<Dd> One - way windows can be made to work with polarized light in the laboratory without violating the second law . This is an apparent paradox that stumped some great physicists, although it does not allow a practical one - way mirror for use in the real world . Optical isolators are one - way devices that are commonly used with lasers . </Dd> <P> With the sun as light source, a mirror can be used to signal by variations in the orientation of the mirror . The signal can be used over long distances, possibly up to 60 km on a clear day . This technique was used by Native American tribes and numerous militaries to transmit information between distant outposts . </P> <P> Mirrors can also be used for search to attract the attention of search and rescue helicopters . Specialized type of mirrors are available and are often included in military survival kits . </P> <P> Microscopic mirrors are a core element of many of the largest high - definition televisions and video projectors . A common technology of this type is Texas Instruments' DLP . A DLP chip is a postage stamp - sized microchip whose surface is an array of millions of microscopic mirrors . The picture is created as the individual mirrors move to either reflect light toward the projection surface (pixel on), or toward a light absorbing surface (pixel off). </P>

When did they stop using silver in mirrors