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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 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 the Dutch mathematician, astronomer, and physical scientist Christian Huygens. Huygens’ principal 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.

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Increasing use of computers in the early 1960s provided the impetus for the development of operating systems, which consist of system-resident software that automatically handles input and output and the execution of jobs. The historical development of operating systems is summarized below under that topic. Throughout the history of computers, the machines have been utilized in two major applications: (1) computational support of scientific and engineering disciplines and (2) data processing for business needs. The demand for better computational techniques led to resurgence of interest in numerical methods and their analysis, an area of mathematics that can be traced to the methods devised several centuries ago by physicists for the computations they made to validate their theories. Improved methods of computation had the obvious potential to revolutionize how business is conducted, and in pursuit of these business applications new information systems were developed in the 1950s that consisted of files of records stored on magnetic tape. The invention of magnetic-disk led not only to more cleverly designed file systems but also, in the 1960s and '70s, to the concept of the database and the development of the sophisticated database management systems now commonly in use.

Text №3 Computers used in cad/cam

Application programs for CAD/CAM can be run on almost any computer consisting of a central processing unit (CPU), memory and some type of input and output units. The CPU carries out program instructions to perform operations on data Programs and data are stored in memory, and so- called data buses carry signals between the CPU, memory, and input and output devices. Input typically consists of devices for manual data entry such as keyboards, while output is usually graphics on CRTs-or printers and plotters.

Computers commonly are divided into several classes. Speed, accessible memory, and processor are used to classify computers. Both speed and memory depend mostly on word length; the number of bits that can pass at a time on the data bus. Internal word length generally determines processing speed, while external word length determines the amount of main memory that can be directly accessed. Internal and external word lengths are often the same, but may differ depending on the system.

Computers are generally grouped into one of four main classes: microcomputers, minicomputers, mainframes, and supercomputers. Microcomputers are defined by a single-chip CPU, which has either an 8-or 16-bit word length. Main memory sizes range from 64 to 512 Kbytes. Newer micros work with 32-bit length, and can access 1 Mbyte or more of main memory. Most microcomputers used for CAD/CAM are personal computers - general-purpose machines based on microprocessors. Software converts the personal computer into a CAD/CAM workstation. More recently personal computers have become so powerful that they are being applied as turnkey CAD/CAM systems. In addition to personal computers, microprocessors provide processing power in many engineering workstations. These systems are dedicated microcomputers optimized for CAD/CAM applications.