The document discusses the principles of geometric optics, focusing on the behavior of light as it interacts with different surfaces. It explains the concepts of luminous versus illuminated objects, types of reflections (specular and diffuse), and introduces the law of reflection and refraction. Additionally, it highlights applications of these principles, such as in photography and night driving, and covers critical angles and total internal reflection.

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

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. 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. Two Kinds of Reflection
• Diffuse reflection: When rays are
reflected from a rough surface, they are
reflected in many directions and no clear
image is formed. None of the normal drawn
to the surface (at the point at which the
incident light ray strikes the surface) are
parallel.

9. Two Kinds of Reflection
Regular reflection or specular reflection:
When rays are reflected from a smooth
surface, they are reflected so that a clear
image is formed. The reflected rays are
nearly parallel. The normal drawn to the
surface (at the point at which the incident
ray strikes the surface) are nearly parallel.

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. • Whether the surface is microscopically rough or
smooth has a tremendous impact upon the subsequent
reflection of a beam of light.

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

13. Applications of Specular and Diffuse
Reflection
There are several interesting applications of this
distinction between specular and diffuse reflection.
One application pertains to the relative difficulty of
night driving on a wet asphalt roadway compared to a
dry asphalt roadway. Most drivers are aware of the
fact that driving at night on a wet roadway results in
an annoying glare from oncoming headlights.

14. Applications of Specular and Diffuse
Reflection
The glare is the result of the specular reflection of the
beam of light from an oncoming car. Normally a
roadway would cause diffuse reflection due to its
rough surface. But if the surface is wet, water can fill
in the crevices and smooth out the surface.

15. Applications of Specular and Diffuse
Reflection
• Rays of light from the beam of an oncoming car hit
this smooth surface, undergo specular reflection and
remain concentrated in a beam. The driver perceives
an annoying glare caused by this concentrated beam
of reflected light.

17. Example of specular reflection

19. Applications of Specular and Diffuse
Reflection
A second application of the distinction between
diffuse and specular reflection pertains to the field of
photography. Many people have witnessed in person
or have seen a photograph of a beautiful nature scene
captured by a photographer who set up the shot with a
calm body of water in the foreground.

20. Applications of Specular and Diffuse
Reflection
The water (if calm) provides for the specular
reflection of light from the subject of the photograph.

21. Applications of Specular and Diffuse
Reflection
Light from the subject can reach the camera lens
directly or it can take a longer path in which it reflects
off the water before traveling to the lens.
• Since the light reflecting off the water undergoes
specular reflection, the incident rays remain
concentrated (instead of diffusing).

22. 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.

23. Question
If a bundle of parallel incident rays undergoing
diffuse reflection follow the law of reflection, then
why do they scatter in many different directions after
reflecting off a surface?

24. Each individual ray strikes a surface which has a
different orientation. Since the normal is different for
each ray of light, the direction of the reflected ray will
also be different.

25. Perhaps you have observed magazines which have
glossy pages. The usual microscopically rough
surface of paper has been filled in with a glossy
substance to give the pages of the magazine a smooth
surface. Do you suppose that it would be easier to
read from rough pages or glossy pages? Explain your
answer.

26. It is much easier to read from rough pages which provide
for diffuse reflection. Glossy pages result in specular
reflection and cause a glare. The reader typically sees an
image of the light bulb which illuminates the page. If you
think about, most magazines which use glossy pages are
usually the type which people spend more time viewing
pictures than they do reading articles.

27. 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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28. 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

29. Reflection of light
• When light meets an interface between two media, its
behavior depends on the nature of the two media
involved. Light may be absorbed by the new medium
or transmitted onward through it or it may bounch
back into first medium. This bouncing of light at an
interface is called Reflection.
 (M.A MATIN = 21)
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30. 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.

31. 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

32. 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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33. Direction of light
1) Divergence (minus vergence)
2) Zero vergence (straight)
3) Convergence (plus vergence)
 (M.A MATIN = 20)
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35. Laws of Reflection
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,
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37. Refraction
Q) What is refraction?
Ans) Refraction of light is a phenomenon of change in
the path of light when it passes from one medium to
another due to change in velocity.

38. 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.

39. 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.

40. Snell’s Law

41. 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.

42. Total Internal Reflection (TIR)

43. 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.

44. Different uses of TIR
3) Total internal reflection is the operating principle of
automotive rain sensors, which control
automatic windscreen/windshield wipers

45. 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

46. • Critical angle of glass is 48.60, diamond is 240 (refractive
index is 2.42) and water is 48.750. An incident ray when
passing through a slab of glass with air on either side will exit
the slab as refracted ray and will be parallel to incident ray.