Tinfoil hat
A tin-foil hat is a piece of headgear designed, according to some people, to shield the brain from mind control and/or mind reading by surrounding it with a metallic lining (typically, aluminium foil or tin-foil).
While there have been and still are many people who believe in the actual utility of such devices, the wearing of tin-foil hats has become a popular stereotype and term of derision, particularly in Internet culture. This draws on the stereotypical image of belief in mind control by ESP, microwave radiation or other technological means as part of the symptoms of paranoid schizophrenia. The delusion of "mind control rays" generally seems very real to those thus afflicted, and the making and wearing of improvised defences against the imagined rays is a commonly observed phenomenon in people afflicted by severe paranoid delusions.
There is a small amount of truth or reason to be found in the tin-foil hat story. A well constructed tin-foil enclosure would approximate a Faraday cage, reducing the amount of (notionally harmless) radiofrequency electromagnetic radiation inside. A common high school physics demonstration involves placing an AM radio on tinfoil, and then covering the radio with a metal bucket. This leads to a noticeable reduction in signal strength. The efficiency of such an enclosure in blocking such radiation depends on the thickness of the tin-foil, as dictated by the skin depth, the distance the radiation can propagate in a particular non-ideal conductor. For half-millimeter-thick tin-foil, radiation above about 20 kHz (i.e., including both AM and FM bands) would be partially blocked. The effectiveness of the tin-foil hat in stopping radio waves is greatly reduced by the fact that it is not a complete enclosure. Placing an AM radio under a metal bucket without a conductive layer underneath demonstrates the relative ineffectiveness of such a setup. Indeed, because the effect of an ungrounded Faraday cage is to partially reflect the incident radiation, a radio wave that is incident on the inner surface of the hat (i.e., coming from underneath the hat-wearer) would be reflected and partially 'focused' towards the user's brain. While tin-foil hats may have originated in some understanding of the Faraday cage effect, the use of such a hat to attenuate radio waves belong properly to the realm of pseudoscience.
A notable appearance of tin-foil hats is the 2002 movie Signs where the children of the lead character wear tin-foil hats to avoid mind control.
See also
