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Pepper’s Ghost

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Fig 1: Commercially available screens which both transmit and reflect light
Fig 1

Summary

An optical illusion to show that glass can both reflect and transmit light.

Operation

The original Pepper's Ghost was used to great effect in theatres since the 1860s to create ghostly apparitions. A large plate glass 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.

Arrange a glass of water, a lit candle and a clear pane of glass as in the diagram within a dark box (without a lid) or with a dark background in a dark room. 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 experiment, only a small amount of light is reflected but because of the darkness, this small amount is enough to produce a conclusive effect.

Safety

 CautionIf using a sheet of glass from a window or picture frame, ensure all sharp edges are taped or filed.

Be aware of the dangers of naked flames.

The contents of this page are for information only. Please refer to CLEAPSS or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.

Notes

While John Henry Pepper is regarded as inventing this illusion, some say it should have been called Dirck's Ghost. English engineer Henry Dirck had first created a working model of the effect but it was impractical for stage use.