1. Human Vision and Vision Correction (PHR 177)Course Prof. Dr. Moustafa. M. Mohamed Vice Dean Faculty of Allied Medical Science Pharos University Alexandria Dr. Mervat Mostafa Department of Medical equipments Pharos University

2. The Physics of light Properties of Light The Eye Image Formation Glasses

3. Light The Electromagnetic Spectrum

4. Radio Waves - communication Microwaves - used to cook Infrared - “heat waves” Visible Light - detected by your eyes Ultraviolet - causes sunburns X-rays - penetrates tissue Gamma Rays - most energetic

5. Properties of Light

6. Wave model – Classical sinusoidal wave – Can travel through a vacuum –Describes reflection, refraction, diffraction, interference, and Doppler Effect phenomena, etc. Particle model – “photon” –Describes absorption and emission phenomena

7. The eyes mediate sight Function – Sensory organ for sight –Detects light and converts it into neural responses that the brain interprets

8. Anatomy of the Human Eye

9. Eye Anatomy Anatomy – Light enters the eye through the pupil – Photoreceptors (light-sensing cells) are located in the retina – Retina acts like the film in a camera How are images formed?

10. Image Formation: Apertures Apertures – “openings” Basis of a pinhole camera – Dark box small “pinhole” to let in light Image screen on opposite side of hole – All light rays from a scene pass through single point (focusing)

11. The Pupil is an Aperture Pupil –Opening in the center of the eyeball – Bounded by the Iris The iris controls the size of the pupil –Opening through which light enters the eye

12. Image Formation: Apertures To achieve a clear image on an image screen, the aperture must be very small Problems: –Smaller aperture: Fewer photons get through. Ratio of pinhole diameter to image distance should be less than 1/100. –Image screen must be large. –Eye would have to be MASSIVE Solution??

13. Lenses are the Solution to the Aperture Problems Lenses focus of the light waves past the aperture Focuses the image on the screen Allows for wider apertures Produces smaller images

14. Lenses of the Eye Cornea Crystalline Lens Primary function – To focus the image on the back of the retina

15. Refraction Bending of the path of a light wave as it passes across the boundary separating two media – Cause: Change in the speed of the light wave No change in speed = no refraction! Optical Density Optical density of a material determines the speed of a wave passing through it ↑ Op9cal density = ↓ Speed

16. Index of Refraction Abbreviated as “n” Indicator of optical density

17. Index of Refraction refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium. This can be written mathematically as: n = speed of light in a vacuum / speed of light in medium.

18. Another common definition of the refractive index comes from the refraction of a light ray entering a medium. The refractive index is the ratio of the sines of the angles of incidence θ1 and refraction θ2 as light passes into the medium or mathematically: Index of Refraction

19. For example, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in vacuum as it does in water. As light moves from a medium, such as air, water, or glass, into another it may change its propagation direction in proportion to the change in refractive index. This refraction is governed by Snell's law,

20. Snell’s Law –Quantitative answer to the question of “By how much does the light ray refract?” n i *sin(θ i ) = n r *sin(θ r ) n i = index of refraction of incident media n r = index of refraction of refractive medium θ i = angle of incidence θ r = angle of refraction If n i = n r, then no refraction!!

21. Object-Image Relationship Image location changes depending on object distance for a given lens’ focal length The Lens Equation 1/f = 1/d object + 1/d image Image size is limited by short image distance Most vision restricted to small region of the retina Distance-Size Relationship

22. Problem Retina is a fixed distance from the cornea-lens system (~22 mm or 2.2 cm) Lens Equation – 1/f = 1/d object + 1/d image – In the eye, d image is fixed = distance between cornea lens system and the retina d object is fixed = distance between the eye and the object being viewed Solution??

23. The Solution is Accommodation Accommodation –The ability of the eye to change its focal length (f) –Mediated by the lens and ciliary muscles

24. Nearby Objects Have a longer dimage Shorten the focal length Ciliary muscles contract Squeeze the lens into a more convex (fat) shape Pushes cornea bulge out further = greater curvature Distant Objects Nearby Objects Have a shorter dimage Lengthen the focal length Ciliary muscles relax Lens assumes a flatter (skinnier) shape Cornea is not pushed out = less curvature

25. Near Point and Far Point Near Point Closest point at which an object can be brought into focus by the eye – Ideally ~25 cm – Recedes with age (can lead to farsightedness) Far Point Farthest point at which an object can be brought into focus by the eye Typically is infinity Decreases with age

26. Hyperopia INABILITY of the eye to focus on NEARBY objects “Can see far” – no difficulty focusing on distant objects Images of nearby objects are formed at a location BEHIND the retina Near point is located farther away from the eye

27. Hyperopia: Causes Shortened eyeball (retina is closer than normal to the cornea lens system) Cornea is too flat Lens can not assume a highly convex (fat) shape

28. Hyperopia: Correction Need to refocus the image on the retina – Decrease the focal length of the cornea-lens system Add a converging lens

29. Myopia Inability of the eye to focus on DISTANT objects “Can see near” – no difficulty focusing on nearby objects Images of distant objects are formed in front of the retina

30. Causes of Myopia Not usually caused by aging Elongated eyeball (retina is farther away than normal from the cornea-lens system Bulging cornea (greater curvature)

31. Correction of Myopia Need to refocus the image on the retina – Increase the focal length of the cornea-lens system Add a diverging lens

32. Presbyopia “After – 40” vision Progressively diminished ability to focus on near objects as one ages – Similar to hyperopia, but different cause Cause = diminished power of accommodation due to natural process of aging – Reduced elasticity of the lens –Weakening of the ciliary muscles – Changes in lens curvature due to continued growth

33. Astigmatism Most common refractive error Blurred or sometimes distorted vision at any distance Cause: – Irregularly shaped cornea or lens More oblong than spherical Refractive power differs between regions Correction –Glasses Lenses with different radii of curvature in different planes