The Human Eye

1. The Human Eye PHY2054: Chapter 26B Eye (OpenStax)

2. Chapter 26 Contents (+ extra) ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ • The ray aspect of light • Reflection of light • Refraction of light: Snell’s law • Total internal reflection • Dispersion: Rainbows and lenses • Images from lenses • Images from mirrors • Single lens systems • Magnifying glass • Projector • Camera • Human eye PHY2054: Chapter 26B Eye (OpenStax)

3. Chapter 26 Contents (+ extra) • Two lens systems • Microscope • Telescope PHY2054: Chapter 26B Eye (OpenStax)

4. Structure of the Eye • Essential parts of the eye • Cornea – transparent outer structure • Pupil – opening for light • Iris– regulates pupil size • Lens – partially focuses light • Retina – location of image • Optic nerve – sends image to brain • Eye focuses light on retina • Most refraction at cornea • Rest of refraction at lens PHY2054: Chapter 26B Eye (OpenStax)

5. Closeup of Eye Components PHY2054: Chapter 26B Eye (OpenStax)

6. Operation of the Eye(Shown in Relaxed State) 24 mm PHY2054: Chapter 26B Eye (OpenStax)

7. Image of Object on Retina 24 mm PHY2054: Chapter 26B Eye (OpenStax)

8. Operation of Eye • Cornea-lens system focuses light onto retina (back surface) • Retina contains receptors called rods (110M) and cones (7M) • Rods & cones send impulses to brain via optic nerve (1M fibers) • Brain converts impulses into our conscious view of the world PHY2054: Chapter 26B Eye (OpenStax)

9. Picture of Retina PHY2054: Chapter 26B Eye (OpenStax)

10. Rods, Cones Close Up (Retina Cross Section) Rods Cones https://www.pinterest.com/explore/cone-cell/ PHY2054: Chapter 26B Eye (OpenStax)

11. Rods and Conesin Retina Light sensitive parts of rods,cones are inside the retina. Note that light must passthrough cell body first, a well-known design problem. PHY2054: Chapter 26B Eye (OpenStax)

12. Structure of Rods and Cones PHY2054: Chapter 26B Eye (OpenStax)

13. Color Perception in Rods and Cones • One type of rod • Monochromatic vision • Only used for night vision • Highly sensitive • 3 types of cones • Color vision! • 3 primary colors • Not as sensitive as rods Cones Rods PHY2054: Chapter 26B Eye (OpenStax)

14. Focusing Process: “Accommodation” • Distant objects • The ciliary muscle is relaxed • Focal length of eye is maximum ~ 16 mm • Nearby objects • The ciliary muscles tense, squeezing the lens • The lens bulges and the focal length decreases (lens power 1/fincreases) • This adjustment process is called “accommodation” • Focal length of eye (normal) • f≅16.3 mm at rest • 1/f=1 / 0.0163m ≅60 “diopters” • During accommodation, lens power increases as p decreases q fixed: ~ 16.3 mm PHY2054: Chapter 26B Eye (OpenStax)

15. Accommodation in Normal Eye • Distant object • Eye muscles relaxed • No accommodation • Nearby object • Eye muscles tense, causinglens to thicken  1/f larger • This is accommodation So the eye adjusts to distance by changing focal length! PHY2054: Chapter 26B Eye (OpenStax)

16. Accommodation Range Range = 60 – 64~ 4D Range ~ 12D See next slide • “Normal” adult: can see objects from 25 cm to ∞ • Lens to retina = q = 16.3 mm  1/q ≃ 60 diopters (D) • Adolescent: can see objects from 8 cm to ∞ PHY2054: Chapter 26B Eye (OpenStax)

17. Accommodation and Age: Presbyopia • Presbyopia is the reduction in accommodation range with age • Cause: hardening of lens • Accommodation range is max for infants (60 – 73 diopters) • Shrinks with age, noticeable effect on reading after 40. Almost 0 at 65. • Can be corrected with converging lenses (reading glasses) Data from Duane, 1912 PHY2054: Chapter 26B Eye (OpenStax)

18. Image Size on Retina • Example: A tree is 50 m tall and 2 km distant. How big is the image on the retina? PHY2054: Chapter 26B Eye (OpenStax)

19. 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 • Objects closer than dnear appear blurry • It increases with age (presbyopia) because of hardening of lens • Use dnear= 25 cm as a useful average • 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 • Objects farther than dfar appear blurry • For normal vision, far point is at infinity (dfar= ∞) • Nearsightedness (myopia): dfaris finite, so anything beyond dfar is blurry Individual near and far points are basic criteria for designingcorrective lenses PHY2054: Chapter 26B Eye (OpenStax)

20. Correcting Farsightedness (Hyperopia) Uncorrected With lens 1/f > 0 PHY2054: Chapter 26B Eye (OpenStax)

21. Correcting Farsightedness: Example Note the + sign, like all hyperoptic prescriptions • Converging lens placed in front of the eye can correct hyperopia • Example: assume dnear= 200 cm = 2.0 m • What we want: be able to see object at 25 cm (normal near point) • Strategy: object is at 25 cm, but make virtual image appear at near point. (Virtual image q = –2.0 m ) • If eyeglasses instead of contacts, correct for eye distance PHY2054: Chapter 26B Eye (OpenStax)

22. Correcting Nearsightedness (Myopia) Uncorrected Myopia is very common condition (excessive reading, comp. screen, etc.) With lens 1/f < 0 PHY2054: Chapter 26B Eye (OpenStax)

23. Correcting Nearsightedness: Example • Note the – sign, like all myopia prescriptions • Diverging lens placed in front of the eye can correct myopia • Example: assume dfar= 30 cm = 0.30 m • What we want: be able to see object at infinity (normal far point) • Strategy: object is at infinity, but make virtual image appear at far point. (Virtual image q = –dfar–0.30 m ) • If eyeglasses instead of contacts, should correct for eye distance PHY2054: Chapter 26B Eye (OpenStax)