Refraction: TIR and Dispersion

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RefractionTIR and Dispersion

• AP Physics M. Blachly
• Light and Optics

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Nature of Light

• What is light?
• Light is a wave
• But what is the medium?
• Applet
• Light is also a particle
• The particle is called a photon.
• The particle nature of light was first proposed
  by Einstein to explain the photoelectric effect.

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Reflection

• A mirror and a piece of white paper both reflect
  essentially all light that hits them. What is the
  difference?

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Reflection

• Mirror is example of specular reflection

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Reflection

• Diffuse reflection from a rough surface

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Reflection

• Microscopic view of a sheet of paper

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Reflections
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The Law of Reflection
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Reflection Headlights

• Why do oncoming headlights seem so bright?
• Why dont headlights work in the rain?

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Transmission

• Light can enter an optically transparent
  material.
• The light slows down as it is absorbed and
  emitted by the atoms

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Speed of Light

• n the called the index of refraction
• it can be thought of as the slow down factor
  for light

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Index of Refraction
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Index of Refraction
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Are you awake?

• What is the speed of light in Water ?

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Refraction

• What happens if a wave of light approached an
  interface where it must slow down?

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Refraction

• Refraction is the bending of a wave when it
  travels from one medium to another.
• Animation showing how light bends

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Refraction Examples

• Slice your finger example
• The bent straw
• Spear fishing

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Refraction Examples
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Catching fish
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Refraction Terminology
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Refraction

• Light bends towards the normal when it enters a
  material with a slower speed (higher n)

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Refraction

• Light bends away the normal when it enters a
  material with a higher speed (lower n)

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Refraction Snells Law
The angle of refraction depends on the indices of
refraction, and is given by Snells law
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Example Problem

• Light, traveling in glass, strikes an optical
  interface at an angle of 50 ? as measured from
  the normal. What is the angle of refraction?

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Refraction of Sound
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Dispersion

• Dispersion is the separation of light into colors
  by refraction
• Animation

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Dispersion
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Dispersion
The index of refraction of a material varies
somewhat with the wavelength of the light.
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Dispersion

• Most famous example of dispersion is the rainbow

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Rainbow
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Rainbow
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Total Internal Reflection

• Simulator
• When going from a slower to a faster medium,
  light cannot escape if the angle is too big.

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Applications

• Diamonds

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Applications

• Light Pipes

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Are you conscious still?

• Light, traveling in plastic, strikes an optical
  interface at an angle of 65.? as measured from
  the normal. What is the angle of refraction?

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Total Internal Reflection
If light passes into a medium with a smaller
index of refraction, the angle of refraction is
larger. There is an angle of incidence for which
the angle of refraction will be 90 this is
called the critical angle
(23-5)
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Total Internal Reflection
If the angle of incidence is larger than this, no
transmission occurs. This is called total
internal reflection.
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Example

• The index of refraction for glass is 1.33. What
  is the critical angle for TIR for a beam of light
  traveling in glass and striking an optical
  boundary with air ( n 1 )?

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Total Internal Reflection
Binoculars often use total internal reflection
this gives true 100 reflection, which even the
best mirror cannot do.
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Total Internal Reflection
Total internal reflection is also the principle
behind fiber optics. Light will be transmitted
along the fiber even if it is not straight. An
image can be formed using multiple small fibers.
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Dispersion
Wavelengths of visible light 400 nm to 750
nm Shorter wavelengths are ultraviolet longer
are infrared
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Dispersion
This variation in refractive index is why a prism
will split visible light into a rainbow of colors.
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Dispersion
Actual rainbows are created by dispersion in tiny
drops of water.