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OPTICS Nancy Kwon O.D. Dept. of Ophthalmology
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Page 1: Lecture 1.new optics

OPTICS

Nancy Kwon O.D.

Dept. of Ophthalmology

Page 2: Lecture 1.new optics

1. Refraction of light at interfaces2. Prisms3. Vergence4. Real vs. virtual objects and images5. Refractive errors6. Accommodation7. Astigmatism8. Contact lens9. Low vision

Page 3: Lecture 1.new optics

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

Page 4: Lecture 1.new optics

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

Page 5: Lecture 1.new optics

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

Page 6: Lecture 1.new optics

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

Page 7: Lecture 1.new optics

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’

Page 8: Lecture 1.new optics

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

Page 9: Lecture 1.new optics

What happens in these scenarios ? complete

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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

Page 13: Lecture 1.new optics

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

Page 14: Lecture 1.new optics

Application of polarization

• Haidenger brush phenomenon

useful to localize the fovea during sensory testing , state of nfl in Henle at the macula

• Polarizing sunglasses

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Page 16: Lecture 1.new optics

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

Page 17: Lecture 1.new optics

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)

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Page 19: Lecture 1.new optics

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

Page 20: Lecture 1.new optics

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

Page 21: Lecture 1.new optics

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

Page 22: Lecture 1.new optics

Optical and lens aberrations

Characteristics of thick lenses

• Spherical aberrations

Distortions• Coma• Chromatic aberration

Page 23: Lecture 1.new optics

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

Page 24: Lecture 1.new optics

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

Page 25: Lecture 1.new optics

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

Page 26: Lecture 1.new optics

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”

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Page 29: Lecture 1.new optics

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

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Page 31: Lecture 1.new optics

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

Page 32: Lecture 1.new optics

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

Page 33: Lecture 1.new optics

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

Page 34: Lecture 1.new optics

Questions GO1

1.What is the wavelength of a wave train with a frequency of 20,000 cycles/sec traveling at 20cm/sec?

Page 35: Lecture 1.new optics

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?

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3. High index glass has an index of 1.87. What is the speed of light in the glass?

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4. What is the size of the 20/40 letter on a printed acuity chart? (5’ at 40 ft)

Page 38: Lecture 1.new optics

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

Page 39: Lecture 1.new optics

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

Page 40: Lecture 1.new optics

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

Page 41: Lecture 1.new optics

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

Page 42: Lecture 1.new optics

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 “

Page 43: Lecture 1.new optics

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