Reflection and Refraction
Physics
Mrs. Coyle
Reflection
When a wave reaches a boundary it is:
– Partially reflected (bounces off surface)
– Partially transmitted through surface.
Law of Reflection
Angle of incidence = Angle of reflection
Note
• The angle of incidence is formed
between the incident ray and the
normal.
• The angle of reflection is formed
between the reflected ray and the
normal.
Specular Reflection
• A reflection produced by a smooth
surface.
Diffuse Reflection
• Reflection from a rough surface.
http://micro.magnet.fsu.edu/primer/java/reflection/specular/specularjavafigure1.jpg
Reflection from a Plane Mirror
di=do
hi=ho
Type of Image
• Real or Virtual
• Enlarged or Reduced
• Erect or Inverted
What is the minimum mirror
height required for a person
to see their full self in the
mirror?
Hint: Compare AB to CD
C
A
B
D
Refraction
• When a wave crosses a boundary
between Medium 1 and Medium 2,
the wave changes direction because
it changes velocity.
• Frequency remains constant.
• Velocity changes as a result of
wavelength change.
•
Simulations
•
http://www3.interscience.wiley.com:8100/legacy/college/halliday/0471320
005/simulations6e/index.htm
• http://www.walter-fendt.de/ph14e/refraction.htm
*******
Refraction
Example
Incident
Ray
Less Rigid
Medium
Refracted Ray
More Rigid
Medium
Refracted
ray bends
towards
the
normal.
Index of Refraction, n
n=c/v
• c : the speed of light in a vacuum,
3 x 108 m/sec
• v : speed of light in the medium.
• n : medium's index of refraction
• n>1 (Why?)
Indices of Refraction
Vacuum
Air
Water
Ethanol
Crown glass
Quartz
Diamond
1.00
1.0003
1.33
1.36
1.52
1.52
2.42
Note
• The speed of light has a lower speed
in a more optically dense medium.
Problem
• What is the speed of light in quartz?
• Answer: 1.97 x 10
8
m/s
Snell’s Law
• n1 sin q1 = n2 sin q2
Note
When the incident ray travels from
a less optically dense medium
(low n)
to
a more optically dense medium
(higher n)
then the refracted ray bends
towards the normal.
Problem
• What is the angle of refraction when
a ray from air with an angle of
incidence of 25 o is incident to water?
• Draw the ray diagram.
• Answer: 18.5 o
Total Internal Reflection
• Can occur when ray goes from
higher n to lower n.
• Above a Critical angle (of incidence)
the ray is reflected, not refracted
• http://www.walter-fendt.de/ph14e/refraction.htm
• For problems, set the angle of
refraction to 90, and solve for critical
angle
Problem
• Find the critical angle for a light ray
that is incident from water to air.
• Answer: 48.8
o
Dispersion
• The index of refraction of glass is different
for the colors that make up white light
because the speed of light is slightly
different in glass for each frequency of
light. (In vacuum all colors have speed
c=3x108m/s.)
Problem
The index of refraction for crown
glass for red light is 1.514.
What is the speed of red light in
crown glass?
Answer: 1.98 x 10
8
m/s
Mirage
http://www.warrenwilson.edu/~physics/PhysPhotOfWeek/2007PPOW/20070921RoadMirage/IMG_
1444MirageCrop500.jpg
Mirage
Total internal reflection occurs because
hot air has a lower n, than cold air.
http://www.warrenwilson.edu/~physics/PhysPhotOfWeek/2007PPOW/20070921RoadMirage/index.html