This is an optical prism made of acrylic glass and filled with water.

Despite it's far to be a perfect prism, you can use it for personal or classroom basic optical experiments.

As it is filled with water, its refractive index is of about 1.33.

If it was entirely made of glass or acrylic glass, its refractive index would be about 1.5.

Actually, in this prism, four refractions occurs :
1 - air (1.0) to glass (1.5)
2 - glass (1.5) to water (1.33)
3 - water (1.33) to glass (1.5)
4 - glass (1.5) to air (1.0)

Step 1: What Do You Need ?

- a board of acrylic glass
- tools required to cut acrylic glass
- transparent adhesive tape (scotch-tape)
- something to glue acrylic glass
- a tool to drill acrylic glass
- a rubber stopper (or anything that will do the job)
- a funnel ( see here if required : https://www.instructables.com/id/Improvised-Tiny-Funnel/ )

Note :
- It is, of course, possible to use real glass instead of acrylic glass. Though, it would be difficult to drill it.

Step 2: Three Rectangles ...

- Cut three rectangles of the same size.

Mine are squares of 9 by 9 centimeters, and I used a manual saw.
If you use an electric saw, be careful : acrylic glass may melt because of the friction.
Of course, it must be possible to use classical glass cutting techniques.

- Chain your rectangles with transparent adhesive tape.
And build-up your prism.

The adhesive tape should be inside, as you're supposed to glue the external edges.

- Glue the edges.

Be careful or you'll get leaks !

- And let it dry.

Once dry, it is possible to leave adhesive tape inside the prism. It does not really matter as long as it is transparent and don't cover too much of the surface.

Step 3: Two Triangles ...

- Cut the two triangles that will cap your prism.

- Drill a hole into one of the two triangles.

Actually, it is possible to drill a hole in both triangles. This would help to easily flush the prism if required.
Make the hole near one of the points. This will help to easily evacuate an eventual bubble of air.

- Glue them on each side of your prism.

- Let it dry.

Step 4: Fill It ...

- Fill it slowly with water.

I recommend not to fill it directly under the faucet (water tap). If you don't keep it dry, you may find difficult to detect eventual leaks.
You could use a funnel. If you don't have one, you could improvise one like I did.
It is also recommended to fill it with water that does not contains dissolved air (more about that in step #5).

- Plug the rubber stopper into the hole.

Try not to create a big bubble of air when doing so.

- Dry it if required, and abandon it on a dry table for few minutes to be sure there is no leaks.

In case of eventual leaks, try to fill them with glue ... Empty your prism if required ...

Step 5: Eventually ...

Eventually, lots of tiny bubbles will appears against the inner surfaces of your prism.

They appears because of the dissolved air into the water. Patrick Pending recommend to let the water sit overnight before filling the prism with.
According to the " Make crystal clear ice ! " it is possible to get rid of the air contained into the water by boiling and letting it cooling several times.

Though, if you don't want to bother boiling water etc, I found it's possible to get rid of most bubbles (once they appeared) by shaking the prism. All the tiny bubbles will join to form a big bubble that you'll eventually evacuate through the rubber stopper.

Step 6: Congratulations !

Voila ... It's ready for use.

Later, I will make an instructable about how to decompose light with this prism.
The pictures are ready, but it's time to sleep for me now ... =o)