1. The Human Visual System The Eye

2. Anatomy of the Human Eye Cornea Pupil Iris Sclera Retina Optic Nerve Lens

3. Human Visual System Refraction Cornea & Lens Light Control Iris (& Pupil) Light Detection Retina Processing Brain

4. Image Formation u The image projected onto the retina is upside down. Object Image

5. Cornea u The outer wall of the eye is formed by the hard, white sclera. u The cornea is the clear portion of the sclera. u Most of the refraction takes place at the cornea. Why? Sclera Cornea

6. Iris and Pupil u The colored iris controls the size of the opening (pupil) where the light enters. u The iris controls the amount of light, like the aperture of a camera. Iris Pupil

7. Iris open Dilated pupil Iris closed Constricted pupil

8. Lens u The lens is transparent. u The lens can change shape. u This is what allows you to focus on objects at different distances. u For close objects, the lens becomes thicker. u For far objects, the lens becomes thinner. Lens Ciliary muscle Transparent Fibers Cross section of the eye lens

9. Accommodation u The lens is attached to the ciliary muscle. u When the muscles contract, the lens becomes thicker. u When the muscles relax, it becomes thinner. u The process by which the lens changes shape to focus is called accommodation. Relaxed muscle Taut ligaments Distant object Near object Contracted muscle Slack ligaments

10. Muscle relaxes Distant object Lens gets thinner Object is in focus

11. Muscle contracts Near object Lens gets thicker Object is in focus

12. Aqueous Humor and Vitreous Humor u Transparent gelatinous liquid filling the eye. u Provides nutrients to the cornea and eye lens. u Also helps maintain the eyeball shape with its pressure. Vitreous Humor Aqueous Humor

13. Retina u The Retina is the light detector for the eye. u Two types of receptors in the retina: u rods for low light level u cones for color. u Signals from the receptors leave through the optic nerve to the brain. Retina Optic Nerve

14. Blind Spot u The retina has a hole in it where the optic nerves exit the eye. u This is because the nerves run in front of the retina. u Our brain fills in the blank spot with surrounding colour, so it’s hard to notice “Hole” in retina Photo receptors Optic Nerve Light

15. Rods and Cones u Only black, grey, white u Only in low light levels. Rod cell Cone cell u Three types of cones in order to detect colour u Requires a lot of light

16. Mach Bands Actual brightness Perceived by you

17. Eye Defects* u Image focuses on the retina for a normal eye. u Distant objects look blurry for a myopic (near sighted) eye. u Near objects look blurry for a hyperopic (far sighted) eye. Normal Myopic Hyperopic Object at infinity

18. Myopia – Near-sightedness u Distant objects look blurry because the eye cannot relax any farther so that the image is focused before the retina. u Near object in focus without accommodation. u Corrected with a diverging lens. Myopic eye relaxed Blurry Myopia corrected with a negative lens Far object Near object Myopic eye relaxed In focus Far object

19. Hyperopia – Far-sightedness u Near objects look blurry because the eye cannot accommodate enough for near objects. u Far object in focus. u Corrected with a converging lens. Hyperopic eye Partially accommodated In focus Hyperopia corrected with a positive lens Far object Hyperopic eye Fully accommodated Blurry Near object

20. Contact Lens u Contact lens is an alternative to corrective lenses. u Changes the curvature of the cornea by adhering to the surface with some fluid. Cornea Contact lens Fluid

21. Presbyopia - “Old eye” u Lens hardens with age. u Eye cannot adequately accommodate near objects. u Bifocals (lens with two focal lengths) contains a concave lens for distance (if needed) and a convex lens for near objects. Near objects magnified Far objects Concave for distance correction (if needed) Convex for near object correction

22. Astigmatism u The cornea is not spherical; Focal length different from one plane to a perpendicular plane. F’ horizontal F’ Vertical Object Image at F’ Vertical Image at F’ Horizontal Cornea Direction of blur

23. Astigmatism u Correction of astigmatism is done through the use of a cylindrical lens. u Cylindrical lens converge rays in one plane but not the perpendicular plane. Cylindrical lens Rays in the vertical plane are undeviated Rays in the horizontal plane are focused

24. Common Eye Diseases* u Cataract - Clouding of the lens. u Symptom: Loss of vision u Cure: Lens replacement u Glaucoma - Pressure buildup in the eye, damaging the retina. u Symptom: Loss of vision first in the periphery. u Cure: Surgery to drain fluid from the eye. u Loss of vision is usually permanent

25. Common Eye Diseases (cont’d)* u Detached retina - portion of the retina detaches from the back of the eye. u Symptom: Perception of flashes, Loss of vision u Cure: Laser surgery to reattach retina u Pink eye - Infection of the surface of the eye. u Symptom: Irritation u Cure: Antibiotics Ow!

26. Your eye care Go see a doctor if you think there is something wrong with your eye- Early detection is essential to keeping damage low and preventing permanent loss of your vision.