch on the torch

- Adjust the ball head with CD disc and try to find position when the reflected light will project rainbow on the wall

- Secure ball head with the M4 bolt

Step 6: Rainbow!

On the picture above you can see various rainbow patterns I have managed to create using this mini rainbow projector. By changing the angle of the CD, the angle and height of the torch and the distance of the device from the wall, you can create different rainbow patterns.

You can also play with the torch. The one I used (which I shared a link with you) has 5 white LED's, but you can try to find one with just one LED or classic filament mini bulb.

If you wish to experiment more you can try to use CD-RW, DVD or Blu-ray as all of them have different density (distance between 0 and 1) and will produce slightly different patterns.

Lastly, you can replace mini CD with different materials like aluminium foil which will give interesting patterns. You can also use Petri dish instead of CD, just glue it to the ball head and pour small amount of water with wash-up liquid and see what happens :)

Happy experimenting!