Without applications of physics such as counter-weighted sets and backdrops, inclined planes, stage lighting instruments, and other mechanisms for deus ex machina, dramatic productions would revert to the words only—fine for Shakespeare and Becket, but not good for audiences who are accustomed to experiencing plays with the eye as well as the ear. Pepper's Ghost is a 19th-century stage illusion, based on basic optical principles, that can find its way into your introductory classroom.

1.

The fraction of the light reflected at various angles from the air-glass interface can be calculated using Fresnel's equations, which are discussed in all upper-division optics textbooks. The fraction also depends on the polarization of the incoming light. Fortunately, at angles of incidence near Brewster's angle of about 57°, the amount of light reflected by the component of the optical signal oriented with its electric field vector in the plane of incidence is fairly small; most of the reflected light comes from the component perpendicular to this plane, thus making the calculations somewhat easier.

2.
Thomas B.
Greenslade
 Jr.
, “
Nineteenth century textbook illustrations XVI: Illusions with unsilvered mirrors
,”
Phys. Teach.
15
,
360
361
(
Sept. 1977
).
3.
Gaston
Tissandier
,
Popular Scientific Recreations
(
Ward, Lock, and Co.
,
London
, n.d.), p.
138
. This book was originally published in French in
1880
.
4.
G. A.
Wentworth
and
B. A.
Hill
,
A Textbook of Physics
(
Ginn & Co.
,
Boston
,
1898
), p.
385
.
5.
Ref. 3, p.
139
.
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