Optics and the Eye

1. Optics and the Eye

2. The Visible Spectrum

4. Some similarities between the eye and a camera

5. Refraction - the basis of optics Light bends when it goes from one medium to another The amount or bending, or refraction, depends on the angle incidence, and the nature of the two media

6. Objects-Lens Distance

7. Optical Power greater the power the closer the image is to the lens

8. Convex lenses have positive optical power Concave lenses have negative optical power

9. The precise distance from the lens to the focused image depends on lens power and the distance to the object according to the following equation: P = 1/F = 1/do + 1/di (lensmaker equation) P = lens power F = the focal distance do = the distance to the object di = the distance to the image (all distances are expressed in meters)

11. Cornea - does most refracting Lens - does fine focussing

12. Focussing --Accomodation In humans, fine focussing is controlled through changing the shape of the lens.

13. Accomodation • Changing the lens shape is controlled by the ciliary muscles

15. Average human eye’s power is 60 diopters (cornea and lens together) Focal distance of such optics is 1/60 = 0.0167 m = 16.7 mm Posterior nodal distance of average eye is 16.7 mm - good fit!

19. Near and Far Points • Near point - closest distance that an object can still be kept in focus. • Will change with age • Far point • Normally at optical infinity

21. Astigmatism Astigmatism Target Sometimes a lens can be emmetropic for light waves in one orientation (say, for horizontal lines), but be badly hyperopic or myopic for other orientations (say, vertical lines). Thus, people with astigmatic vision see sharp lines and contours in some orientations, and blurry contours in others.

22. Someone with an astigmatism in the vertical orientation might see the target such that the horizontal lines are in focus and of high contrast, but as the lines become more vertical they go out of focus, becoming blurry and lower in contrast.

23. The Human Eyeball

25. The Retina The cells of the RETINA act as transducers. A transducer changes one form of energy into another.

27. Membranous discs contain photopigment

28. Rod Photoreceptors • about 120 million per eye • only one kind • most sensitive to light of about 505 nm • approx. 10 times more sensitive than cones • used in night/scotopic vision • psychophysical & physiological data indicate that rods can respond to a single photon!

29. Cone Photoreceptors • about 8 million per eye • 3 kinds, each most sensitive to 440, 530 or 560 nm • basis of colour vision • approx. 10 times less sensitive than rods • used in day/photopic vision

30. Distribution of Rods & Cones • rods are most dense in periphery • no rods in the center of the macula • cones are most dense in fovea • no receptors in blind spot

32. Eye Movements

33. Why move our eyes? One reason we move our eyes is that we have a relatively narrow field of vision and must move our eyes around to sample the visual world extensively. A second reason we move our eyes is that our retinas are not uniformly sensitive. Our retinas have the highest acuity in the region called the fovea. To see something clearly we orient our eyes so that the image will be projected onto the high resolution fovea.

34. Two Main Classes of Eye Movements 1. Conjugate (Version) - Both eyes move to the same degree and in the same direction 2. Vergence - eyes rotate in opposite directions - e.g. inward to look at a close object

35. Conjugate Eye Movements

36. Vergence Eye Movements