Chapter 25 Optical Instruments 25 2 The Eye 25 mm Determine the focal length of your eyes when looking at an object far away Object is far away p Want image at retina q 25mm 1 1 1 25 mm f f relaxed 25 mm Everyone has a far point and near point Pfar and Pnear 25 cm are considered reasonable Normal eyes can see objects from 25 cm to Namely f can be adjusted so that objects between 25 cm to can have images at q 25 mm at the retina For near point of a normal eye Tense 250 mm 25 mm Determine the focal length of your eye when looking at an object up close 25 cm 1 1 1 Object is up close p 25cm 250mm Want image at retina q 25mm Far sightedness 25 mm f ftense 22 7 mm f relaxed 25 mm If you are nearsighted far point is closer than Too far for near sighted eye to focus Near sightedness If one can only see as far as pfar far point rather than move with the lens to pfar 250 mm Example if you can t see farther than 2 m away do dfar If one can only see as near as pnear near point rather than 25 cm move p 25 cm to pnear with a lens Key eyes don t see the object they only see the image of the lens no different from seeing through a telescope or microscope Want to have virtual image of distant object p at the far point q pfar 1 Refractive Power of Lens Example An individual is nearsighted his near point is 13 0 cm and his far point is 50 0 cm a What should be the focal length of the lens b When the lenses are in use what is this person s near point Diopter 1 fglass in meters Person with far point of 5 meters would need contacts with focal length 5 meters 1 5m 0 20 Diopters Near sightedness is when a person 1 2 3 4 has a near point not at has a far point not at has a near point not at 25 cm officially becomes a nerd If you are farsighted near point is further than 25 cm Too close for far sighted eye to focus d dne Example pnear 40cm o ar When object is at 25 cm lens must create an virtual image at qnear 1 1 1 p q f 1 1 1 25 cm 40 cm f f 66 7 cm What lens 2 25 6 Resolution of single slit and circular apertures Telescopes Big diameter for longer wavelength Central maximum 1st diffraction minimum Same star or object in general different resolution Diameter D light It has nothing to do with focusing Rain Man s Principle Maxima and minima will be a series of bright and dark rings on screen First diffraction minimum is at sin 1 22 K Mart sucks K Mart s telescopes do suck min D Resolving power 25 4 The compound microscope sin 1 22 D 3 d orbital image STM 4
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