MadSci Network: Physics

Re: what is geometrical optics, please explain using example of 'talking head'

Date: Thu Jan 5 02:13:41 2006
Posted By: Zehra Sarac, Post-doc/Fellow, Optics, Gebze Institute of Technology
Area of science: Physics
ID: 1132019342.Ph


I think this information can be helped you. But teaching is very hard. 
Maybe you can view this web city (
and you can learn how you can teach it..


Talking Head
Reflections from a mirror mounted beneath a table give the illusion that a 
disembodied head is sitting on top of the table. 
table, about 1-meter-square with circular hole near the back 
mirror to fit diagonally between opposite legs of the table 
table cloth reaching to the floor on all sides 
shag carpet or straw (optional) 
The mirror is mounted underneath the table so as to cover the whole space 
from the under side of the table top to the floor[1]. A shag carpet or 
straw on the floor helps conceal the bottom edge of the mirror. Near the 
back of the table in the center and behind the mirror, a circular hole is 
cut through the table top of sufficient diameter to pass the head of the 
subject. The table is initially covered with a cloth that reaches to the 
floor on every side. 
A volunteer is taken from the audience around to the back of the table by 
an assistant who helps the volunteer kneel or sit with the head protruding 
through the hole in the table while the demonstrator, who at this point 
emulates a magician, holds the tablecloth in such a way as to conceal from 
the audience what is happening to the volunteer. A little patter about the 
similarities and differences between science and magic fits in well here. 
A few grunts and groans from behind the cloth add to the drama. Finally, 
when the volunteer is in place, the cloth is removed, and the audience is 
presented with the illusion that a disembodied head is resting on the 
table. One can carry on a conversation with the head, culminating perhaps 
in a pun about restoring the body while the victim is "ahead." In a magic 
show, the volunteer would be removed from the table, and the audience 
would be left to wonder how it was done. However, in a science 
demonstration, the volunteer should be removed in view of the audience 
while the demonstrator explains the trick. 

The illusion is most effective if the mirror is very clean and its edges 
are concealed. The table should be placed well away from other 
obstructions that would be eclipsed by the mirror. If a carpet is used, it 
must be aligned carefully so that there is no discontinuity at its edge 
where it goes behind the mirror. One should be careful not to stand 
directly in front of the table lest the reflection of one's legs be seen 
in the mirror. The table should not be placed too close to the audience to 
avoid reflections of the audience. The illusion works best if the audience 
is seated slightly above the level of the top of the table. The proximity 
of the audience to the head would seem to favor discovery of the trick, 
but, on the contrary, it is indispensable to its success. 

Although this demonstration is more amusing than educational, it serves to 
introduce, motivate, and illustrate the idea that the angle of incidence 
is equal to the angle of reflection in geometrical optics. It also 
illustrates the danger of being deceived in making observations of nature 
and of the importance of considering all possible explanations of a 
phenomenon before reaching a conclusion. 
There are no significant hazards with this demonstration. Care should be 
taken when moving the mirror to avoid breaking it, and the volunteer 
should be cautioned not to lean or push against the mirror. 
1. D. H. Charney, Magic, Strawberry Hill Publishing Company: New York 

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