A glance in the distance

When you look into that 'window' you see a long tunnel with lamps. In reality it is a shallow box with two mirrors; one of them is at the bottom and reflects everything and the other mirror placed at the front and is half-transparent. Behind the front window, a line of lamps shines.

The image of those lamps is reflected in the bottom mirror and the image of this image is reflected in the front window creating an impression of an endless line of lamps.

A laser tube operates on the same principle. It ends with two mirrors; one of them reflects all the light/beams and the other only some of them.