1Lecture 15Chap. 6 – Optical Instruments• Single lens instruments– Eyeglasses– Magnifying glass• Two lens– Telescope & binoculars– Microscope• The projector– Projection lens– Field lensOctober 12, 2010 all these vugraphswere used. Lecture plan2Exam II, Nov. 4, will cover • Ch. 4 (camera), • Ch. 5 (the eye), • Ch. 6 (optical instruments), • Ch. 7 (visual perception),• Ch. 9 & 10 (color & color perception). •We save Ch. 8 for later. We are here3Optical instruments• Single lens instruments– Eyeglasses– Magnifying glass• Two lens instruments– Telescope & binoculars– Microscope• The projector– Projection lens– Field lensWe are here4EyeglassesFor nearsighted people (can’t focus far away)Eyeglasses are diverging (thinner in middle)Normal vision: you can focus from 25 cm to infinity (∞)For farsighted people (can’t focus up close)Eyeglasses are converging (thicker in middle)Demo: eyeglasses5Eyeglass prescription is in dioptersOptometrists use diopters to measure the strength of a lensDiopters [or D] = 1 / (focal length in meters)Example: f = 50 cm or f = 0.5 m D = 1/f = 2 diopters (units are 1/meters)6AstigmatismVertical and horizontal lines focus differently This problem is fixed by a cylinder lensSharply focusedOut of focusFocuses in one direction, but not the other!7Action of a cylinder lensFocuses in one direction, but not the other!If a cylinder lens is needed for your eyeglasses, your cornea and eyelens is curved more in one direction than in the other!Focus is a line, not a pointOne lens magnifier (again)89Magnification of a one-lens magnifierMagnification = Example: 5 cm focal length has 5 magnification10 cm focal length has 2.5 magnification2 mm focal length has 125 magnificationcmin length focalcm 2510van Leeuwenhoek’s microscope 25 cm, reading distance, approximatelyFocal length, approximately11van Leeuwenhoek’s microscope Year: 1600sTiny lens with 2 mm focal length(the lens diameter cannot be much bigger than the focal length)Magnification = Problem: image was still small, and very dim. 125cm0.2cm 25=12Robert Hooke’s microscope 1600sDiscovered: Blood cellsMicrobes13Optical instruments• Single lens instruments– Eyeglasses– Magnifying glass• Two lens instruments– Telescope & binoculars– Microscope• The projector– Projection lens– Field lensWe are here14Robert Hooke’s two-lens microscope objectlens 1Nosepieceor Objectivelens 2eyepiecelens 1 imagelens 2 imageA magnifying glass (the eyepiece) magnifies the first image further. The first lens, the nosepiece, is used as a projection lens. Demo: student microscope15Hooke’s discoveriesThe cellDetails of a flea16Robert Hooke’s two-lens microscopeMagnification = M1x M2Example: Eyepiece M2= 25 Nosepiece M1= 40 Final magnification = 40 x 25 = 1000M1, like for any projection lens, is Xi/ XoM2, like for any magnifier, is 25 cm / focal length17Modern studentbinocular microscopeA beamsplitter, a half-silvered mirror, sends half the light to each eyepiece. 18Galileo’s telescope (~1600)Negative lens for eyepiece gives right-side-up image. FobjectiveFeyepiece19Kepler’s telescope (~1600)Positive lens for eyepiece gives upside down image that is easier to see. lens 1Objectivelens 2eyepiecelens 1 imagelens 2 imageFeyepieceFobjective20Galileo’s telescope30 x magnificationTiny lens means not much light entered, so image is dim.Discoveries: SunspotsCraters on the MoonPhases of VenusMoons of JupiterMagnification of a telescope(refractor or reflector)Magnification = Example: Objective focal length = 1 m = 100 cmEyepiece focal length = 1 cmMagnification = Fobjective/Feyepiece= 10021eyepiece oflength focalmirroror lens objective oflength focalUseless magnificationA bigger fuzzy image is not useful (no additional information). A high resolution image requires large lenses and mirrors. 228 inch reflector telescope Hubble telescope, 96 inch mirrorSmallest visible feature ∝ 1/(diameter of lens or mirror)23Yerkes observatory,Largest refractor40 inch lens1897Larger lenses would sag under their own weight. The lens was achromatic (two kinds of glass). 24Newton’s reflector telescope25Newton’s telescopeAdvantage: no chromatic aberration26Telescope drives• Telescopes must rotate once every 24 hrs (approximately) to follow the stars, or the pictures will have streaks. NorthstarStudent telescopes27View of galaxyNGC130028Catadioptric telescopeAlso called Schmidt-CassegrainFront glass lens corrects aberrationsWhy buy this? It’s shorter.29Hubble Space Telescope imageCompare to studenttelescope image.30Palomar reflector - 5 meter mirror31South Africa Large Telescope (10 m)This shows 7 of the 91 mirrors. Diameter is about 10 m.Each mirror is a hexagon so that they pack closely. Former CU student Amanda Gulbisuses this telescope. 32Largest single mirrors are 8 m now33Finished mirror(don’t sneeze)34KeckTelescope36 mirrors10 m dia.35Magellan Telescope (yr. 2018)24.5 m dia. segmented mirror36European Extremely Large Telescope (42 m dia.) would use many smaller mirrorsAssemble your own telescope1. Instructions2. Red end caps, cardboard spacer, foam lens holder3. Cardboard washer4. Objective lens (big)5. Eyepiece lens (small)6. Sliding cardboard tubes Advice: don’t lose the little lens