University Malaysia Pahang

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University Malaysia Pahang

• Faculty of Electrical Electronics Engineering
• BEF0003 Physics
• Chapter 10 Introduction to Waves Optics

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Lecture Outline

• Models of Light Rays Waves
• Reflection
• Refraction
• Total Internal Reflection
• Fiber Optics and Applications

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Model of Light Rays Waves

• Light characteristics
• Rectilinear propagation
• Light travels in straight lines
• Reflection
• When light is incident on a smooth surface it
  turns back into the original medium
• Refraction
• The path of light changes when it enters a
  transparent medium

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Properties of Light

• In vacuum light always travel at the same speed,
• Visible light is just one particular type of
  electromagnetic radiation. Other types of
  electromagnetic radiation include radio waves,
  infrared radiation (heat), ultraviolet radiation,
  x-rays and -rays. The different types of
  radiation are distinguished by their wavelength,
  or frequency

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The Electromagnetic Spectrum
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Properties of Wave
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Wavelength
Wave crest as seen from above
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Rays

• A ray is a line drawn from one wave crest to
  another which intersects each crest at right
  angles, as in figure. For light waves, the rays
  always point in the direction of the motion. Rays
  therefore provide a useful representation for
  describing the motion of light waves

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Reflection

• When light strikes the boundary between two
  media
• Some light incident on surface is reflected
• Some passes into the glass
• Partially absorbed
• Partially transmitted
• Transmitted light undergoes a change in
  direction, called refraction.

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Law of Reflection

• Two basic law of reflection
• The angle of incidence is equal to the angle of
  refraction.
• The incident ray, the reflected ray and the
  normal to the surface all lie in the same plane.
• Light reflection from a smooth surface is called
  regular or specular reflection.
• A rough or irregular surface will spread out and
  scatter the incident light resulting diffuse
  reflection

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Refraction

• Definition bending of a light ray as it passes
  obliquely from one medium to another.

?i angle of incidence ?r angle of refraction
?i
Air
Water
?r
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Index of Refraction

• Index of refraction, n of a particular material
  is the ratio of the free space velocity of light
  to the velocity of light through the material.
• Different value for different material (velocity
  of light in a material is different for different
  wavelength)
• Example nwater1.33, nglass1.50
• Unitless quantity and usually n gt 1

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Law of Refraction
N

• Two basic law of refraction
• The incident ray, the refracted ray and the
  normal to the surface all lie in the same plane.
• The path of a ray refracted at the interference
  between to media is exactly reversible.

?i
Air
Water
?r
N
?r
Air
Water
?i
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Snells Law

• The ratio of the sine of angle of incidence to
  the sine of the angle of refraction is equal to
  the ratio of the velocity of light in the
  incident medium to the velocity of light in the
  refracted medium.

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Wavelength and Refraction

• The wavelength of light is reduced when it enters
  a medium of greater optical density

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Example

• A light ray of wavelength ? 589 nm is incident
  on glass with an angle of incidence of 30 . The
  index of refraction of glass is 1.52.
• What is the angle of refraction?
• What are the speed and wavelength of the light
  inside the glass?

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Solution
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Problem

• Light passes through a flat slab of glass. The
  angle of incidence of the light onto the glass is
  30. What is the angle with which the light
  emerges on the other side of the slab?

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Total Internal Reflection

• Occurs when light passes obliquely from one
  medium to a medium with lower optical density.
• A ray is totally internally reflected when its
  approach a surface at an angle which is greater
  than the critical angle.
• Critical angle,?c
• Limiting angle of incidence in a denser medium
  that result in an angle of refraction of 900
• Angle of incidence angle of reflection
• Total internal reflection only occur only when
  light incident from a denser medium (n1gtn2)

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Total Internal Reflection
A
B
A ?i?r0 B ?ilt?r C ?i?c ?r900 D
?i?r gt ?c
n2
C
n1
D
n1gtn2
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Example

• A fish lives at the bottom of a lake 10 m deep
  filled with water whose index of refraction 1.52.
  
• At what angle relative to the normal must the
  fish look up towards the surface of the water in
  order to see a fisherman who is sitting on a
  distant shore?
• What is the closest distance that another fish
  living at the bottom of the lake can approach in
  order that the first fish can see it by looking
  towards the surface?

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Solution
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Solution

• As indicated in the figure, the incoming light
  from the fisherman must be coming almost parallel
  to the surface of the lake. Thus, the ray that
  hits the fish's eye must be coming at close to
  the critical angle for water i.e. the angle of
  incidence is approximately 90

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Solution
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Fiber Optics Applications

• Larger bandwidth than copper wire
• gtinformation transmitted during fixed time period
• Increased information carrying capacity
• Based on the concept of total internal reflection
• Light passing through one medium encounters 2nd
  medium of lower optical density
• Fiber optic structure
• ncoregtncladding

Core
Buffer for protection
Cladding
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Fiber Optics Applications

• Advantages
• Immunity to electromagnetic interference
• Improved data security
• Higher transmission speed
• Increased signal bandwidth
• Applications
• Communication
• Sensors
• Medical instruments
• ???

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Fiber Optic Internal Reflection
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Fiber Optic Structure
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Fiber Optic Cables
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Summary Learning Outcomes

• Models of Light Rays Waves
• Describe the properties of light, waves and rays
• Reflection
• Define and illustrate the concept of reflection
• State the law of reflection
• Refraction
• Define index of refraction and state the law of
  refraction
• Apply Snells Law to solve problem involving
  transmission of light in two or more media.
• Determine change of velocity or wavelength of
  light as it moves in different media
• Total Internal Reflection
• Explain the concept of total internal reflection
  and the critical angle and use this idea to solve
  problems
• Fiber Optics and Applications
• Know how fiber optic work, its advantages and its
  applications.