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In August 1908, a white mob attacked Springfield, Illinois’s Black community, destroying homes and businesses and lynching two Black men. National outrage over the violence helped spur the movement that created the NAACP the following year.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
Optical path (OP) is the trajectory that a light ray follows as it propagates through an optical medium. The geometrical optical-path length or simply geometrical path length (GPD) is the length of a segment in a given OP, i.e., the Euclidean distance integrated along a ray between any two points.[1] The mechanical length of an optical device can be reduced to less than the GPD by using folded optics. The optical path length in a homogeneous medium is the GPD multiplied by the refractive index of the medium.
Path of light in medium, or between two media is affected by the following:
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Optical path (OP) is the trajectory that a light ray follows as it propagates through an optical medium. The geometrical optical-path length or simply geometrical path length (GPD) is the length of a segment in a given OP, i.e., the Euclidean distance integrated along a ray between any two points. The mechanical length of an optical device can be reduced to less than the GPD by using folded optics. The optical path length in a homogeneous medium is the GPD multiplied by the refractive index of the medium.
In optics, optical path length (OPL, denoted Λ in equations), also known as optical length or optical distance, is the vacuum length that light travels over the same time taken to travel through a given medium length. For a homogeneous medium through which the light ray propagates, it is calculated as taking the product of the geometric length of the optical path followed by light and the refractive index of the medium. For inhomogeneous optical media, the product above is generalized as a path integral as part of the ray tracing procedure. A difference in OPL between two paths is often called the optical path difference (OPD). OPL and OPD are important because they determine the phase of the light and govern interference and diffraction of light as it propagates. In a medium of constant refractive index, n, the OPL for a path of geometrical length s is just Λ = n s . {\displaystyle \Lambda =ns.} If the refractive index varies along the path, the OPL is given by a line integral Λ = ∫ C n d s , {\displaystyle \Lambda =\int _{C}n\mathrm {d} s,} where n is the local refractive index as a function of position along the path C. This can be re-written as Λ = n ¯ | C | {\textstyle \Lambda ={\bar {n}}\left|C\right|} where n ¯ = ∫ C n d s | C | {\textstyle {\bar {n}}={\frac {\int _{C}n\mathrm {d} s}{\left|C\right|}}} is the average refractive index over the path C of geometric length |C|. An electromagnetic wave propagating along a path C has the phase shift over C as if it was propagating a path in a vacuum, the length of which is equal to the OPL of C. For single frequency (monochromatic) light, the phase shift over C is Δ φ = k 0 Λ = k 0 ∫ C n d s {\textstyle \Delta \varphi =k_{0}\Lambda =k_{0}\int _{C}n\mathrm {d} s} where k0 is the vacuum angular wavenumber. Thus, if a wave travels through several different media, the optical path lengths of the individual segments may be added to obtain the total OPL. In wave interference, the difference between the optical path lengths of two coherent waves (for example, a laser beam split into two paths by a beam splitter) determines the corresponding phase difference at their common destination, and thus the corresponding interference patterns. For a monochromatic wave emitted from a point source, a wavefront is a surface of constant phase. In geometrical optics, this means that the optical path length from the source to each point on a given wavefront is the same, up to an integer multiple of the wavelength. Fermat's principle states the physical ray path is one for which the optical path length is stationary with respect to nearby paths. In many elementary cases, this means that the path light takes between two points is the path that has the minimum OPL.
Differential interference contrast (DIC) microscopy, also called Nomarski interference contrast (NIC) or Nomarski microscopy, is an optical microscopy technique used to enhance the contrast in unstained, transparent samples. DIC works on the principle of interferometry to gain information about the optical path length of the sample, to see otherwise invisible features. A relatively complex optical system produces an image with the object appearing black to white on a grey background. This image is similar to that obtained by phase-contrast microscopy, but without the bright diffraction halo. The technique was invented by Francis Hughes Smith. The "Smith DIK" was produced by Ernst Leitz Wetzlar in Germany and was difficult to manufacture. DIC was then developed further by Polish physicist Georges Nomarski in 1952. DIC works by separating a polarized light source into two orthogonally polarized mutually coherent parts which are spatially displaced (sheared) at the sample plane, and recombined before observation. The interference of the two parts at recombination is sensitive to their optical path difference (i.e. the product of refractive index and geometric path length). Adding an adjustable offset phase determining the interference at zero optical path difference in the sample, the contrast is proportional to the path length gradient along the shear direction, giving the appearance of a three-dimensional physical relief corresponding to the variation of optical density of the sample, emphasising lines and edges though not providing a topographically accurate image.
A microscope is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisible to the eye unless aided by a microscope. There are many types of microscopes, and they may be grouped in different ways. One way is to describe the method an instrument uses to interact with a sample and produce images, either by sending a beam of light or electrons through or onto a sample in its optical path, by detecting photon emissions from a sample, or by scanning across and a short distance from the surface of a sample using a probe. The most common microscope (and the first to be invented) is the optical microscope, which uses lenses to refract visible light that passed through a thinly sectioned sample or to refract light reflected from the surface of an object to produce an observable image. Other major types of microscopes are the fluorescence microscope, electron microscope (both the transmission electron microscope and the scanning electron microscope) and various types of scanning probe microscopes.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Mae Jemison, aboard Space Shuttle Endeavour in 1992.
