Nature of Light
Optical Spectrum: The field of optics began with descriptions of the part of the electromagnetic spectrum which the human eye can detect. Later studies extended this description to shorter (ultraviolet) and longer (infrared) wavelength portions of the spectrum. Later still, the optical spectrum was shown to be part of a the general electromagnetic spectrum. Optical wavelengths include wavelengths between about 1 nm and 105 nm and interact with materials in similar ways. For instance, the physical mechanisms behind many optical sources are electronic and molecular transition and the methods of beam control often depend on reflection and refraction from interfaces. Recall that wavelength (as defined in a vacuum) and frequency are related by l f = c. Light is subdivided into the ultraviolet, visible, and infrared bands. There are not precise wavelength divisions between these bands. The wavelength divisions and spectrum can be seen in Figure 1 and �Figure 2 respectively.
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Response of the Human Eye
Chromatic Response (in order of increasing wavelength) Violet, Blue, Green, Yellow, Orange, Red | |
| The human eye responds to light in the visible part of the spectrum. The sensitivity of the eye peaks in the “green” portion of the visible spectrum around a wavelength of 500 nm and falls off toward the edges of the visible spectrum. The limiting wavelengths of about 450 nm and 700 nm are typical values; the actual response will differ for individuals and as a function of age. The color response of the eye is determined by wavelength. A given wavelength will be perceived as a single color, however, the same sensation of color can be produced by a combination of wavelengths. The photosensors in the eye are called rod and cones due to their physical shape. The cones are concentrated on the back of the eye and are provide our color discrimination. The rods are more distributed and provide sensitivity to movement and low light. The physiology of vision is not fully understood and will not be further discussed. Light measurement for illumination is weighted by the spectral or chromatic response of the “average” young human eye (and thus only applies to the visible spectrum) and is called photometry with units of luminous intensity being the candela. The measurements of light in the context of this course will be independent of the human eye response and is called radiometry with units of luminous intensity being watts per steradian. | |
Wave-Photon Duality: Light is unusual in that it readily displays both wave-like and quantum behavior. Radio frequencies exhibit wave-like behavior primarily since the wavelengths are large and the quantum energies are extremely small. X-rays exhibit largely quantum particle-like behavior since the wavelengths are extremely small and the energy packets are large. Optical wavelengths display both behaviors. Many of the properties of light can be adequately described by waves. Notable exceptions include some cases of emission and absorption. In particular, photoelectric emission led scientists to introduce the concept of photons which was part of the development of the quantum mechanics. Also, the operation of laser diodes and photodiodes can only be explained using a quantum description of semiconductor and light. The quantum energy Eq = hf = hc/ l where h (Planck’s constant) = 4.136 x 10-15 eV-sec = 6.626 x 10-34 J-sec. A comparison of the parameters and representations are given in Figure 4. | |
Ray Optics - a geometric representation of the behavior of light (also called geometrical optics) which corresponds to the limiting case of l ® 0. Electromagnetic (EM) Optics - or physical optics, an electromagnetics representation of the behavior of light using Maxwell’s equations (limiting case as photon number approach 8). Quantum Optics - the most general representation of the behavior of light in terms of photons, i.e. radiant energy packets, using quantum mechanics. | |
| The ray description of light is usually limited to simple analysis of reflective and refractive optical elements. The electromagnetic description is generally adequate for a wide range of analyses including general propagation, interference, diffraction, and wave-guides. The wave or scalar wave description of light is a subset of electromagnetic optics and applies to cases in which polarization effects are negligible. The quantum description is the most general, but is less intuitive and more complex. It is required for the analysis of lasers and most detectors. | |
Optical Phenomena: The characteristics of light may be changed as it propagates through medium or as it falls on surfaces (interfaces between media). The changes may be active in which the properties of the media depend on an external parameter such as an external field. Passive changes depend on properties inherent to the media itself. The changes include variation in propagation direction, reorientation of polarization state, energy loss, and energy gain.
Changes in the Direction of Light Reflection - the specular (mirror-like) return of light as it is incident on a different medium. Refraction - the bending of obliquely-incident light as it passes into a different medium. Diffraction - deviations from rectilinear paths not due to reflection or refraction. Scatter - change in the spatial distribution of a wave from interaction with a surface or heterogeneous medium. (Usually applied to interaction with microirregulaities in a surface or medium that are described statistically.)
Other Passive Changes Interference - systematic attenuation and reinforcement of overlapping waves. (This effect does not require a medium and may occur in vacuum.) Retardation – the reduction in phase velocity while propagating in a medium. (The velocity becomes c/sqrt( e r ) = c/n in a dielectric). Dispersion – the separation of wave components due to variation in velocity with l . Optical Activity – the rotation of polarization from interaction with a medium.
Changes in the Irradiance of Light Absorption - loss due to energy conversion as light passes through a material (this effect is usually passive, but may be active). Amplification - gain; increase in the irradiance of a wave as it transverses media without significant distortion of the wavefront. (The associated medium is called an active medium.) Emission - conversion of energy into light.
Other Active Changes Electro-optic (EO) - the technology which uses applied low-frequency electric fields to control optical radiation. (It can also refer to devices which convert electrical signals to optical signal and vice versa such as laser diodes and photodiodes). Acousto-optic (AO) - the technology which employs the interaction between sound waves and optical radiation typically to control the direction of propagation. Magneto-optic - the technology which uses applied magnetic fields to control optical radiation. Nonlinear optics (NL) - the technology in which optical radiation has complex interactions with material and can produce amplitude-dependent behavior, can alter its frequency, etc. | |
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