The lecture of optics based on postgraduate ophthalmic student. Here it describes the reflection, refraction and physiological defect of eye

1. Geometrical Properties of Light
Md Anisur Rahman (Anjum)
Professor & Head
(Ophthalmology)
Dhaka Medical College, Dhaka
Second Lecture

2. Geometric Optics
 Geometric Optics deals with the formation of images by using
such optical devices as lenses, prisms and mirrors and with the
laws governing the characteristics of these images, such as
their size, shape, position and clarity.
 Rays of light
 Pencil of light
 Beam of light
• (M.A MATIN P=19)
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3. Some basic aspect
Before discussion of reflection we have to clear some
basic idea:
Luminous versus Illuminated Objects
Opaque. Transparent & Mirror surface
Specular reflection & diffuse reflection
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4. Luminous versus Illuminated Objects
Luminous objects are objects that generate their own
light
Illuminated objects are objects that are capable of
reflecting light to our eyes.
The sun is an example of a luminous object, while the
moon is an illuminated object.
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5. Opaque. Transparent & Mirror surface
1) Some substances absorb light which fall on them.
These are called Opaque substances.
2) Some substances allows the light to pass through
them. These are called Transparent substances,
such as glass.
3) Some others such as mirror surfaces reflect the light
backward.
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6. Q. What happened to the light when it
strikes a surface?
Ans) 3 things may happen. It may be:
 Absorbed
 Reflected
 Or Refracted

8. Reflection
 The law of reflection, evidently first stated by Euclid
around 300 BC, states that when light encounters a
flat reflecting surface the angle of incidence of a ray
is equal to the angle of reflection

9. Defination of Reflection
 Reflection is defined as the change of path of light
without any change in the medium.
 All the reflections end up in producing images of the
object kept in front of the reflecting surface.

10. Specular reflection & diffuse reflection
Reflection of smooth surfaces such as mirrors or a
calm body of water leads to a type of reflection
known as specular reflection.
Reflection of rough surfaces such as clothing, paper,
and the asphalt roadway leads to a type of reflection
known as diffuse reflection.

11. specular reflection & diffuse reflection
The diagram depicts two beams of light incident upon
a rough and a smooth surface.

14. Applications of Specular and Diffuse
Reflection
The light is thus able
to travel together to the
lens of the camera and
produce an image (an
exact replica) of the
subject which is strong
enough to perceive in
the photograph. An
example of such a
photograph is shown.

15. Whether the surface is microscopically rough or
smooth has a tremendous impact upon the
subsequent reflection of a beam of light.
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16. 1) The incidence ray and
the reflected ray lie in
the same plane which
is perpendicular to
the mirror surface at
the point of incidence.
2) When light is
reflected off any
surface, the angle of
incidence is always
equal to the angle of
reflection,
Laws of Reflection

17. Optical density
In space light maintains a constant speed of about
184,000 miles/second but as it travels through the
substance of a transparent body it will encounter
resistance and its speed will be lowered. This
property of offering resistance to light is known as
Optical density.
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18. Direction of light
1) Divergence (minus vergence)
2) Zero vergence (straight)
3) Convergence (plus vergence)
 (M.A MATIN = 20)
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20. Refraction
Q) What is refraction?
Ans) Refraction of light is a phenomenon of change in the
direction of light when it passes from one medium to another
due to change in velocity.

21. Terms used in refraction
1) NORMAL: This is a line right angles to the interface
2) INCIDENCE RAY: The ray that strikes the interface at the
base of the normal in an angular fashion.
3) REFRACTED RAY: This is the deviated ray in the second
medium.

22. 4) ANGLE OF INCIDENCE: Angle between the normal and the
incident ray
5) ANGLE OF REFRACTION: The angle between the refracted
ray & the normal is called ANGLE OF REFRACTION
6) The two angles are never equal.

23. Snell’s Law

24. Total Internal Reflection (TIR)
• The complete reflection of a light ray reaching an interface
with a less dense medium when the angle of incidence exceeds
the critical angle.

25. Total Internal Reflection (TIR)

27. Critical Angle
Critical angle is the angle of incidence above which total internal
reflection occurs.
It is defined as the angle when the incidence ray is of such an
angle that the refracted ray is at right angles to the normal

28. • Critical angle of glass is 48.60, diamond is 240 (refractive
index is 2.42) and water is 48.750.

29. Different uses of TIR
1) Gonioscopy employs total internal reflection to view the
anatomical angle formed between the eye's cornea and iris.
2) Total internal reflection is the operating principle of optical
fibers, which are used in endoscopes and telecommunications.
3) Total internal reflection is the operating principle of
automotive rain sensors, which control automatic
windscreen/windshield wipers

30. Different uses of TIR
• The phenomena is used in many optical instruments like
telescope, microscope, binocular, periscope etc
• The brilliance of diamond is due to TIR
• The phenomena of mirage explained by TIR (an optical
illusion caused by atmospheric conditions, especially the
appearance of a sheet of water in a desert or on a hot road
caused by the refraction of light from the sky by heated air)

32.  If we think eye as an optical device, mathematically, it is not a
perfect optical device, but functionally it is an unique optical
device.
 Obviously there are some physiological optical defects:

33. What are the physiological defects of eye?
1) Diffraction of light
2) Chromatic aberration
3) Spherical aberration
4) Decentring
5) Peripheral aberration
 Ref: Duke-Elders p: 35
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34. 1) Diffraction of light
• The diffraction of light was first explained by
the English scientist Thomas Young in the
early 1800s.
• He made a pin-hole in his window shutter
and observed diffraction of the sunlight on
the board.

35. Thomas Young Experiment: Diffraction

36. 1) Diffraction of light
Diffraction is the bending of waves around obstacles,
or the spreading of waves by passing them through an
aperture, or opening.
Smaller the pupil more diffraction will be occur
https://www.vocabulary.com/dictionary/diffraction

37. 1) Diffraction of light: The Airy disc
When light passes through a circular aperture, a
circular diffraction pattern is produced.

38. • This consists of a bright central disc surrounded
by alternate dark and light rings. The central
bright zone is known as the Airy disc.
• (Elkington: 10)

39. 1) Diffraction of light: Applied aspect
a) Diffraction effects are most marked with small
apertures, and occur in all optical systems
including lenses, optical instruments and the
eye.
In the case of lenses and instruments, the
diffraction effect at the apertures used is negligible
compared with the other errors or aberrations of
the system (Elkington).

40. Diffraction of light: Applied aspect
b) In the case of the eye, diffraction is the main
source of image imperfection when the pupil is
small. (Elkington). The image formed on the retina
from a distant small source has the form a blur
circle the Airy disc (Khurana)
c) The principle of diffraction is used in the design
of some multifocal intraocular lenses (Elkington)

41. Diffraction of light: Further reading
a) Diffractive multifocal intraocular lenses
b) Second-generation diffractive multifocal
intraocular lenses
(AAO: Vol: 3. P 303-304 2016-17)
Types of diffraction: 2 Types (Khurana P: 6)
1) Fresnel diffraction
2) Fraunhofer diffraction

43. 2. Chromatic aberration
When white light is refracted at an optical
interface, it is dispersed into its component
wavelengths or colours.
The shorter the wavelength of the light, the more it
is deviated on refraction.
Thus a series of coloured images are formed when
white light is incident upon a spherical lens

44. Chromatic aberration:
RED> GREEN>BLUE

45. When lenses are used in instruments, it is desirable
to eliminate chromatic aberration.
BUT HOW?
Do you know about Achromatic lenses?

46. By combination of crown & flint glass

47. How to overcome chromatic aberration in glass?
Flint glass gives a dispersion nearly double that of the
crown glass, and its R,I is 1.7, while the RI of crown glass
is 1.5; hence if we combine a convex lens of crown glass
with a concave lens of half the strength composed of flint
glass, the dispersion will be neutralized, while a
considerable part of the refractivity of the crown glass will
still remain. Such a combination forms an achromatic lens.

48. Achromatic lens
• Flint glass:
• Refractive Index = 1.7
• Dispersive power: 2
• Crown glass:
• Refractive Index = 1.5
• Dispersive power: 1
• Suppose we take
• +10 D (Crown)
• - 5 (Flint)
• Ref Duke- Elder 33

49. Before that we have to know the Dispersion of Light & dispersive power
White light is composed of various wavelengths.
Light of shorter wavelength is deviated more than
light of longer wavelength, e.g. blue light is
deviated more than red.

50. • The refractive index of a material is normally
taken to mean that for the yellow sodium flame.
• The angle formed between the red and blue light
around the yellow (Fig. 3.12) indicates the
dispersive power of the medium (cf. chromatic
aberration).

52. Diagram to show dispersion of light. (The angles
involved are exaggerated.) (Fig: 3.12 Elkington)

53. Dispersive power
The ratio of angular dispersion to the angle of
deviation for the mean wavelength (yellow colour)
is called dispersive power of the material.
The difference in the angle of deviation of extreme
colours i.e. red and violet is known as
angular dispersion.

54. Duochrome Test
In clinical practice the chromatic aberration of the
eye is made use of in the duochrome test.
The patient views the letters by means of red and
green light respectively, and can easily tell which
appear clearer. The test is sensitive to an alteration
in refraction of 0.25 D or less.

55. A myopic eye
sees the red
letters more
clearly than
the green
while a
hypermetropic
eye sees the
green letters
more
distinctly

56. 3) Spherical aberration
 What is spherical aberration in lenses?
• The shape of a spherical lens causes a problem
called spherical aberration. In spherical aberration,
parallel light rays that pass through the central
region of the lens focus farther away than light rays
that pass through the edges of the lens. The result is
many focal points, which produce a blurry image
• https://www.google.com/search?q=what+is+spherical+aberration%3F&rlz=1C1CH
BF_enBD875BD875&oq=what+is+spherical+aberration%3F&aqs=chrome..69i57j0l
7.18166j0j8&sourceid=chrome&ie=UTF-8

57. Aberration of Spherical lens

59. How to minimize chromatic & spherical
aberration?
The best forms are the so-called periscopic and meniscus
lenses, where the radius of curvature of the posterior
surface is greater than that of the anterior surface.
This error can be eliminated by grinding lenses so that
their curvature gradually decreases from the center to the
periphery. Such lenses are called Aplantic lenses.

60. What is “Best formed lens?”
To eliminate the spherical aberration, curvature of the lens
is reduced periphery by grinding. In other way in
periphery of the lens is attached by a plane lens which
eliminate the peripheral ray.
By combining a flint and crown glass in appropriate
power an achromatic and Aplantic combination can
reduced both chromatic and spherical aberration which is
also called the “ Best formed lens”.
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62. To form an ideal image by eye, the optical system
have to be accurately centered (cornea & two surfaces
of lens)
But virtually the center of the cornea is 0.25 mm
below the axis of the lens.
On the other hand the fovea is not situated on the
optical axis. 1.25 mm below & downward (temporal)
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64.  When we look at an object, we do not look directly
along optic axis (line passing through corneal center,
lens & meets retina on nasal side of fovea. ANR)
 But along a line joining the object with the fovea and
passing through the nodal point. This line is called the
visual axis. (ONF)

65. Decentering
• FIXATION AXIS: line joining fixation point and
centre of rotation
•  VISUALAXIS: line joining fixation point,
nodal point and fovea
•  OPTICAL AXIS: line passing through centre of
cornea, lens and meets retina on nasal side of
fovea

66. Axes of the eye
Optic axis: AR: Line passing through the centre of cornea
(P) , centre of lens (N) and meets the retina on the nasal
side of the fovea. (R)
Visual axis: OF: Line joining the fixation point (O), nodal
point (N) and fovea (F).
Fixation axis: OC: Line joining the fixation point (O) and
the centre of rotation (C)

67. Visual angle
• Angle alpha ONA: Angle form between optic axis
and visual axis at the nodal point.
• Angle gamma OCA: Angle formed optic axis and
fixation axis at the centre of rotation of the eyeball.
• Angle kappa OPA: Line joining the fixation point (O)
and the centre of rotation (C)

68. Angle alpha: The angle formed at the nodal point
between these two axes, that is the angle ONA is called
the angle alpha, it’s average size is 5 degree.
When the visual axis cuts the cornea, as it usually does,
on the nasal side of the optic axis, the angle is designated
positive, when the two axes coincide it is nil, sometimes
the visual axis cuts the cornea on the temporal side of the
optic axis, in which case the angle alpha is negative.

69. Peripheral aberration
Peripheral aberration causes:
 Coma
 Oblique astigmatism
 Distortion of images
It is neutralize by peculiar shape of the eye.
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