Article Outline
Optics, branch of physical science dealing with the propagation and behavior of light. In a general sense, light is that part of the electromagnetic spectrum that extends from X rays to microwaves and includes the radiant energy that produces the sensation of vision (see Electromagnetic Radiation; Energy; Spectrum; X Ray). The study of optics is divided into geometrical optics and physical optics, and these branches are discussed below.
Radiant energy has a dual nature and obeys laws that may be explained in terms of a stream of particles, or packets of energy, called photons, or in terms of a train of transverse waves (see Photon; Radiation; Wave Motion). The concept of photons is used to explain the interactions of light and matter that result in a change in the form of energy, as in the case of the photoelectric cell or luminescence. The concept of transverse waves is usually used to explain the propagation of light through various substances and some of the phenomena of image formation. Geometrically, a simple transverse wave may be described by points that oscillate in the same plane back and forth across an axis perpendicular to the direction of oscillation such that at any instant of time the envelope of these points is, for example, a sine function that intersects the axis (see Geometry; Trigonometry). The wave front progresses, and the radiant energy travels along the axis. The oscillating point may be considered to describe the vibration of the electric component, or vector, of the light wave. The magnetic component vibrates in a direction perpendicular to that of the electric vector and to the axis. The magnetic component is ineffective and may be ignored in the study of visible light. The number of complete oscillations, or vibrations (see Oscillation), per second of a point on the light wave is known as the frequency. The wavelength is the linear distance parallel to the axis between two points in the same phase, or occupying equivalent positions on the wave, for example, the distance from maximum to maximum in the case of a sine function representation. Differences in wavelength manifest themselves as differences in color in the visible spectrum. The visible range extends from about 350 nanometers (violet) to 750 nanometers (red), a nanometer being equal to a billionth of a meter, or 4 × 10-8 in. White light is a mixture of the visible wavelengths. No sharp boundaries exist between wavelength regions, but 10 nanometers may be taken as the low wavelength limit for ultraviolet radiation. Infrared radiation, which includes heat energy, includes the wavelengths from about 700 nanometers to approximately 1 mm. The velocity of an electromagnetic wave is the product of the frequency and the wavelength. In a vacuum this velocity is the same for all wavelengths. The velocity of light in material substances is, with few exceptions, less than in a vacuum. Also, in material substances this velocity is different for different wavelengths, as a result of dispersion. The ratio of the velocity of light in vacuum to the velocity of a particular wavelength of light in a substance is known as the index of refraction of that substance for the given wavelength. The index of refraction of a vacuum is equal to 1; that of air is 1.00029, but for most applications it is also taken to be 1.
The laws of reflection and refraction of light are usually derived using the wave theory of light introduced by Dutch mathematician, astronomer, and physical scientist Christiaan Huygens. Huygens’s principle states that every point on an initial wave front may be considered as the source of small, secondary spherical wavelets that spread out in all directions from their centers with the same velocity, frequency, and wavelength as the parent wave front. When the wavelets encounter another medium or object, each point on the boundary becomes a source of two new sets of waves. The reflected set travels back into the first medium, and the refracted set enters the second medium. It is sometimes simpler and sufficient to represent the propagation of light by rays rather than by waves. The ray is the flow line, or direction of travel, of radiant energy, and the assumption is made that light does not bend around corners. In geometrical optics the wave theory of light is ignored and rays are traced through an optical system by applying the laws of reflection and refraction.
This area of optical science concerns the application of laws of reflection and refraction of light in the design of lenses (see Lenses below) and other optical components of instruments. If a light ray that is traveling through one homogeneous medium is incident on the surface of a second homogeneous medium, part of the light is reflected and part may enter the second medium as the refracted ray and may or may not undergo absorption in the second medium.
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Reflection and Refraction
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The amount of light reflected depends on the ratio of the refractive indexes for the two media. The plane of incidence contains the incident ray and the normal (line perpendicular) to the surface at the point of incidence (see Fig. 1). The angle of incidence (reflection or refraction) is the angle between the incident (reflected or refracted) ray and this normal. The laws of reflection state that the angle of incidence is equal to the angle of reflection and that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane. If the surface of the second medium is smooth or polished, it may act as a mirror and produce a reflected image. If the mirror is flat, or plane, the image of the object appears to lie behind the mirror at a distance equal to the distance between the object and the surface of the mirror. The light source in Fig. 2 is the object A, and a point on A sends out rays in all directions. The two rays that strike the mirror at B and C, for example, are reflected as the rays BD and CE. To an observer in front of the mirror, these rays appear to come from the point F behind the mirror. It follows from the laws of reflection that CF and BF form the same angle with the surface of the mirror as do AC and AB. If the surface of the second medium is rough, then normals to various points of the surface lie in random directions. In that case, rays that may lie in the same plane when they emerge from a point source nevertheless lie in random planes of incidence, and therefore of reflection, and are scattered and cannot form an image.
Not all of the light that strikes a mirror is reflected; some of the light can pass through the mirror or be absorbed by the mirror. Many scientists thought a perfect mirror—one that reflects 100 percent of the light that strikes it—could not exist. However, in 1998, scientists made a perfect mirror by stacking up microscopic layers of tellurium and the plastic polystyrene.
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