Lecture 38: WED 22 APROptical Instruments: the human eyeOptical instruments: combination of several (thin) lensesCorrective GlassesMagnifying lensPowerPoint PresentationSlide 7ExampleReflective telescopesLecture 38: WED 22 APRLecture 38: WED 22 APRPhysics 2102Jonathan DowlingOptics: ImagesOptics: ImagesQuickTime™ and a decompressorare needed to see this picture.The human eyeThe human eye consists of a variable-geometry lens (crystalline) which produces a real image on a “screen” (retina) which is transmitted to the brain via the optical nerve.The cristalline automatically adjusts itself so we see well any objectplaced between infinity and a distance called “near point” (about 25cmfor a typical 20 year old). The “image distance” is the eye diameter~2cm.Optical Instruments: the human eyeOptical Instruments: the human eyeOptical instruments: combination Optical instruments: combination of several (thin) lensesof several (thin) lenses F1F1F2F2If lenses are very close, the compound lens has 1/f~1/f1+1/f2Corrective GlassesCorrective GlassesA farsighted person needs a convergent lens.A nearsighted person needs a divergent lens.The “power” of a lens is measured in dioptres: P=1/f with f is in m.Glasses with -6D are divergent glasses with f=1/6D =0.17m=17cmThe dioptres add! Two lenses have 1/f=1/f1+1/f2 D=D1+D2The magnification of an object is m=i/p~i/h, but i=eye diameter. Maximum magnification: m~2cm/25cm (!?) Angular magnification (different from lateral): m’fcmfhcmh 25m ' 25=≅=θθθMagnifying lensMagnifying lensWe’d like to make p smaller (move the object closer). We use a magnifying lens to produce a (larger) image than our eye can see: Very near the focus!Microscope:To increase the magnification of a lens, one wants to have a short focal length. That means small radii of curvature (very curved lens). This, in turn implies a lot of aberration (one is immediately out of the thin lens approximation). A solution to this is obtained by combining two lenses. The resulting device is called microscope.pim −=Object O is magnifiedby the objective:And its image is magnifiedby the eyepiece:fcmm25=θTotal magnification:eyobfcmfsmmM25−==θTelescope:Telescopes are arrangement of lenses that improve vision of objectsvery far away. They are configured like a microscope. However, theobjective forms an image essentially at its focus, and therefore theeyepiece’s focus has to be placed at that same point.The magnification is given bythe ratio ey/ob, and sinceobobfh /'=θeyeyfh /'=θeyobffm −=Refracting telescopes are of limiteduse (chromatic aberration). Reflectingtelescopes built with mirrors are preferred in astronomy.ExampleExampleThe world’s largest refracting telescope is at the Yerkes Observatory of the University of Chicago at Williams Bay, Wisconsin. The objective has a diameter of 102cm and a focal length of 19.5m. The focal length of the eyepiece is 10cm. What is its magnifying power?1951.05.19−=−=−=mmffmeyobWhy so large (102cm)? Because the larger the objective, the more light it gathers.Reflective telescopesReflective telescopesKeck observatory (Mauna Kea, Hawaii) and the Hale-Bopp comet.Largest optical telescope, composed of 36 (!) hexagonal mirror segments performing as a single mirror 10m
View Full Document
Unlocking...