Download 4 Reflection and Refraction

Reflection and
Refraction
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.
Reflection

Regular reflection – occurs when parallel
light waves strike a surface and reflect in
the same parallel directions
 “Smooth” surfaces

Diffuse Reflection – occurs when parallel
light waves strike a rough uneven surface
and reflect in many different directions.
Regular Reflection

A reflection produced by a smooth
surface.
Diffuse Reflection

Reflection from a rough surface.
Diffuse Reflection

When the surface is rough, images are
not formed, but the light still reflects
Regular
Regular
http://micro.magnet.fsu.edu/primer/java/reflection/specular/specularjavafigure1.jpg
Diffuse Reflection


Bumps on the surface must be less
than about 1/8 the wavelength of light
in order to have a smooth surface
Big satellite dishes are made of wire
mesh and appear smooth to the longer
wavelength microwave signals
Satellite Dish
Satellite Dish
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
Creating Images
Plane Mirrors




We trace rays from the object
A plane mirror always produces a virtual
image
 Virtual image – copy of an object
formed at location from which the light
rays appear to come
The image is the same distance behind
the mirror as the object is in front of it
The object and the image are the same
size
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


Refraction
Phet Refraction
Refraction
Refraction
Incident Ray
Example
Less Dense
Medium
More Dense
Medium
Refracted Ray
Index of Refraction
Index of Refraction
Refraction

When light slows down at a boundary,
it bends toward the normal (less dense
to more dense)

When light speeds up at a boundary, it
bends away from the normal (more
dense to less dense)
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
Problem

What is the speed of light in quartz?

n = 1.54

Answer: 1.97 x 10 8 m/s
Snell’s Law

n1 sin q1 = n2 sin q2
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
Total Internal Reflection
This is used in fiber-optic cables to transmit
data signals. The light inside the cable cannot
escape so no energy (signal) is lost as it travels.
Why We Love Diamonds?
Total Internal Reflection in a
Diamond
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.)

As white light passes through a prism,
shorter wavelengths refract more than
longer wavelengths, and the colors
separate
 Longer wavelength move faster as it
enter the prism (red light) so it bends
the least
 Violet is bent the most
Rainbows
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
Refraction

This bending of light can play tricks on
our minds
Mirages – false or distorted image
Mirage
http://www.warrenwilson.edu/~physics/PhysPhotOfWeek/2007PPOW/20070921RoadMirage/IMG_
1444MirageCrop500.jpg
Occurs because
hot air has a lower n, than cold air.
Another Trick
Light travels faster at
higher altitudes
because the air is less
dense.