Lecture 1.new optics
- 1. OPTICS Nancy Kwon O.D. Dept. of Ophthalmology
- 2. 1. Refraction of light at interfaces 2. Prisms 3. Vergence 4. Real vs. virtual objects and images 5. Refractive errors 6. Accommodation 7. Astigmatism 8. Contact lens 9. Low vision
- 3. Refraction of light at interfaces • Light slows down when entering refractive media • Refractive index = n = speed of light in vacuum Speed of light in material n is always > 1 n vacuum = 1 ( exactly ) n air = 1.0003 n water = 1.33
- 4. Two laws of refraction: SNELLS LAW n sin i = n’ sin r Incident ray, normal to the surface and refracted ray all lie in the same plane
- 5. When a light ray passes from a medium with a lower refractive index (n) to a medium with a higher refractive index ( n’), it is bent toward the normal •When passing from a higher refractive index ( n ) to a lower refractive index ( n ‘) it is •bent away from the normal
- 6. Index of Refraction • Index of refraction of lens materials • Air 1.00 • Water 1.33 • Aqueous / vitreous 1.34 • Cornea 1.37 • Lens 1.42 • Plastic ( CR-39 ) 1.49 • Crown glass 1.52 • High index plastic 1.7 – 1.9
- 7. Greater than critical angle ; get total internal reflection Critical angle Critical angle • Only occurs when light passes from a higher index to a lower index medium n n’ n n’
- 8. Total internal reflection and the critical angle • The angle at which all light is reflected instead of refracted (“bent”) into the medium with a higher refractive index • Light from the angle is typically reflected internally by the cornea and tear film • Gonioscopy contact lens and methycellulose alters the index of refraction of light to allow to see iris root angle
- 9. What happens in these scenarios ? complete
- 12. Examples of total internal reflection GONIOSCOPY Lens replacing the tear air interface with plastic KOEPPE LENS: changes the radius of curvature of the eye Direct gonioscopy
- 13. Polarization • “ light waves moving through a picket fence”--- • -Waves of certain direction come through, others blocked • Unpolarized light– mixture of various plane polarized beams • Partial polarization– mix of light, plane,circular, elliptical
- 14. Application of polarization • Haidenger brush phenomenon useful to localize the fovea during sensory testing , state of nfl in Henle at the macula • Polarizing sunglasses
- 16. 11. Diffraction 2. AIRY DISC • Smallest circular aperture that can still give resolution of a point light source ----- airy disc • Sets a limit on VA when pupil size less than 2.5 mm • bright rings---- constructive interference • dark rings----- destructive interference 1. Light waves are bent when they encounter a physical aperture such as a circle or pupil
- 17. Diffraction - continued Image resolution degrades through a small pupil because of diffraction Pupils less than 2.5 mm cause decreased acuity, can even be 1.5 mm small before VA decreased 3. Small pupils 4. Clinical examples of diffraction Pinhole refraction : BVA may not be better than 20 /25 bc of diffraction Squinting Stenopeic slit Diffractive multifocal IOL (Restor, Rezoom)
- 19. 12. Rayleigh scattering • Explains why sky appears “ blue” • Shorter wavelengths of light scattered more than longer wavelengths • Violet is the shortest wavelength in the visible range • Of the three cones in retina, blue chromophore absorbs the shortest wavelength of light • We see scattered blue light in atmosphere (instead of violet sky) while we see unscattered red light from the sun
- 20. Scattering • Occurs because of irregularities in the light path, such as particles or inclusions, large or small • Scattering varies according to wavelength • Large particles scatter light more—less dependent on wavelength • Small particle scatter--- depends on wavelength , shorter wavelengths scatter more
- 21. Scattering Scattering in the human eye by what pathological conditions? Corneal haze--- excess water in stroma , disrupts the close packed collagen structure Early cataract---large molecules cause scattering Anterior chamber flare---protein in the aqueous humor
- 22. Optical and lens aberrations Characteristics of thick lenses • Spherical aberrations Distortions • Coma • Chromatic aberration
- 23. 13. Optical and lens aberrations 1. Spherical aberration a. Light rays in the lens periphery are refracted more than the center b. Large pupil especially at night are prone to spherical aberration (night myopia) c. Avoid LASIK/ PRK in large pupils d. Human eye compensates for spherical aberrations by pupil constriction flatter radius of curvature in corneal periphery alteration of index of refraction in lens
- 24. 2. Distortion a. Optical aberration of thick lenses b. Higher spherical power, the more periphery is magnified or minified c. Plus (aphakic) lenses Magnification of images Pincushion distortion a. Minus lenses Barrel distortion Minification of images
- 25. Optical Aberrations 3. Coma Off axis light rays cause a comet shaped image/ aberration Off- axis spherical aberration Similar to spherical aberration but occurs in nonaxial rays Type of higher order wavefront aberration (LASIK) Seen in keratoconus, decentered corneal transplants, decentered LASIK ablations
- 26. 4. Chromatic aberration Each wavelength of visible light has a different index of refraction Shorter wavelengths (blue) are refracted more 5. Duochrome Test Subjective monocular test Based on chromatic aberration Used to prevent giving too much minus “when in doubt, leave them in the red”
- 29. LASER Laser light is : • Monochromatic • All photons have the same wavelength ( less chromatic aberration through the lens system) • Coherent • The emitted photons are oscillating in the same direction at the same time “ in phase” • Able to produce interference pattern • Polarized - linear • Directional - emits narrow beam that spreads slowly • Intensity - Can deliver large amount of energy to a small area • Brightness per unit area Light Amplification by Stimulated Emission of Radiation
- 31. Laser components Three basic ingredients 1) Pulsed power source to supply energy - makes light coherent 2) Active medium ( photon emitter) • makes light monochromatic 3) Chamber with mirrors at opposite ends ( has to reflect 90% and one has to let light through ) • one partially transmits • reflects photons multiple times before release • makes light directional
- 32. Laser light damage : Mechanisms Three ways 1) Photocoagulation • Energy is absorbed by the tissue • Local rise in temperature • I.e. Argon, Krypton dye, holmium etc 2) Photodisruption • Plasma formation: the target is ionized • Shock wave • I.e. Nd:Yag
- 33. 3) Photoablation • Sublimation- disruption of covalent bonds • High energy photon of 193 UV light exceeds the covalent bond strength of corneal proteins • No heat or force • I.e. excimer laser
- 34. Questions GO1 1.What is the wavelength of a wave train with a frequency of 20,000 cycles/sec traveling at 20cm/sec?
- 35. 2. What is the index of refraction of a material , if the speed of light within the medium is 2.7 x 10 8 m / s?
- 36. 3. High index glass has an index of 1.87. What is the speed of light in the glass?
- 37. 4. What is the size of the 20/40 letter on a printed acuity chart? (5’ at 40 ft)
- 38. Answers to GO 1 1. Wavelength = 0.001 cm/cycle 2. n=1.11 3. V=1.6 x 100,000,000 m/s 4. X=0.696 inches
- 39. 1. What is the wavelength of a wave train with a frequency of 20,000 cycles/sec traveling at 20cm/sec? Answer: Wave velocity (υ) = λ f velocity of light (c) =3 x 10 8 m/s velocity = 20 cm /sec frequency = 20,000 cycles/sec λ = υ / f = 20 cm /sec x 1/ 20000 cycles/sec = 0.001 cm/ cycles
- 40. 2. What is the index of refraction of a material, if the speed of light within the medium is 2.7 x 10 8 m/s ? Answer: n= c / v n= 3 x 10 8 m/s / 2.7 x 10 8 m/s n= 1.11
- 41. 3. High index glass has an index of 1.87. What is the speed of light in the glass? Answer: n= c / v n= 1.87 v = c / n = 3 x 10 8 m /sec / 1.8 = 1.6 x 107
- 42. 4. What is the size of the 20/40 letter on a printed acuity chart? (5’ at 40 ft) Answer: Tan 5’ = 0.0145 = h / 240 “ 20’x 12” h = 0.0145 x 240” = 0.348 This is size of the 20/20 letter on the visual acuity chart 20 feet away Ratio wise the 20/40 letter at the same distance would be larger Tan 5’ = 0.0145 = h / 480” 40’ x 12” h= (0.0145) (480”) = 0. 696 “
- 43. concepts and principles 1. Properties of light Wave theory Particle theory Quantum optics – light as wave and particle 2. Quantum theory and ultraviolet light a. Light is composed of photons that behave as a wave Wavelength Amount of energy per wave ( frequency) a. UV light 280-400 nm : shorter wavelength (i.e. - UV ) contains more energy more potential for tissue damage