Similarly, mirrors can be used in optical telescopes or some optical communications systems to collect light and focus it onto special light detectors. In much the same way that the incoming radio or light signals are processed, the outgoing signals can also benefit from specially shaped antennas or lenses. The radio or light source, when positioned at the focal point of a reflector, can shape the outgoing signal into a narrow beam. The larger the antenna or lens, the narrower the beam becomes. A narrow light beam insures that more of the desired signal is directed toward the distant receiver for better efficiency.
Lenses are usually much cheaper, readily available and much easier to align than mirrors.
Useful lenses can be found in hardware stores, bookstores, office supply stores and even grocery stores. All of the discussions in this book will center on the use of lenses, although some of the techniques used for lenses can also be applied to mirrors. Small glass or plastic lenses are great for short range applications. However, glass lenses larger than about 3 inches become too heavy and expensive to be practical. Fresnel lenses can be purchased with diameters ranging from one to more than 36 inches.