Optometric Optics

1. LIGHT = life
G K K
M Optom – CU
Asst Prof
MLUH

2. Light is a form of energy whose interaction with retina gives the
sensation of sight. It is visible portion of the electromagnetic
radiation spectrum.
Light enables us to see.
-luminous objects
-non- luminous objects
It lies between ultraviolet and infrared portions, from 400 nm at the violet end of
the spectrum to 700 nm at the red end .
The white light consists of seven colours denoted by
VIBGYOR
1. Violet
2. Indigo
3. Blue
4. Green
5. Yellow
6. Orange
7. Red

3. ELECTROMAGNETIC SPECTRUM

4. Colour Wavelength (order)
Red
Orange
Yellow
Green
Blue
violet
620-780nm
590-620nm
570-590nm
500-570nm
450-500nm
380-450nm

5. Properties of light
• Propagated as electromagnetic wave, i.e., it does not require medium
for its propagation.
• Speed of light in free space (vacuum) is 3 × 108 m/s (186,000 miles/s).
• Speed in a medium is less than in the vacuum.
• Transverse in nature, so can be polarized.
• Monochromatic light refers to light of a single wavelength.
•White light is heterochromatic.
• Not deflected by electric and magnetic field.

6. Nature of Light
Wave theory of light ( Huygen’s theory).
Particle theory of light (Newton’s theory-
corpuscles)
Modern theory of light --- Quantam Theory
(depending on the situation explaining both
theories)

15. Visible light and the eye
• Media of the eye are uniformly permeable to the visible rays between
600 nm and 390 nm.
• Cornea absorbs rays shorter than 295 nm. Therefore, rays between 600
nm and 295 nm only can reach the lens.
• Lens absorbs rays shorter than 350 nm. Therefore, rays between 600
nm and 350 nm can reach the retina in phakic eyes; and those between
600 nm and 295 nm in aphakic eyes.
• Eye is most sensitive to yellow-green light (wave length 550 nm).
•The sensitivity decreases on both sides of this wavelength, so it is
minimum for violet and red light

16. Sensitivity of eye to visible spectrum of light

17. 1. Physical Optics
Physical character and behavior of light and its interaction with matter.
Physical optics takes into consideration the basic dual nature of light, the
waveform and the particle (photon or quantum) form, and thus includes:
A. Wave optics.
B. Quantum optics.
2. Geometrical Optics
Image forming properties of lenses, mirrors and prisms
A. Reflection.
B. Refraction.

18. Light is a transverse wave
One transverse wave has a crest and a trough
Wave optics

19. Longitudinal Waves
Compression Rarefaction

21. Transverse Wave
Phase
Any point on the wave that we want to
discuss.
Amplitude
Maximum displacement of a wave
Wavelength
Distance between 2 symmetrical part of
wave motion
Phase Difference
It refers to fraction of cycle

22. Time period
Time taken for a complete vibration is called time period

23. Frequency
Number of oscillations/cycles per unit time is called
frequency
Unit = hertz
Never changes when light moves from one medium to
another (5.45x1014 Hz)
Intensity
Energy delivered or carried by the wave per unit area per
unit time
I = (energy delivered)/[(area/time)]

24. Phenomena based on wave optics are:
• Interference
• Diffraction
• Polarization

25. Interference
Superposition principle
•When two or more low energy waves are superposed on the same space or medium
•The resultant displacement at each position is the
algebraic sum of the displacements due to each wave.
Constructive Interference
In Phase
Crests of both the component waves line up
Resultant amplitude is equal to the sum of
the two component amplitudes
Ar = (A1+A2)

26. Destructive Interference
Out of Phase
Crest of one wave lines up with the trough of
another component wave
a.
b.
Resultant amplitude is equal to the
subtraction of two component waves
Ar = (A1 – A2)

28. Applications
• Holography
• Laser Interferometry
• Anti Reflection Coatings
• Excitation filter and Barrier filter used in Fundus Fluorescein
Angiography(FFA)
• Optical Coherence Tomography (OCT)

29. Diffraction
The ability of a wave to propagate around corners is called diffraction

30. Applications
•Limits the visual acuity when the pupil size is less than 2.5mm.
•Resolution of an image

31. Polarization
The phenomenon of conforming the vibrations of a wave in
specific direction perpendicular to the direction of wave motion
is called polarization

32. Applications
•Haidinger Brushes (Orthoptic Exercises)
• Slit lamp (↓unwanted reflections)
•Polarizing projection charts (malingering patients)
Uses
• Titmus Fly Test (stereopsis)
• Recording and Reproducing 3D
pictures
• Polaroid Sunglasses

33. Quantum optics
Quantum optics treats light as a particle (localized energy pocket)
called photon.
Phenomena based on quantum optics are:
• Transmission and absorption of light
• Scattering of light
• LASER (Light amplification of stimulated emission of radiation)
• Fluorescence.

34. Absorption
The amount of light lost in a medium while passing through it

35. Transmission
The total amount of light coming out from a medium in the incident direction

36. Scattering
When a beam of light passes through a medium (gas) a part of it
appears in the direction other than the incident direction
(heterogeneous medium) this phenomenon is called scattering of light
Depends on
• Size of the particle
• Distance between particles
• Strength of the interaction

37. Scattering of light

38. Small Particles < λ/10
• Wavelength dependent
• Small wavelength(more)
• Large wavelength(less)
eg: Air molecules
Applications
• Blue color of sky
• Corneal haze in oedema
• Appearence of Cataract
• Cells and Flare
• Glare

39. LASER
Light Amplification by Stimulated Emission of Radiation
Characteristics
•Monochromatic
•Coherent
•Collimated

40. Monochromatic
Same Wavelength
Coherent
Same phase difference

41. Collimated
All the rays are exactly parallel to each other

42. A laser is a device that emits light through a process of
optical amplification based on the stimulated emission of
electromagnetic radiation.

43. Applications of LASER
•Excimer Laser (Refractive Surgery)
•Nd: YAG (Neodymium Yttrium Aluminum Garnet)
Dissection of Posterior Capsule, Peripheral Iridotomy
•Argon and Krypton lasers for retinal photocoagulation
•Carbon dioxide lasers for photo vaporization

44. 2. Geometrical Optics
Image forming properties of lenses, mirrors and prisms
A. Reflection.
B. Refraction.

45. Reflection of light
Reflection of light is a phenomenon of change in the path of light
rays without any change in the medium.
 The light rays falling on a reflecting surface are called incident rays
and those reflected by it are reflected rays.
A line drawn at right angle to the surface is called the normal.
Laws of reflection
1. The incident ray, the reflected ray and the normal at the point of
incident, all lie in the same plane.
2. The angle of incidence (i) is equal to the angle of reflection (r).

46. Laws of reflection

47. REGULAR REFLECTION OR SPECULAR REFLECTION
mirror-like reflection of light.
the reflected rays are also parallel to each other.

48. IRREGULAR REFLECTION OR DIFFUSED REFLECTION
•the different portions of the surface reflect the incident
•light in different directions
• no definite image is formed
• the surface becomes visible.
• non-luminous objects visible

49. Refraction of light
Refraction of light is the phenomenon of change in the path of light, when it goes from
one medium to another.
The basic cause of refraction is change in the velocity of light in going from one
medium to the other.
Laws of refraction
1. The incident (i) and refracted (r) rays are on opposite sides of the normal (N) and all the
three are in the same plane.
2. The ratio of sine of angle of incidence (i) to the sine of angle of refraction (r) is constant for
the part of media in contact.
This constant is denoted by the letter m and is called ‘refractive index’ of the medium
In which the refracted ray lies with respect to medium 1 (in which the incident ray sin i lies), i.e.
sin r = m2.
When the medium 1 is air (or vaccum), then n is called the refractive index of the medium
This law is also called Snell’s law of refraction.

50. Laws of refraction. N1 and N2 (normals); I (incident ray); i (angle of incidence); R
(refracted ray, bent towards normal); r (angle of refraction); E (emergent ray,
bent from the normal)

51. Total internal reflection
When a ray of light travelling from an optically denser medium to an optically-rarer
medium is
incident at an angle greater than the critical angle of the pair of media in contact, the
ray is totally reflected back into the denser medium.
This phenomenon is called total internal reflection.
Critical angle refers to the angle of incidence in the denser medium, corresponding to
which angle of refraction in the rare medium is 90°.
It is represented by C and its value depends on the nature of media in contact.
Application.
1. fibroptic lights
2. applanation tonometer
3. gonioscope.

52. Refraction of light (1-1’)
path of refracted ray at critical angle, c (2-2’)
Total internal reflection (3-3’)