The original Pepper's Ghost was used to great effect in theatres since the 1860s to create ghostly apparitions. A large glass plate was mounted at the front of the stage at a 45° angle which reflected brightly lit objects placed off stage essentially super-imposing these objects with the set and actors.
A common example of a 'Pepper's Ghost' can be seen when looking out of a window on a dark evening, a lamp or bright object in the room will seem to 'float' in the garden showing that the glass not only allows you to see through it but is also reflecting light from the object in the room.
This practical involves using a glass of water, a lit candle and a clear pane of glass at 45 degrees to the viewer within a dark box (without a lid) or with a dark background in a darkened room. The candle needs to be one side and the beaker the other side of the glass. At the correct angle, the lit candle will seem to be inside of the glass of water.
When light travels from one medium to another (such as from air to glass) part of the light is reflected from the surface and the other transmitted through the medium. The amount reflected depends upon the 'refractive index' of the medium and also the 'angle of incidence' In this practical, only a small amount of light is reflected but because of the darkness, this small amount is enough to produce a good effect.