We live in a world immersed in all manner of electromagnetic energy. Portions of this energy can be directly sensed by one or more of our biologic senses. Visible light and radiant hear are two forms which are daily sensed. Other forms of electromagnetic energy can not be sensed directly. Among these are non-visible light (Ultraviolet and Infra red), penetrating radiation (X-rays, gamma rays, atomic particles) and radio waves (microwaves, etc.). This energy can only be sensed indirectly. The most sensitive and sophisticated biological sensor is the eye. The word "optics" originally pertained to the eye, or sense of vision. Soon it also came to include things which aid the eye, composed of lenses, mirrors, prisms, etc. Historically, things associated with vision received the most careful scientific investigation. As the investigation continued, more and more forms of energy were detected, but it took a long period of time to fit the puzzle together and conclude that all of the forms of energy were alike, and that the only distinguishing element was wavelength. This ultimately led to the expanded electromagnetic spectrum. The Nature of Light Until about the middle of the 17th century it was generally believed that light consisted of a stream of corpuscles. These corpuscles were emitted by light sources, such as the sun or a candle flame, and traveled outward from then source in straight lines. They could penetrate transparent materials and were reflected from the surfaces of opaque materials. When the corpuscles entered the eye, the sense of sight was stimulated. If the test of the adequacy of any theory is its ability to account for the known experimental facts with a minimum of hypotheses, the corpuscular theory was an excellent one. The theory was called on to explain 1) why light appeared to travel in straight lines, 2) why it was reflected from a smooth surface such as a mirror with the reflection equal to the angle of the incidence, and 3) why and how it was refracted at a boundary surface such as that between air and water or air and glass. for all of these phenomena, a corpuscular theory provided a simple explanation. By the end of the 17th century, Christian Huygens showed that the laws of reflection and refraction could be explained on the basis of a wave theory and that such a theory furnished a simple explanation of the recently discovered phenomenon of double refraction. This theory failed at the time because it was objected that if light were a wave motion, one should be able to see around corners since waves can bend around obstacles in their path. We know that the wave lengths of light waves are so short that the bending, while it does occur, is so small that it is not ordinarily observed. As a matter of fact, the bending of a light wave around the edges of an object, a phenomenon known as diffraction, was noted as early as the middle of the 17th Century, but the significance was not realized at the time. It was not until 1827 that the experiments of Thomas Young and Augustin Fresnel on interference and the measurements of the velocity of light in liquids demonstrated the existence of optical phenomena for whose explanation a corpuscular theory was inadequate. Young's experiments enabled him to measure the wave lengths of the waves and Fresnel showed that the rectilinear propagation of light, as well as the diffraction effects, could be accounted for by the behavior of waves of short wave length. In 1842, Joseph Henry, Professor of Natural History at the Princeton University, studied the discharge of charged leyden jars through coils of wire and discovered that electrical oscillations were taking place. He theorized that these oscillations could give use to electromagnetic waves The discovery by Hans Christian Oersted of the magnetic field around a current-bearing wire had united the two previously unrelated subjects of electricity and magnetism. Much work was done trying to show a relationship between light and magnetism. Finally, after 20 years Michael Faraday of the Royal Institution in London showed that when a beam of plane-polarized light was passed through apiece of lead glass in a direction parallel to the lines of force of the magnetic field, the plane of polarization was rotated. The next great step forward in the theory of light was the work of the Scottish scientist, James Clerk Maxwell. In 1873, he showed that an oscillating electrical circuit should radiate electromagnetic waves. The velocity of propagation of the waves could be computed from purely electrical and magnetic measurements and it turned out to be very nearly 3 X 108 m/sec. Within the limits of experimental error, this was equal to the measurement velocity of propagation of light. Fifteen years later, Heinrich Hertz, using an oscillating circuit of small dimension, succeeded in producing short wave length waves (we call them microwaves) of undoubted electromagnetic origin and showed that they possessed all the properties of light waves. Maxwell's electromagnetic theory of light and its experimental justification by Hertz constituted one of the triumphs of physical science. These new indirect detectors extended the range of the human eye. Photographic plates, thermopiles, and bolometers extended the range to include Ultraviolet and Infra red non-visible electromagnetic radiation. Wilheim Konrad Roentgen discovered X-radiation while investigating low pressure discharges in cathode ray tubes. He wanted to detect any cathode rays coming out of the tube, so wrapped then tube in black paper and coated the tube with Barrium-Plantino-Cyanide which would fluoresce if any rays came through the Tube. He accomplished this and noted that the rays came from the anode of the tube rather than from the cathode. He called these new rays X-rays. Sources of Electromagnetic Waves Spectrum Analysis of Emitted Radiation The emission of radiation from a source is not uniform over the whole spectrum, but has a characteristic emission spectrum. If we combine a detector with equal responses to all frequencies with a narrow, tunable bandpass filter, we can measure relative emission amplitude as a function of frequency (wavelength). In obtaining such a spectral representation, we can tell a great deal about the source of the electromagnetic radiation. The visible light is a very small portion of the electromagnetic spectrum. In light, we talk about wavelength rather than frequency 
A. The Wave Concept 
The radiation from excited atoms does not in general form a coherent radiation field. This is in general due to the fact large numbers of atoms are involved and act independently in producing the radiation field. Thus, with the light from fluorescent bulbs and tungsten filaments, the excited atoms is incoherent. The radiation from electronic oscillators and laser oscillators is coherent.
Given any point on a propagating wave, whether or not we will be able to predict when the next peak occurs will depend on the degree of temporal and spatial coherence of the field. There are many sources of coherent radiation in the R.F. spectrum, but until the advent of the laser oscillator, there were no coherent sources in the light spectrum. C. The Wave Nature of Light Because light is a wave, when two coherent waves exist in the same region of space, constructive and destructive interference will occur in proportion to the degree of coherence. When a wave passes around an obstacle, the wave "diffracts" or bends some what. When light passes through a small opening, a diffraction pattern is observed. In general, both diffraction and interference occurs simultaneously. Since light energy will propagate between two points in space or from a source to a distant point in space, we may employ the light energy as a means of communication between the source and the distant point. We can then indicate the basic requirements for an optical communications system. The transmission of light energy between two points does not in itself constitute a communication of intelligence. It is only when a signal intelligence modulates the transmission of the energy that communications is accomplished. In addition, it is only when this modulated energy is converted back to the original signal intelligence the communications process is complete. Let's consider a simple, usable source of light energy, an incandescent tungsten filament. A tungsten filament is heated by a direct current to incandescence. The bias current passes through a modulation transformer which supplies power from a modulation amplifier to raise or lower the temperature of the filament in step with the signal intelligence. The ability of the emitted light intensity to follow the modulation as a function of frequency is limited by the thermal time constant of the filament. The emitted light intensity is a function of T5, hence a small change in temperature results in a large change in intensity. In linear systems the bandwidth is approximately  where Tr = rise time to a step input. If the temperature time constant is .1 seconds, then the intensity time constant is (.1)5 or 105 seconds.  Thus, such a system can pass a signal intelligence of 50,000 Hz/s. To complete the communication system, it is only necessary to illuminate a photo electric transducer with the received energy and amplify the transducer output to a sufficient level for reproduction. Every propagating electromagnetic wave can be described by stating its frequency or wavelength, its amplitude, and the plane or planes of the electric field vector. In particular, the propagation of light through many materials is strongly dependent upon the polarization of the wave. If an electromagnetic wave is randomly polarized, then the electric field vector has components in all planes. If the wave is plane polarized, then the electric field vector lies in only one plane. Many materials will propagate a wave with out attenuation in only one preferred plane and will linearly attenuate a wave oriented from 0 to 90° to the preferred plane. In addition, the preferred plane may be a function of the electric or magnetic fields impressed across the material. By utilizing this effect, electronic modulation can be accomplished. The advantage of this system is that the modulation and generation of light are mode separate functions and result in bandwidths of many gigacycles. D. Optical Antennas In addition to the generation and detection of light energy, for a practical system, the energy must be efficiently transmitted and received. To do so, we employ lenses, mirrors and prisms to collect, focus, and collimate light energy. Most of the laws of optics can be reduced to geometry and the laws of geometric optics. Let's postulate an ideal isotropic point source of light.  Ideal point source An ideal point source emits light in all directions isotropically. Using this basic idea, we can then examine the laws of simple lenses and mirrors. Basic Lens Laws: Lens equations for simple lens.  Then a simple lens will image the light from an object placed at the focal distance at ƒ, conversely light coming from ƒ will be imaged at the focal point. We can then use a simple lens as an effective means of transmitting and receiving light energy. Converging Mirrors For a given size, it is both cheaper and easier to grind a one sided mirror than a two sided lens. The same law applies to converging mirrors. Photo Electric Transducers Devices which convert light energy into electrical signals suitable for amplification and/or demodulation. |