Physics 1230: Light and Color Exam 4 cancelled: Exam extra credit assignment will be due Wed. at 5PM Extra credit to improve exam scores! HW9: Due today, Monday, 5PM FCQ at end of lecture. Lecture 13: The Eye, eyeglasses and other optical instruments, start retina signal processing. Reading: Chap. 9,10 Color perception. 1
Review: Parts of the human eye Specialized optical instrument and image analysis computer.
The Eye: Analogy to the Camera Lens and cornea Iris (diaphram) Ciliary muscle (focus) Retina (FILM??) We will see that the retina does FAR more than film or CCD 3
The Lens system: Imaging &Focusing Most converging of light is by the cornea. Assisted to varying degrees by eyelens focus. Ciliary muscles puff up to relax the lens for close focusing 4
What does accommodation of the eye have to do with looking at me or your thumb? How does it work? (Lens represents combined cornea-eyelens system) Thumb is out of focus Focusing your eye on a nearby thumb requires shorter focal length (more bulgy) eyelens than focusing on the Prof far away, since rays must be bent more for image to fall on retina. large f Prof is in focus Thumb is in focus smaller f Prof is out of focus
The Eye as a cool instrument: The eyelens We know that lenses suffer from various aberrations. What happens in the eyelens? Spherical aberration is mostly corrected Cornea is not spherical surface (aspherical) Iris cuts out rays through the edge of the lens Index of refraction is not uniform. Curvature of field retina is curved to correct for this Chromatic aberration: Bluest light is absorbed Many of these tricks are now used in technology. 6
The Retina: Detecting the light and processing the images The retina and optic nerves are recognized as actually parts of the brain (like your olifactory bulb in the nose). They start development IN the brain and migrate Has 10 8 nerve endings to detect image rods, for high sensitivity (night vision) cones, for color and detail, 7 million optic nerve = 10 6 transmission lines fovea, region of best vision (cones) More nerves in your retina than some creatures have in their entire brains. Processing Power. 7
Human versus cat retinas http://webvision.med.utah.edu/anatomy.html Lots of specialization here for detection and processing. More in the next couple of lectures Light comes in from here
Very common eye problems: Issues in the lens focusing affect many of us: Myopia, see close objects clearly, only fixed by a negative lens Hyperopia, see things far, only fixed by a positive lens Presbyopia, stiff lens, no accommodation Bifocal glasses have near and far foci. How do we fix these problems? 10
The remaining lectures: We are here Ch. 5 (the eye), Ch. 6 (optical instruments), Ch. 7 (Retina and visual perception), Ch. 9 & 10 (color & color perception). 11
Optical instruments we ll cover: We are here Single lens instruments Eyeglasses Magnifying glass Two lens Telescope & binoculars Microscope 12
A near-sighted or MYOPIC eye produces an image that is not far enough behind the lens, so is blurry on the retina. Therefore, the eye lens focal length is: A) Too long for a focused image. B) Too short for a focused image. C) Actually, the iris is closed too much D) None of these. 13
You have a lens with a short focal length and you wish it was longer. You can make it longer by using a second lens. The correct choice for this case is: A) A focusing lens of negative power B) A diverging lens of positive power C) A focusing lens of positive power D) A diverging lens of negative power 1 1 1 Recall: ftotal f1 f2 OR D D D TOTAL 1 2 14
Eyeglasses: Our most common optical instrument Normal vision: you can focus from 25 cm to infinity ( ) For nearsighted people (can t focus far away) Eyeglasses are diverging (thinner in middle) For farsighted people (can t focus up close) Eyeglasses are converging (thicker in middle) 18 th century HUGE improvement in quality of life Demo: eyeglasses 15
Eyeglass prescription is in diopters Optometrists use diopters to measure the power of a lens Diopters [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) The example above would be: (A) reading glasses (B) distance glasses 16
Astigmatism Vertical and horizontal lines focus differently This problem is fixed by a cylinder lens Sharply focused Out of focus Focuses in one direction, but not the other! 17
Action of a cylinder lens Focuses 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! 18
Play with your eyeglasses to see what they can do! 19
The Magnifying glass (again): New insight! Our first effort to explain: Ray optics lets us determine the ray paths. A model of the observer lets us predict an image where rays converge. 20
The Magnifying glass (again): Another view The eye perceives via focused images: 25 cm Typical closest focus is 25 cm from the eye. A magnifying glass is like READING GLASSES: It lets you focus on closer things. 21
The Magnifying glass (again): Another view The eye perceives via focused images: 25 cm Best focused image alone. Or: By similar triangles: Magnifying glass M 25 cm f cm Focal length Bigger image Demo with thumb and eyepiece from telescope kits. 22
Magnification of a one-lens magnifier Example: 5 cm focal length has a magnification of A) 5 B) 4 C) 25 D) 1/5 E) None of these focal 25 cm length in cm Interesting thought: 2 mm focal length has 125 magnification 23
van Leeuwenhoek s microscope Focal length, approximately 25 cm, reading distance, approximately 24
van Leeuwenhoek s microscope Years developed: 1660s Tiny lens with 2 mm focal length (a lens cannot be much bigger than the focal length) 25cm Magnification = 125 0.2cm Technology edge: Outstanding single lenses Problem: image was still small, and very dim. 25
Robert Hooke s microscope, also circa 1660 Discovered: Blood cells, Microbes, etc. A TWO LENS system. No existing pictures of Hooke van Leeuwenhoek 26
Robert Hooke s two-lens microscope A magnifying glass (the eyepiece) magnifies the first image further. lens 2 image lens 1 image object lens 2 eyepiece lens 1 Nosepiece or Objective The first lens, the nosepiece, is used as a projection lens. 27
Hooke s discoveries The cell Detailed structure of creatures. Example: The flea (plague). 28
Modern binocular microscope is very much the same as Hooke s. A beamsplitter, a halfsilvered mirror, sends half the light to each eyepiece. 29
Many optical instruments can be understood step by step, as we did for Hooke s microscope: The first lens collects light and produces an image. The second lens produces a new image of the first image. The third lens produces a new image of the second image And so on. 30
Galileo s telescope (~1600) Negative lens for eyepiece gives rightside-up image. 31
lens 1 image Kepler s telescope (~1600) lens 2 image lens 2 eyepiece lens 1 Objective Positive lens for eyepiece gives upside down image. It s upside down, but brighter and is easier to see. 32
Your telescope kit makes a: A) Galilean telescope B) Keplerian telescope C)Another type that we have not seen. 33
Galileo s telescope 30 x magnification Tiny lens means not much light entered, so image is dim. Discoveries: Sunspots Craters on the Moon Phases of Venus Moons of Jupiter 34
Magnification of a telescope (refractor or reflector) Magnification = focal length of objective lens or mirror focal length of eyepiece Example: Objective focal length = 1 m = 100 cm Eyepiece focal length = 1 cm Magnification = 100 35
Limits to magnification A bigger fuzzy image is not useful (no additional information). A high resolution image requires large lenses and mirrors. 8 inch reflector telescope Hubble telescope, 96 inch (2 meter) mirror Smallest visible feature 1/(diameter of lens or mirror) 36
Yerkes observatory, Largest refractor 40 inch lens 1897 Larger lenses would sag under their own weight. The lens was achromatic (two kinds of glass). 37
Yerkes Today Still available for observations in the original observatory. Lake Geneva Wisc. COLD winters yield great seeing. Constant pressure from developers for the lakeshore. 38
Newton s reflector telescope 39
Newton s telescope Advantage: no chromatic aberration 40
Palomar reflector - 5 meter mirror 41
South Africa Large Telescope (10 m) Former CU student Amanda Gulbis uses this telescope. This shows 7 of the 91 mirrors. Diameter is about 10 m. Each mirror is a hexagon so that they pack closely. 42
Largest single mirrors are 8 m now Prof. Roger Angel s group. University of Arizona 43
Finished mirror (don t sneeze) 44
Keck Telescope 36 mirrors 10 m dia. 45
Magellan Telescope (yr. 2018) 24.5 m dia. segmented mirror 46
European Extremely Large Telescope (42 m dia.) would use many smaller mirrors 47
The remaining lectures: Ch. 5 (the eye), Ch. 6 (optical instruments), We are here Ch. 7 (Retina and visual perception), Ch. 9 & 10 (color & color perception). 48
Stop here for the FCQs 49
The retina is where the image falls at the back of your eyeball Inverted image falls on retina instead of film. Can demonstrate this inversion Open left eye only. Press gently with left finger on eyeball just above tear duct. Observe dark spot in lower left corner of your field of view. Rods & cones packed into retina. Sensitive to light like camera film Optic nerve Nerve fibers connect rods & cones to brain. (transform light into electrical signals) Blind spot is where optic nerve leaves eyeball. Demo. How are rods and cones distributed in the retina? Fig. 5.12 The fovea is the small region near the center of the retina Used for sharp, detailed viewing. Has the most cones (precise, color vision) Has no rods (used for low light, less precise viewing). Looking at someone means their image is on your fovea If their image is not on your fovea you see them "out of the corner of your eye." Eyeball moves to see a sharp image It scans to make all parts of an image eventually fall on your fovea Like TV image scanning
Retina has 10 8 nerve endings to detect image rods, for night vision cones, for color and detail, 7 million optic nerve = 10 6 transmission lines fovea, region of best vision (cones) 51
Human and cat retinas http://webvision.med.utah.edu/anatomy.html Light comes in from here
Rods and cones Rhodopsin, a photochemical, responds to light It is destroyed and reformed. Signal goes to a synapse, a gap between nerve cells There are 3 kinds of cones for 3 colors red, green, blue 54
Retina details Choroid, outside layer with blood supply Photoreceptors: rods and cones Plexiform layer, inside layer with nerves Photopic vision, in bright light, cones are used Scotopic, in low light, rods are used more rods per nerve combines signals 55
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References http://www.costaricacoffeeart.com/alternative_photograph y_make_your_own_negative_film_or_plates.php http://unblinkingeye.com/articles/emulsion/emulsion.html http://en.wikipedia.org/wiki/charge-coupled_device http://www.camerapedia.org/wiki/ccd 57
Robert Hooke s two-lens microscope Magnification = M 1 x M 2 Example: Eyepiece M 2 = 25 Nosepiece M 1 = 40 Final magnification = 40 x 25 = 1000 M 1, like for any projection lens, is X i / X o M 2, like for any magnifier, is 25 cm / focal length 58
Telescope drives Telescopes must rotate once every 24 hrs (approximately) to follow the stars, or the pictures will have streaks. North star 59
Student telescopes View of galaxy NGC1300 60
Catadioptric telescope Also called Schmidt-Cassegrain Front glass lens corrects aberrations Why buy this? It s shorter. 61
Hubble Space Telescope image Compare to student telescope image. 62
Binoculars Simply folded telescopes for each eye. 63
What does 7 x 50 binoculars mean? 7x is the magnification 50 is the diameter of the front lens (the objective lens) in millimeters 6 x 30 binoculars are easy to carry 7 x 50 binoculars are heavy, but these give a brighter image 15 x 80 binoculars need a tripod 64
35 mm slide projector Field lens is used to put the most light on the slide. Field lens Color slide Projection lens Mirror 65
Viewgraph projector Mirror Projection lens Fresnel lens (condenser) and viewgraph location Curved mirror 66
Eye problems Loss of accomodation: ability to focus from 10 inches to infinity Cataracts = cloudy eyelens, replacement lens does not accommodate Floaters = dead cells floating in vitreous humor (seen against a clear sky) Diseases of the eye components. Nerve damage Etc. 67
A dash past The Iris Low light levels (say at night!): Wide open, f/2 or f/3 Get more of the available light! We lose our color vision. More aberrations from the larger lens opening Less depth of field Higher light levels (say daytime): Closed down, f/8 We sense colors! Fewer aberrations More depth of field You can check depth of field: Try it: Close one eye, hold up thumb, stuff behind thumb is out of focus. 68