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UF PHY 2054 - Optical Instruments

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Chapter 25Optical InstrumentsThe CameraCamera OperationCamera Operation, IntensityCamera, f-numbersCamera, f-numbers, contThe EyeThe Eye – Parts, contThe Eyes – Parts, finalThe Eye – OperationThe Eye – Operation, contThe Eye – FocusingThe Eye – Focusing, contThe Eye – Near and Far PointsConditions of the EyeFarsightednessCorrecting FarsightednessNearsightednessCorrecting NearsightednessPresbyopia and AstigmatismDioptersSimple MagnifierThe Size of a Magnified ImageAngular MagnificationMagnification by a LensCompound MicroscopeCompound Microscope, contMagnifications of the Compound MicroscopeOther Considerations with a MicroscopeTelescopesRefracting TelescopeAngular Magnification of a TelescopeDisadvantages of Refracting TelescopesReflecting TelescopeReflecting Telescope, Newtonian FocusExamples of TelescopesResolutionRayleigh’s CriterionJust ResolvedBarely Resolved (Left) and Not Resolved (Right)Resolution with Circular AperturesResolving Power of a Diffraction GratingResolving Power of a Diffraction Grating, contMichelson InterferometerMichelson Interferometer, schematicMeasurements with a Michelson InterferometerChapter 25Optical InstrumentsOptical InstrumentsAnalysis generally involves the laws of reflection and refractionAnalysis uses the procedures of geometric opticsTo explain certain phenomena, the wave nature of light must be usedThe CameraThe single-lens photographic camera is an optical instrument ComponentsLight-tight boxConverging lensProduces a real imageFilm behind the lensReceives the imageCamera OperationProper focusing leads to sharp imagesThe lens-to-film distance will depend on the object distance and on the focal length of the lensThe shutter is a mechanical device that is opened for selected time intervalsMost cameras have an aperture of adjustable diameter to further control the intensity of the light reaching the filmWith a small-diameter aperture, only light from the central portion reaches the film, and spherical aberration is minimizedCamera Operation, IntensityLight intensity is a measure of the rate at which energy is received by the film per unit area of the imageThe intensity of the light reaching the film is proportional to the area of the lensThe brightness of the image formed on the film depends on the light intensityDepends on both the focal length and the diameter of the lensCamera, f-numbersThe ƒ-number of a camera is the ratio of the focal length of the lens to its diameterƒ-number = f/DThe ƒ-number is often given as a description of the lens “speed”A lens with a low f-number is a “fast” lensCamera, f-numbers, contIncreasing the setting from one ƒ-number to the next higher value decreases the area of the aperture by a factor of 2The lowest ƒ-number setting on a camera corresponds to the aperture wide open and the maximum possible lens area in useSimple cameras usually have a fixed focal length and a fixed aperture size, with an ƒ-number of about 11Most cameras with variable ƒ-numbers adjust them automaticallyThe EyeThe normal eye focuses light and produces a sharp imageEssential parts of the eyeCornea – light passes through this transparent structureAqueous Humor – clear liquid behind the corneaThe Eye – Parts, contThe pupilA variable aperture An opening in the irisThe crystalline lensMost of the refraction takes place at the outer surface of the eyeWhere the cornea is covered with a film of tearsThe Eyes – Parts, finalThe iris is the colored portion of the eyeIt is a muscular diaphragm that controls pupil sizeThe iris regulates the amount of light entering the eye by dilating the pupil in low light conditions and contracting the pupil in high-light conditionsThe f-number of the eye is from about 2.8 to 16The Eye – OperationThe cornea-lens system focuses light onto the back surface of the eyeThis back surface is called the retinaThe retina contains receptors called rods and conesThese structures send impulses via the optic nerve to the brainThe brain converts these impulses into our conscious view of the worldThe Eye – Operation, contRods and ConesChemically adjust their sensitivity according to the prevailing light conditionsThe adjustment takes about 15 minutesThis phenomena is “getting used to the dark”AccommodationThe eye focuses on an object by varying the shape of the crystalline lens through this processAn important component is the ciliary muscle which is situated in a circle around the rim of the lensThin filaments, called zonules, run from this muscle to the edge of the lensThe Eye – FocusingThe eye can focus on a distant objectThe ciliary muscle is relaxedThe zonules tightenThis causes the lens to flatten, increasing its focal lengthFor an object at infinity, the focal length of the eye is equal to the fixed distance between lens and retinaThis is about 1.7 cmThe Eye – Focusing, contThe eye can focus on near objectsThe ciliary muscles tensesThis relaxes the zonulesThe lens bulges a bit and the focal length decreasesThe image is focused on the retinaThe Eye – Near and Far PointsThe near point is the closest distance for which the lens can accommodate to focus light on the retinaTypically at age 10, this is about 18 cmIt increases with ageThe far point of the eye represents the largest distance for which the lens of the relaxed eye can focus light on the retinaNormal vision has a far point of infinityConditions of the EyeEyes may suffer a mismatch between the focusing power of the lens-cornea system and the length of the eyeEyes may beFarsightedLight rays reach the retina before they converge to form an imageNearsightedPerson can focus on nearby objects but not those far awayFarsightednessAlso called hyperopiaThe image focuses behind the retinaCan usually see far away objects clearly, but not nearby objectsCorrecting FarsightednessA converging lens placed in front of the eye can correct the conditionThe lens refracts the incoming rays more toward the principle axis before entering the eyeThis allows the rays to converge and focus on the retinaNearsightednessAlso called myopiaIn axial myopia the nearsightedness is caused by the lens being too far from the retinaIn refractive myopia, the lens-cornea system is too powerful for the


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UF PHY 2054 - Optical Instruments

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