1. Physics 1230: Light and Color Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Phone:
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
Class # 15
Chapter #5: The human eye & vision

2. 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

3. 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

4. Ch. 5 The Eye

5. 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

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

7. 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

8. Helpful concept map of vision:

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

10. 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

11. 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?

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

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

14. 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

15. 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.

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

17. 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

18. 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

19. Which has the shorter focal length: Cornea or eyelens?

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. What about the stuff inside?

27. 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.

28. 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.

29. 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.

30. 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)

31. 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)

32. 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

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

34. Retina Sensitivity

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

36. 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

37. 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.

38. 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)

39. 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

40. 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.

41. 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.

42. 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?

43. 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. …

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

45. References