Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F-521 Email: ivan.smalyukh@colorado.edu Phone: 303-492-7277 Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang jhihan.yang@colorado.edu  Class # 15 Chapter #5: The human eye & vision

Review: For photography of objects at different distances from us we use A. Camera with CCD that has at least 5 megapixels; B. Small aperture (large f-number); C. Large aperture (small f-number); D. Short exposure; E. A, B, C, & D

Review: For photography of moving objects we use A. Camera with CCD that has at least 5 megapixels; B. Small aperture (large f-number); C. Large aperture (small f-number); D. Short exposure and larger aperture; E. A, B, C, & D

Ch. 5 The Eye

Announcements New HW assigned today; Will discuss the outcome of the midterm on Tuesday; Overall everybody did a very good job! Reading – Textbook Chapter 5

We will learn Anatomy: The parts of the eye How it “works” Night vision Response time Eye problems and fixes 6

Eye parts that you see Pupil Opening to the inside of the eye Iris changes its opening to adjust light the pupil is the opening Sclera the white outer wall

Helpful concept map of vision: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/visioncon.html#c1

Lens + Cornea Sclera Retina Lens Film/CCD Iris Diaphragm pupil Light tight box Iris Diaphragm pupil aperture

The Eye: Analogy to the Camera Lens and cornea Iris (diaphram) Ciliary muscle (focus) Retina (film/CCD??) We will see that the retina does FAR more than film or CCD

Image is upside down and right-left reversed (like a simple camera) After a few days, your brain would flip the world right side up. The brain adjusts. Do we see things upside down? Why?

Comparison of eye to camera The camera Lens Diaphragm (f-stop) Focusing knob Film/CCD The eye Lens and cornea Iris Ciliary muscle Retina

All these visual system still exist in various creatures. Evolution of the eye All these visual system still exist in various creatures. Wikipedia 13

Iris Wide open at night - less depth of field aberrations Closed down in daytime or for close work, 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. Try it: Pinhole pupil

The Lens system: Imaging We have two lenses: Cornea and lens Cornea accounts for most of optical power (so, does cornea or lens have greater focal length?) Cornea has fixed focal length. Lens = fine-tuning. Cornea has 2/3 of optical power of eye. 43 diopters.

The Lens system: Focusing mostly by cornea assisted to varying degrees by eyelens ciliary muscles puff up to relax the lens for close focusing

How thin lenses add Ftot = final focal length F1 = focal length lens 1 Diverging lenses (concave) have negative focal lengths This is the same as adding powers: Dtot = D1 + D2 D is diopters Demo: put together some lenses

Question You have two converging lenses, used close together. The effective lens has a focal length that is : Dtot = D1 + D2 D = 1/F Larger than either alone Shorter... The same as one of them.. So the cornea + eyelens gives a shorter focal length (thus more power) than either alone

Which has the shorter focal length: Cornea or eyelens?

How and where is the image produced in your eye? Dual (compound) lens system Cornea (on outside) does most of the ray bending Eyelens does fine tuning: The image is focused on your retina We'll use an equivalent single lens in examples Cornea does most of ray bending Retina F1 F1' Final image is on retina Eyelens behind cornea does fine-tuning; focuses by changing its (long) focal length

How does eye focus? A camera moves the lens relative to screen In the eye the “screen” (ie., retina) is at a fixed distance So, the eye changes the focal length of the lens! (eyelens only, not cornea) = “accomodation” The “focusing knob” is the ciliary muscle Camera: Eye: 1/f = 1/xo + 1/xi 1/f = 1/xo + 1/xi changes as xo changes fixed number varies varies fixed number

Accommodation is achieved by the eyelens' ability to change its focal length by changing its bulge Less bulge means less bending power and longer focal length More bulge means more bending power and shorter focal length large f smaller f

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 me far away, since rays must be bent more for image to fall on retina. Prof is in focus large f Thumb is in focus Prof is out of focus smaller f

Too long for a focused image. Too short for a focused image. 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: Too long for a focused image. Too short for a focused image. Actually, the iris is closed too much None of these. They’re likely to have trouble with this. The key is that the longer the focal length, the further back the image. Lensmaker’s equation is one way to think about it but I haven’t emphasized that. I’ve emphasized that a longer focal length lens will make an image that is further away from it. So what kind of corrective lens do they need? Convex (f>0) Concave (f<0) Remember, Dtot = D1 + D2 and D = 1/f So, we want to lengthen the effective focal length of their eye

Nearsighted (myopic; can’t see far) = concave lenses Diverging Lens: f < 0 Farsighted (hyperopic; can’t see near) = convex lenses This is the first time they’ve seen that f<0 for diverging lens f>0

What about the stuff inside?

The Retina (the “film” of the eye) Light-sensitive cells = “rods” and “cones” Cones = color, precise Rods = low light, B&W Fovea = high density of cones That’s where you “look at” someone. Unlike camera, we “scan” to get image of room Signal gets out through optic nerve No rods and cones here = blind spot.

But the retina needs blood supply Blood vessels are in front of the cones and rods…. Red eye = light bounces off blood vessels and see red. Red eye reduction pre-flash to close pupil so see less red eye.

Will learn much more about retina later Cones in foveal pit are more densely packed. No blood supply here. Direct passage of light to receptors, not touching the nonreceptive cells in the middle. Ganglion cells are those which send signal to optic nerve. Transmit from the photoreceptor to the ganglion.

Dark adaptation Rods = dark vision When you are in the light, you use your cones In the dark, you use your rods Detection threshold: When you go into the dark, you take about 7 minutes for your eyes to switch to using all rods, no cones (25 min for full adaptation)

Retina has 108 nerve endings to detect image rods, for night vision cones, for color and detail, 7 million optic nerve = 106 transmission lines fovea, region of best vision (cones)

Rods and cones Rhodopsin, a photochemical, responds to light It is destroyed and reformed. There are 3 kinds of cones for 3 colors red, green, blue

Retina details Photopic vision, in bright light, cones are used Scotopic, in low light, rods are used more rods per nerve combines signals

Retina Sensitivity

More about the eye Night vision Response time Eye problems hyperopia, myopia, presbyopia, astigmatism

Dark adaptation (night vision) Time to adapt to dark: ~20-30 minutes Night adaptation: shifts from rods to cones You see blue better in the dark

Persistence of vision Images remain on receptors for 1/25 second in low light 1/50 second in bright light Movies do 24 frames per second in a darkened room. TVs do 60 frames per second, ok in lighted room.

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)

Eye problems continued 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 focal lengths

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.

The Retina: Detecting the light and processing the images Has 108 nerve endings to detect image rods, for high sensitivity (night vision) cones, for color and detail, 7 million optic nerve = 106 transmission lines fovea, region of best vision (cones) Do this slide QUICKLY, emphasizing that the retina is part of the brain and that we are going to come back to it for more depth. More nerves in your retina than some creatures have in their entire brains. Processing Power.

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?

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. …

Review: Parts of the human eye Specialized optical instrument and image analysis computer.

References http://www.costaricacoffeeart.com/alternative_photography_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