OS2 Refraction

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OS2 Refraction

• Conceptual Physics pp. 448-459 Chapter 30

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Refraction

• Refraction-The bending of a wave as it enters a
  new medium
• Medium-The material the wave travels through
• Why does light refract?
• It enters the new medium at an angle
• It changes speed

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Terms Associated with Refraction

• Angle of incidence (?i or ?i) -The angle between
  the incident ray and the normal line
• Angle of refraction (?r or ?r) -The angle between
  the refracted ray and normal line

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Refraction
Normal line an imaginary line perpendicular to
where the light strikes the interface.
Incident ray
?i
Interface the boundary between substances
?r
Imaginary line showing the path that the light
would take if it could go straight. (used as a
reference line)
Refracted ray
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Refraction Example
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Index of Refraction (n)

• Index of refraction (n) the ratio of the speed
  of light in a vacuum to the speed of light in
  that medium
• Example nglass speed of
  light in vacuum
• speed of light in glass
• nglass 3.00 x 108 m/s
• 2.00 x 108 m/s
• nglass 1.50

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Index of Refraction (n)

• The higher the n value (index of refraction)
• The slower the light will travel through the
  material
• The greater the optical density
• The more light will refract (bend)

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Common Values
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Index of Refraction

• A few index values that we will be using often in
  class (Pg. 15 in workbook)
•  
• nglass 1.50
• nwater 1.33
• nair 1.00
• ndiamond 2.42

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Refraction of Light

• Light will NOT refract when
• 1. the two materials have the same index
• If the materials have the same index, the speed
  of light is the same in each, so no refraction
  takes place

n1.3
n1.3

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Refraction of Light

• Light will NOT refract when
• 2. the light strikes perpendicular (normal) to
  the surface

n1.2 n1.4
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Terms Associated with Refraction

• Critical Angle-The angle of incidence that
  produces a 90º angle of refraction
• (can ONLY be produced going from more to less
  optically dense)

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Rules for Refraction

• 1. Light passing from less to greater index
• Light bends toward the normal line

Angle of incidence (measured from the normal)
?i
n 1.2 n 1.5
?r Angle of refraction
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Example
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Rules for Refraction

• 2a. Light passing from greater to lesser index
• Light bends away from the normal line

As the angle of incidence increases, the angle of
refraction also increases and the refracted ray
moves closer to the interface.
(?i is less than critical angle)
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Rules for Refraction

• 2b. Light passing from greater to lesser index
• Light travels along interface

(?i equals / at critical angle)
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Rules for Refraction

• 2c. Light passing from greater to lesser index
• Total internal reflection

(?i is greater than critical angle)
The light doesnt refract. Instead, it reflects
back into the first substance. This is how fiber
optics work.
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Total Internal Reflection
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Lenses

• Two Types of Lenses
• 1. Convex (converging)
• 2. Concave (diverging)
• Lenses are simply rectangular and triangular
  prisms put together.

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Triangular Prism

• When light passes through a triangular prism, it
  bends toward the thicker portion of the prism.

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Rectangular Prism

• Light bends when it enters and unbends by the
  same amount when it exits the prism
• The path is unchanged- it is simply displaced
  sideways.

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Example of Rectangular Prism
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Lenses

• Putting the triangular and rectangular prisms
  produce the 2 types of lenses (double concave and
  double convex)

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Terms Associated with Ray Diagrams Review

• Focal point-The point where refracted rays from
  parallel incident rays intersect
• Focal length-The distance between the focal point
  and the lens
• Real image-An image formed from the intersection
  of actual light rays
• Virtual image-An image that is NOT formed by the
  intersection of actual light rays

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Convex Lenses (converging)

• Have real foci
• Two focal points at equal distances on each side
  of the lens

F
F
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Rules for Lens Ray Diagrams Convex Lenses

• 1. Draw the ray parallel to the axis, hit the
  lens, and refract through the focal point on the
  other side.
• 2. Draw the ray through the center of the lens
  without bending.
• 3. Draw the ray through the first focal point,
  hit the lens, and refract parallel to the axis.

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Concave Lenses (diverging)

• Have virtual foci.

F
F
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Rules for Lens Ray Diagrams Concave Lenses

• 1. Draw the ray parallel to the axis, hit the
  lens, and refract away from the first focal
  point.
• 2. Draw the ray through the center of the lens
  without refracting.
• 3. Draw the ray toward the focal point on the
  other side of the lens, hit the lens, and refract
  parallel to the axis.

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Rules for Lens Ray Diagrams Concave Lenses

• The image with a concave lens will always be
  smaller, upright, and virtual
• It will also always be found between the focal
  point and the lens. (similar to a convex mirror)

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Real and Inverted Images

• Rules to determine whether the image is real or
  virtual
• 1. Real images are always inverted
• 2. Virtual images are upright.
• (Except for combinations of lenses)

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Lenses

• 2 Types
• 1. Double convex
• 2. Double concave

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Double Convex Lens

• Converging lens
• Examples
• Magnifying glasses
• Microscopes
• Correct farsightedness (hyperopia)

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Farsightedness
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Double Concave Lens

• Diverging Lens
• Examples
• Peep hole
• Correct nearsightedness (myopia)

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Nearsightedness
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MATH EXAMPLE

• Light traveling through air hits glass at a 28
  degree angle. What is the refracted angle?
• 1. Draw a picture
• 2. List variables
• 3. Write formula
• 4. Substitute
• 5. Show some work
• 6. Solve and label
• 7. Check if the answer is reasonable!!!

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Group Practice

• 1. Light strikes the surface of a piece of glass
  from air with an incident angle of 70?. What is
  the angle of refraction?
• Person 1 Draw a diagram
• Person 2List the variables
• Person 1 Write the formula and substitute
• Person 2 Solve

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Group Practice

• 2. Calculate the critical angle for the
  substances water and glass.
• Person 2 Draw a diagram
• Person 1 List the variables
• Person 2 Write the formula and substitute
• Person 1 Solve

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Part III
Object Distance Image Distance Larger/smaller/same Erect/Inverted Real/Virtual