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Ch 2 Optics
Contents
1- The Nature of Light
2 Reflection and Refraction
3 The Law of Refraction
4 Total Internal Reflection, Binocular, Optical Fiber
5 Dispersion and Prisms, Color Matching
6 Eye vision
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Objectives
• 1- Understand the nature, origin and
sources of light
• 2-Use the laws of Reflection and Refraction
to understand some optical devices
• 3- Discuss the theory of operation of some
medical instruments.
• 4- Understand light dispersion and the
theory of Color Matching
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Nature of Light Theories
• Light was though to be a stream of particles
(Corpuscular theory)
• James Clerk Maxwell (1831-1879) developed the
electromagnetic theory and pronounced that light
is a form of high frequency electromagnetic wave
• Max Planck (1858-1947) put forward the quantum
theory of light (light is emitted in the form of
photons)
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What is the light?
Is it?
Waves
Or
Particles
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Properties of light - two models
Light ray model
Wave model
• Particle-like view
• Photons travel in
straight lines
• Applications
– Mirrors
– Prisms
– Lenses
• Traces motions of wave
fronts
• Best explains
– Interference
– Diffraction
– Polarization
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Light is an electromagnetic wave.
• The electric (E) and magnetic (B) fields
are in phase.
• The electric field, the magnetic field, and the
propagation direction are all perpendicular.
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Waves can interfere.
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Light is not only a wave, but also
a particle.
• Photographs taken in dimmer light look
grainier.
• When we detect very weak light, we find that
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it’s made up of particles.
We call them photons.8
Where does light come from?
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Sources of light
Accelerating charges emit light
1- Linearly accelerating charge
2-Synchrotron radiation—
light emitted by charged
particles deflected by a
magnetic field
B
3-Bremsstrahlung (Braking
radiation)— light emitted when
charged particles collide with other
charged particles
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4-But the vast majority of light in the universe
comes from molecular vibrations emitting light.
•Electrons vibrate in their motion around nuclei
High frequency:
~1014 - 1017 cycles per second.
•Nuclei in molecules vibrate
with respect to each other
Intermediate frequency:
~1011 - 1013 cycles per second.
•Nuclei in molecules rotate
Low frequency:
~109 - 1010 cycles per second.
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Polarized and unpolarized media
On the right, the displacements of the charges are correlated, so it
is polarized at any given time (and its polarization is oscillating).
Unpolarized medium
Polarized medium
Note that matter’s polarization is analogous to the
polarization of light. Indeed, it will cause the emission
ofMaylight
with the same polarization
direction.
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Sources of light
• Matter constantly emits
and absorbs radiation
• Emission mechanism
• Different accelerations
lead to different
frequencies
• Luminous
– Accelerated, oscillating
charges produce
electromagnetic waves
– Producing light
– The Sun versus the
nonluminous Moon
• Absorption mechanism
• Incandescent
– Oscillating
electromagnetic waves
accelerate charges within
matter
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– Glowing with visible light
from high temperatures
– Examples: flames,
incandescent light bulbs
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The Interaction of Light and Matter:
•The interaction of light and matter is what makes life interesting.
•Everything we see is the result of this interaction.
•Why is light absorbed or transmitted by a particular medium?
•Light causes matter to vibrate. Matter in turn emits light, which
interferes with the original light.
•Traces motions of wave fronts
•Best explains
– Interference
– Diffraction
– Polarization
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Light interacts with matter
• Interaction begins at
surface and depends on
– Smoothness of surface
– Nature of the material
– Angle of incidence
• Possible interactions
– Absorption and
transmission
– Reflection
– Refraction
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Geometrical Optics:
Study of reflection and refraction of light from
surfaces using the ray approximation.
1-The ray approximation states that:
light travels in straight lines until it is reflected or refracted
and then travels in straight lines again.
2-The wavelength of light must be small compared to the
size of the objects or else diffractive effects occur.
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Reflection details
• Angles measured with
respect to the
“surface normal”
– Line perpendicular to
the surface
• Law of reflection
qi = qr
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Light refraction
Experiment shows that the path of a light ray through a refracting surface is
reversible. For example, the ray in Figure a travels from point A to point B.
If the ray originated at B, it would follow the same path to reach point A,
but the reflected ray would be in the glass.
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Refraction
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The Fundamental Law
Snell’s
Law
n1 sin q1   n2 sin q2 
Sin 0=0
Sin 90= 1
n2
q1
q2
n1
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Critical Angle
n2
θ2
θ1
θ2
θ1 θ
C
θ1
θ1
n1
P
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Total internal reflection
n1 sin q i   n2 sin q r 
at q r  90 0 sin q r   1
n1 sin q i   n2
n1
 sin q c  ,
n2
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for n1  n2
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Refraction, cont.
• Critical angle
– Light refracted parallel to
surface
– No light passes through
surface - “total internal
reflection”
– Applications - fiber optics,
gemstone brilliance
Substance
Index of
refraction
Light speed
Air
Approx. 1
~c
Water
1.333
0.75c
Glass
1.5
0.67c
Diamond
2.4
0.42c
18,000,000
38 mph!
BE
condensate
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Optical Instruments
Binoculars
Many optical instruments, such as binoculars, periscopes, and
telescopes, use glass prisms and total internal reflection to turn a
beam of light through 90° or 180°.
Assignment
Write a short report on Binoculars taking into consideration the following points
1- Optical phenomena
2- Structure
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Fiber Optics
Light can travel with little loss in a curved
optical fiber because the light is totally reflected
whenever it strikes the core-cladding interface
and because the absorption of light by the core
itself is small.
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Optical Fiber Structure
nc
ncore
ncore > nc
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Utilizations
Physicians often use fiber-optic cables to aid in
the diagnosis and correction of certain medical
problems without the intrusion of major surgery.
For example, a fiber-optic cable can be
threaded through the esophagus and into the
stomach to look for ulcers.
In this application, the cable consists of two
fiber-optic lines:
one to transmit a beam of light into the stomach
for illumination
and the other to allow the light to be transmitted
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out of the stomach.
Example-Endoscopy
In the field of medicine, optical fiber cables have had
extraordinary impact. In the practice of endoscopy, for instance, a
device called an endoscope is used to peer inside the body.
A colonoscope reveals a polyp (red)
attached to the wall of the colon.
A bronchoscope is being used to look
for signs of pulmonary disease.
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Arthroscopic Surgery
Optical fibers have made arthroscopic surgery possible, such as
the repair of a damaged knee shown in this photograph:
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Answer the following
question
•
• Are there applications in dentistry
•
• Justify your answer
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Light Refraction
•
•
•
•
Here h and h”
are heights of
the body and
its image
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Thin Lenses
Thin Lenses
• The magnification of a thin lens is
•
•
•
•
Combine definition of focal length with lensmaker’s equation
If f > 0, we have a converging lens
If f < 0, we have a diverging lens
What if f = infinity?
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Power of a Lens
• The power of a lens in diopters is the inverse of
its focal length
P = 1/f
f  0.20 m  P  5.0 diopters
f  0.40 m  P  2.5 diopters
Exercises
• Calculate image
positions, si
• Calculate image
heights, hi, for (a) and
(b)
• Calculate image
length for (c)
• Are images real or
virtual, upright or
inverted, and reduced
or enlarged
Fig. 17-24, p. 538
Image Reconstruction (Lens)
1. Rays parallel to the optical axis, go through the focal point.
2. Rays through the focal point, emerge parallel to the optical axis.
3. Rays through the center of a lens or through the center of curvature of a
mirror are undeviated.
Optical Axis
The eye
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Vision
The near point is the closest distance for which the
lens can accommodate to focus light on the retina.
Typically, the near point of the eye is
at age 10 about 18 cm.
at age 20 about 25 cm,
at age 40, 50 cm
at age 60. 500 cm or greater
The far point of the eye represents the farthest
distance for which the lens of the relaxed eye can
focus light on the retina
A person with normal vision is able to see very distant
objects,
such as the Moon, W3
and so has a far point at38
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infinity.
Farsightedness (or hyperopia)
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Nearsightedness (or myopia)
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Assignment
• Write a report, not more than 2 bages
about eye vision including the following
points
• 1-lens system of the eye
• 2-Accomodation
• 3-Resolving power of the eye
• 4-Retina structure and function
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Dispersion and colors
• White light
– Mixture of colors in sunlight
– Separated with a prism
• Dispersion
– Index of refraction varies with
wavelength
– Different wavelengths refract at
different angles
– Violet refracted most (blue sky)
– Red refracted least (red sunsets)
– Example: rainbows
• Wavelength/frequency related
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Dispersion is the tendency of optical
properties to depend on wavelength.
Dispersion of the refractive index allows prisms to separate white
light into its components and to measure the wavelength of light.
Dispersed beam
White light
n(l)
Dispersive
element
Dispersion
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can be good or bad, depending
on what you’d like to do.
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Color Matching
• To understand color you must appreciate the
three dimensional nature of color.
• The dimensions of color are:
• HUE(color) is commonly referred to as color,
E.g. Blue, Orange etc. It is associated with the
wavelength of the light received.
• VALUE (brightness) is how we tell a light hue
from a dark one.
• CHROMA (saturation) is the intensity or
saturation of a hue.
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Hue -colors
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VALUE (brightness)
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Saturation
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Additive primary colors
theory
1 Red
+
1
Blue
=
1 Blue
+
1 Green
=
+
1 Red
=
1 Green
Magenta
Cyan
Yellow
1 Red + 1 Blue + 1 Green = White
2 Red
+
1 Green
=
Orange
2 Green
+
1 Red
=
Lime
1 Blue
+
1 Green
+
4 Red
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=
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Brown
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Important
•A good site to visit
• http://www.dentalxp.com/
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Assignment
• Solve the following problems
• 1-4-7-10-13-15-18
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