The Human Eye PHY 2054 Chapter 26 B
The Human Eye PHY 2054: Chapter 26 B Eye (Open. Stax) 1
Chapter 26 Contents (+ extra) ✔ 1. The ray aspect of light ✔ 2. Reflection of light ✔ 3. Refraction of light: Snell’s law ✔ 4. Total internal reflection ✔ 5. Dispersion: Rainbows and lenses ✔ 6. Images from lenses ✔ 7. Images from mirrors ✔ 8. Single lens systems 1. 2. 3. 4. Magnifying glass Projector Camera Human eye PHY 2054: Chapter 26 B Eye (Open. Stax) 2
Chapter 26 Contents (+ extra) 9. Two lens systems 1. 2. Microscope Telescope PHY 2054: Chapter 26 B Eye (Open. Stax) 3
Structure of the Eye Essential parts of the eye u Cornea – transparent outer structure u Pupil – opening for light u Iris – regulates pupil size u Lens – partially focuses light u Retina – location of image u Optic nerve – sends image to brain Eye focuses light on retina u Most refraction at cornea u Rest of refraction at lens PHY 2054: Chapter 26 B Eye (Open. Stax) 4
Closeup of Eye Components PHY 2054: Chapter 26 B Eye (Open. Stax) 5
Operation of the Eye (Shown in Relaxed State) 24 mm PHY 2054: Chapter 26 B Eye (Open. Stax) 6
Image of Object on Retina 24 mm PHY 2054: Chapter 26 B Eye (Open. Stax) 7
Operation of Eye Cornea-lens system focuses light onto retina (back surface) u Retina contains receptors called rods (110 M) and cones (7 M) u Rods & cones send impulses to brain via optic nerve (1 M fibers) u Brain converts impulses into our conscious view of the world PHY 2054: Chapter 26 B Eye (Open. Stax) 8
Picture of Retina PHY 2054: Chapter 26 B Eye (Open. Stax) 9
Rods, Cones Close Up (Retina Cross Section) Rods Cones PHY 2054: Chapter 26 B Eye (Open. Stax) https: //www. pinterest. com/explore/cone-cell/ 10
Rods and Cones in Retina Light sensitive parts of rods, cones are inside the retina. Note that light must pass through cell body first, a well-known design problem. PHY 2054: Chapter 26 B Eye (Open. Stax) 11
Structure of Rods and Cones PHY 2054: Chapter 26 B Eye (Open. Stax) 12
Color Perception in Rods and Cones One type of rod u Monochromatic vision u Only used for night vision u Highly sensitive 3 Cones Rods types of cones u Color vision! u 3 primary colors u Not as sensitive as rods PHY 2054: Chapter 26 B Eye (Open. Stax) 13
Focusing Process: “Accommodation” Distant objects u The ciliary muscle is relaxed u Focal length of eye is maximum ~ 16 mm Nearby objects u The ciliary muscles tense, squeezing the lens u The lens bulges and the focal length decreases (lens power 1/f increases) u This adjustment process is called “accommodation” Focal length of eye (normal) uf ≅ 16. 3 mm at rest u 1/f = 1 / 0. 0163 m ≅ 60 “diopters” u During accommodation, lens power increases as p decreases q fixed: ~ 16. 3 mm PHY 2054: Chapter 26 B Eye (Open. Stax) 14
Accommodation in Normal Eye Distant object • Eye muscles relaxed • No accommodation Nearby object • Eye muscles tense, causing lens to thicken 1/f larger • This is accommodation So the eye adjusts to distance by changing focal length! PHY 2054: Chapter 26 B Eye (Open. Stax) 15
Accommodation Range “Normal” u Lens adult: can see objects from 25 cm to ∞ to retina = q = 16. 3 mm 1/q ≃ 60 diopters (D) Range = 60 – 64 ~ 4 D Adolescent: can see objects from 8 cm to ∞ Range ~ 12 D See next slide PHY 2054: Chapter 26 B Eye (Open. Stax) 16
Accommodation and Age: Presbyopia is the reduction in accommodation range with age u Cause: hardening of lens u Accommodation range is max for infants (60 – 73 diopters) u Shrinks with age, noticeable effect on reading after 40. Almost 0 at 65. u Can be corrected with converging lenses (reading glasses) Data from Duane, 1912 PHY 2054: Chapter 26 B Eye (Open. Stax) 17
Image Size on Retina Example: A tree is 50 m tall and 2 km distant. How big is the image on the retina? PHY 2054: Chapter 26 B Eye (Open. Stax) 18
The Eye: Near Point and Far Point Near point (dnear) is the closest distance for which the lens can easily accommodate to focus light on the retina u Objects closer than dnear appear blurry u It increases with age (presbyopia) because of hardening of lens u Use dnear = 25 cm as a useful average u Farsightedness (hyperopia): dnear > 25 cm Far point is the largest distance for which the lens of the relaxed eye can focus light on the retina u Objects farther than dfar appear blurry u For normal vision, far point is at infinity (dfar = ∞) u Nearsightedness (myopia): dfar is finite, so anything beyond dfar is blurry Individual near and far points are basic criteria for designing corrective lenses PHY 2054: Chapter 26 B Eye (Open. Stax) 19
Correcting Farsightedness (Hyperopia) Uncorrected With lens 1/f > 0 PHY 2054: Chapter 26 B Eye (Open. Stax) 20
Correcting Farsightedness: Example Converging Example: lens placed in front of the eye can correct hyperopia assume dnear = 200 cm = 2. 0 m u What we want: be able to see object at 25 cm (normal near point) u Strategy: object is at 25 cm, but make virtual image appear at near point. (Virtual image q = – 2. 0 m ) Note the + sign, like all hyperoptic prescriptions If eyeglasses instead of contacts, correct for eye distance PHY 2054: Chapter 26 B Eye (Open. Stax) 21
Correcting Nearsightedness (Myopia) Uncorrected With lens 1/f < 0 Myopia is very common condition (excessive reading, comp. screen, etc. ) PHY 2054: Chapter 26 B Eye (Open. Stax) 22
Correcting Nearsightedness: Example Diverging lens placed in front of the eye can correct myopia Example: assume dfar = 30 cm = 0. 30 m u What we want: be able to see object at infinity (normal far point) u Strategy: object is at infinity, but make virtual image appear at far point. (Virtual image q = –dfar – 0. 30 m ) • Note the – sign, like all myopia prescriptions If eyeglasses instead of contacts, should correct for eye distance PHY 2054: Chapter 26 B Eye (Open. Stax) 23
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