<text><span class="style10">ptics (5 of 5)</span><span class="style7"></span><span class="style10">LASERS</span><span class="style7">The term `laser' is derived from the technical name for the process - Light Amplification by Stimulated Emission of Radiation. Stimulated emission is the emission of a photon - a particle of light.When an amplifying material, such as a gas, crystal or liquid, is placed between appropriate mirrors, photons from a light beam repeatedly pass through it stimulating more photons and thus increasing their number with each pass. The additional photons all have the same frequency, phase and direction. One of the mirrors is made so that a small amount of light passes through it; this is the external laser beam, which can be continuous or pulsed. This beam can be focused onto very small areas and the intensity - the ratio of power to area - can be very great, enabling some lasers to burn through thick metal plates. Lasers have a wide variety of uses, for example in surveying, communications and eye surgery.</span><span class="style10">HOLOGRAMS</span><span class="style7">A hologram is a `three-dimensional' or stereoscopic image formed by two beams of light. Holography differs from conventional photography in that both the amplitude of the light and its phase (a measure of the relative distance the light has traveled from the object) are recorded on the film. It has many scientific uses as well as the more familiar display holograms now being used, for example, on credit cards.</span></text>
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<text><span class="style10">ecording a hologram. </span><span class="style7">When the light beams from sources such as lasers overlap they produce interference fringes due to the wave nature of light. A hologram is produced by recording the fringes from the interference of two beams of laser light. The reference beam (RB) falls directly onto the film, while the object beam (OB) is reflected from the object.</span><span class="style10">Replaying a hologram.</span><span class="style7"> When the hologram is replayed by shining a beam of laser light onto it, the light is diffracted in such a way that it appears to come from the position of the original object. The image can be viewed from a range of angles and is a true three-dimensional reconstruction of the object.</span></text>
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<text>ΓÇó THE HISTORY OF ASTRONOMYΓÇó QUANTUM THEORY AND RELATIVITYΓÇó WAVE THEORYΓÇó ELECTROMAGNETISMΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó PHOTOGRAPHY AND FILMΓÇó RADIO, TELEVISION AND VIDEOΓÇó TELECOMMUNICATIONSΓÇó SEEING THE INVISIBLE</text>
