Reflection and Refraction

PhysicsMrs. 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

A

D

B

C

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://www.walter-fendt.de/ph14e/refraction.htm *******

• http://www3.interscience.wiley.com:8100/legacy/college/halliday/0471320005/simulations6e/index.htm

Refraction

Example

Less RigidMedium

More RigidMedium

Refracted ray bendstowardsthe normal.

Incident Ray

Refracted Ray

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 1.00Air 1.0003Water 1.33Ethanol 1.36Crown glass 1.52Quartz 1.52Diamond 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 1 = n2 sin

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.warren-wilson.edu/~physics/PhysPhotOfWeek/2007PPOW/20070921RoadMirage/IMG_1444MirageCrop500.jpg

MirageTotal internal reflection occurs becausehot air has a lower n, than cold air.

http://www.warren-wilson.edu/~physics/PhysPhotOfWeek/2007PPOW/20070921RoadMirage/index.html