PH 103

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PH 103
Refraction and refractive phenomena
Dr. James van Howe Lecture 2
Colonoscopy photo taken using fiber
bundle, FromDartmouth medical Center
March 12, 2008
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Bonus Name the vocalist

• Robert Plant
• Jack White
• Bon Scott
• Chris Cornell

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Visible light travels more slowly through an
optically dense medium than through a vacuum. A
possible explanation for this could be that the
light

• Is absorbed and re-emitted by the atomic
  structure of the optically dense medium
• Is absorbed and re-emitted by the nucleus of the
  optically dense medium
• Bounces around randomly inside of the optically
  dense medium
• Loses amplitude as it passes through the
  optically dense medium

Problem 71. from MCAT practice test 9
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If a very bright light source shines on a mirror,
the mirror may become warm because

• All of the light is reflected, and, by momentum
  conservation, the molecules in the mirror move,
  producing heat energy
• Some of the light passes through the mirror, and
  by energy conservation, potential energy is
  produced
• Some light is absorbed by the mirror, and, by
  energy conservation, thermal energy is produced
• None of the light is reflected, and, by energy
  conservation, mass is converted to energy

Problem 70. from MCAT practice test 9
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Tacoma Narrows bridge collapse
-Just like mass on a spring, all you need is a
little bit of driving force at the right
frequency to make the mass shake like hell.
-All bridges (objects) have mechanical
resonances, some are sharper than others, Tacoma
bridge way too sharp! Damping makes resonance
broad, less absorption, safe!
sharper, more peak energy
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Shallow areas slow the velocity of water waves
Faster Velocity
Slower Velocity
Shoreline
In the shallow area, the wave

• Bends towards the normal the wavelength
  decreases
• Bends towards the normal the wavelength
  increases
• Bends away from the normal the wavelength
  decreases
• Bends away from the normal the wavelength
  increases

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Shallow areas slow the velocity of water waves
Faster Velocity
Slower Velocity
Shoreline
Water waves almost always come in parallel to
shoreline, because of refraction
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Snells Law
In the figure below, if what is
?
Air,
Water,
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What is the wavelength in water compared to that
in air?
Across the boundary frequency of light stays the
same in general,
In air
In water,
So,
So,
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Refractive Optical Illusions
Goggles at bottom of pool appear to be higher
Your brain traces the light rays from the water
as if they did not bend from refraction
Birds of prey like eagles whose diet contain
fish, learn to compensate and reach a bit lower
to snatch their prey
Tricky part if our eyes are to make an image on
the retina, the light rays need to come from the
object (goggles) No light rays emanate from the
eyes, we dont have mutant powers
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Refractive Optical Illusions
Your legs appear to be shorter
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Broken Pencil
Image appears higher
Imaging rays go from water to air
Like goggles and short legs example
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Special case for slow to fast medium
Water,
Air,
At a special angle called the critical angle,
light does not escape from the denser medium
Total Internal Reflection
For water-to-air,
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Total Internal Reflection
Responsible for light guiding through fiber optic
cable
Demo water jet, tank
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Activity, bouncing beam of light
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Bouncing due to gradient in index of refraction
(gradient in density)
Less dense, lower index
bends away from normal
bends toward the normal
Denser, higher index
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Similar phenomena happens to sound waves
-Why sound waves carry on a cool summer night
Warm Air
Faster Velocity
bends away from normal
Cool Air
Slower Velocity
Ground
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A glass fiber caries a light digital signal long
distance with a minimum loss of amplitude. What
optical property of glass allows this phenomenon?

• Dispersion
• Refraction
• Reflection
• Diffraction

Problem 26. from MCAT practice test 8
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Homework
Ch23 problems 31,38, 43 Corresponding Reading
23-5 through 23-8