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 InstrumentsAnalysis generally involves the laws of reflection and refractionAnalysis uses the procedures of geometric opticsTo explain certain phenomena, the wave nature of light must be usedThe CameraThe single-lens photographic camera is an optical instrument ComponentsLight-tight boxConverging lensProduces a real imageFilm behind the lensReceives the imageCamera OperationProper focusing leads to sharp imagesThe lens-to-film distance will depend on the object distance and on the focal length of the lensThe shutter is a mechanical device that is opened for selected time intervalsMost cameras have an aperture of adjustable diameter to further control the intensity of the light reaching the filmWith a small-diameter aperture, only light from the central portion reaches the film, and spherical aberration is minimizedCamera Operation, IntensityLight intensity is a measure of the rate at which energy is received by the film per unit area of the imageThe intensity of the light reaching the film is proportional to the area of the lensThe brightness of the image formed on the film depends on the light intensityDepends on both the focal length and the diameter of the lensCamera, f-numbersThe ƒ-number of a camera is the ratio of the focal length of the lens to its diameterƒ-number = f/DThe 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, contIncreasing the setting from one ƒ-number to the next higher value decreases the area of the aperture by a factor of 2The lowest ƒ-number setting on a camera corresponds to the aperture wide open and the maximum possible lens area in useSimple cameras usually have a fixed focal length and a fixed aperture size, with an ƒ-number of about 11Most cameras with variable ƒ-numbers adjust them automaticallyThe EyeThe normal eye focuses light and produces a sharp imageEssential parts of the eyeCornea – light passes through this transparent structureAqueous Humor – clear liquid behind the corneaThe Eye – Parts, contThe pupilA variable aperture An opening in the irisThe crystalline lensMost of the refraction takes place at the outer surface of the eyeWhere the cornea is covered with a film of tearsThe Eyes – Parts, finalThe iris is the colored portion of the eyeIt is a muscular diaphragm that controls pupil sizeThe iris regulates the amount of light entering the eye by dilating the pupil in low light conditions and contracting the pupil in high-light conditionsThe f-number of the eye is from about 2.8 to 16The Eye – OperationThe cornea-lens system focuses light onto the back surface of the eyeThis back surface is called the retinaThe retina contains receptors called rods and conesThese structures send impulses via the optic nerve to the brainThe brain converts these impulses into our conscious view of the worldThe Eye – Operation, contRods and ConesChemically adjust their sensitivity according to the prevailing light conditionsThe adjustment takes about 15 minutesThis phenomena is “getting used to the dark”AccommodationThe eye focuses on an object by varying the shape of the crystalline lens through this processAn important component is the ciliary muscle which is situated in a circle around the rim of the lensThin filaments, called zonules, run from this muscle to the edge of the lensThe Eye – FocusingThe eye can focus on a distant objectThe ciliary muscle is relaxedThe zonules tightenThis causes the lens to flatten, increasing its focal lengthFor an object at infinity, the focal length of the eye is equal to the fixed distance between lens and retinaThis is about 1.7 cmThe Eye – Focusing, contThe eye can focus on near objectsThe ciliary muscles tensesThis relaxes the zonulesThe lens bulges a bit and the focal length decreasesThe image is focused on the retinaThe Eye – Near and Far PointsThe near point is the closest distance for which the lens can accommodate to focus light on the retinaTypically at age 10, this is about 18 cmIt increases with ageThe far point of the eye represents the largest distance for which the lens of the relaxed eye can focus light on the retinaNormal vision has a far point of infinityConditions of the EyeEyes may suffer a mismatch between the focusing power of the lens-cornea system and the length of the eyeEyes may beFarsightedLight rays reach the retina before they converge to form an imageNearsightedPerson can focus on nearby objects but not those far awayFarsightednessAlso called hyperopiaThe image focuses behind the retinaCan usually see far away objects clearly, but not nearby objectsCorrecting FarsightednessA converging lens placed in front of the eye can correct the conditionThe lens refracts the incoming rays more toward the principle axis before entering the eyeThis allows the rays to converge and focus on the retinaNearsightednessAlso called myopiaIn axial myopia the nearsightedness is caused by the lens being too far from the retinaIn refractive myopia, the lens-cornea system is too powerful for the
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