Schematic below:
In order, what it does is:
- Light from a coherent source is divided into two beams when incident on the beam splitter.
- The two beams travel towards two mirrors and and reunited at the beam splitter, where they interfere.
- The interference pattern can be viewed directly on the viewing screen.
The 50/50 beam splitter is coated with a nanometre-thin silver coat. This has a low reflectance (only 50%). The rest of the light is transmitted.