This document summarizes key concepts in optics, including: 1. Refraction of light at interfaces and how refractive index is defined. Total internal reflection occurs when light passes from higher to lower index medium at an angle greater than the critical angle. 2. Optical phenomena like diffraction, scattering, polarization are discussed. Refractive errors and accommodation are also covered. 3. Optical aberrations like spherical aberration and chromatic aberration are properties of thick lenses. Laser components and mechanisms of laser tissue damage complete the summary.

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