OPTICS
Nancy Kwon O.D.
Dept. of Ophthalmology
1. Refraction of light at interfaces2. Prisms3. Vergence4. Real vs. virtual objects and images5. Refractive errors6. Accommodation7. Astigmatism8. Contact lens9. Low vision
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
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
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
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
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’
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
What happens in these scenarios ? complete
Examples of total internal reflectionGONIOSCOPY
Lens replacing the tear air interface with plastic
KOEPPE LENS: changes the radius of curvature of the eye
Direct gonioscopy
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
Application of polarization
• Haidenger brush phenomenon
useful to localize the fovea during sensory testing , state of nfl in Henle at the macula
• Polarizing sunglasses
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
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)
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
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
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
Optical and lens aberrations
Characteristics of thick lenses
• Spherical aberrations
Distortions• Coma• Chromatic aberration
13. Optical and lens aberrations
1. Spherical aberration
a. Light rays in the lens periphery are refracted more than the centerb. Large pupil especially at night are prone to spherical aberration (night myopia)c. Avoid LASIK/ PRK in large pupilsd. Human eye compensates for spherical aberrations by
pupil constriction flatter radius of curvature in
corneal periphery alteration of index of refraction
in lens
2. Distortion
a. Optical aberration of thick lensesb. Higher spherical power, the more
periphery is magnified or minifiedc. Plus (aphakic) lenses
Magnification of images Pincushion distortion
d. Minus lenses Barrel distortion Minification of images
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
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 re
d”
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
Laser components
Three basic ingredients1) 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
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
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
Questions GO1
1.What is the wavelength of a wave train with a frequency of 20,000 cycles/sec traveling at 20cm/sec?
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?
3. High index glass has an index of 1.87. What is the speed of light in the glass?
4. What is the size of the 20/40 letter on a printed acuity chart? (5’ at 40 ft)
Answers to GO 1
1. Wavelength = 0.001 cm/cycle2. n=1.113. V=1.6 x 100,000,000 m/s4. X=0.696 inches
1. What is the wavelength of a wave train with a frequency of 20,000 cycles/sec traveling at 20cm/sec?
Answer: Wave velocity = fvelocity of light (c) =3 x 10 8 m/svelocity = 20 cm /secfrequency = 20,000 cycles/sec
f = 20 cm /sec x 1/ 20000 cycles/sec
= 0.001 cm/ cycles
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/sn= 1.11
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 10 7
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 “
concepts and principles1. Properties of light
Wave theory Particle theory Quantum optics – light as wave and particle
2. Quantum theory and ultraviolet lighta. Light is composed of photons that behave as a wave
Wavelength Amount of energy per wave ( frequency)
b. UV light 280-400 nm : shorter wavelength (i.e. - UV ) contains more energy more potential for tissue damage
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