Interference
Focus Question
How do bubbles produce a rainbow effect?
Review Vocabulary
interference: results from the superposition of
two or more waves; can be constructive or
destructive
diffraction: the bending of light around a barrier
Interference
• Depending on their phase
relationship, the two waves
undergo constructive or
destructive interference.
• When the light from the slits
is projected on a screen, the
constructive and destructive
interference results in a
pattern of light and dark
bands called interference
fringes.
Diffraction
Review Vocabulary
diffraction: the bending of light around a barrier
Reflection
Focus Question
Why does light reflected from a mirror
make an image while light reflected from
a piece of paper does not?
New Vocabulary
specular reflection
diffuse reflection
plane mirror
Review Vocabulary
normal: the line in a ray diagram that shows the
orientation of the barrier or mirror and is drawn
at a right angle, or perpendicular, to the barrier
or mirror
ray: a line drawn at a right angle to a wavefront;
represents the direction of wave travel
The Law of Reflection
• The law of reflection
states the angle of
incidence is equal to the
angle of reflection. This
applies to both smooth
and rough surfaces.
Law of Reflection
θi  θr
The Law of Reflection
• Specular reflection is caused by a smooth surface, in
which parallel light rays are reflected in parallel.
• Diffuse reflection is caused by the scattering of light
off a rough surface.
Objects and Plane-Mirror Images
• A plane mirror is a flat, smooth surface from which light
is reflected by specular reflection.
• An object is a source of light rays that are to be reflected
by a mirrored surface, such as the bird shown below.
• Some of the light travels from the bird to the mirror and
reflects. Some of the reflected light from the bird hits
the girl’s eyes.
Quiz
1. Which is caused by a smooth surface, in which
parallel light rays are reflected in parallel?
A
normal reflection
C
specular reflection
B
diffuse reflection
D
virtual reflection
Quiz
2. Which is caused by the scattering of light off a rough
surface?
A
specular reflection
C
diffuse reflection
B
normal reflection
D
virtual reflection
Quiz
3. Which is a flat, smooth surface from which light is
reflected by specular reflection?
A
diffuse mirror
C
plane mirror
B
convex mirror
D
concave mirror
C, C, C
Refraction
Focus Question
What happens to light when it enters a
new medium?
New Vocabulary
index of refraction
critical angle
total internal refraction
dispersion
Review Vocabulary
refraction: the change in direction of waves at
the boundary between two different mediums
Light and Boundaries
• When light encounters a transparent or translucent
medium, some light is reflected from the surface of the
medium and some is transmitted through the medium.
• When light crosses a boundary between two media, it
bends. This phenomenon is called refraction.
Snell’s Law of Refraction
• The angle of incidence (θ1) is the angle at which the
light ray strikes the surface. It is measured from the
normal to the surface.
• The angle of refraction (θ2) is the angle at which the
transmitted light leaves the
surface. It also is measured
with respect to the normal.
• The index of refraction (n)
determines the angle of
refraction between two
mediums.
Snell’s Law of Refraction
Snell’s law of refraction relates the indexes of
refraction, the angle of incidence, and the angle of
refraction for any two substances.
Snell’s Law of Refraction
n1 sin 1  n2 sin 2
Use with Example Problem 1.
Problem
A ray of light is incident upon a layer of flint
glass (n = 1.62) at an angle of 19.0°. What is
the angle of refraction?
SOLVE FOR THE UNKNOWN
• Use Snell’s law to find the angle of refraction.
Response
SKETCH AND ANALYZE THE PROBLEM
• Sketch the situation.
• List the knowns and unknowns.
KNOWN
n1= 1.00
n2 = 1.62
θ1 = 19.0°
UNKNOWN
θ2 = ?
n1 sin 1  n2 sin  2
 n1 sin 1 
 2  sin 

n
2




1 1.00sin 19
 sin 
  11.6

1.62 

1
EVALUATE THE ANSWER
• n1 < n2, so θ2 should be smaller than θ1. This agrees
with our answer.
The Meaning of the Index of Refraction
• By examining refraction using the
wave model of light and using
some geometry, we find that
sin θ2
v2

sin θ1
v1
• By combining this with Snell’s
law, we see that
n1
v2

n2
v1
The Meaning of the Index of Refraction
If n2 = 1.00 (a vacuum), then v2 = c
and we see that the index of
refraction for a medium is related to
the speed of light in the medium.
Index of Refraction
c
n 
v
Total Internal Reflection
• When n1 > n2, the angle of refraction is greater than the
angle of incidence.
• At a certain angle of incidence, known as the critical
angle (θc), the refracted light ray lies along the
boundary of two mediums.
Critical Angle
n2
sin θC 
n1
Total Internal Reflection
Total internal refraction occurs when light traveling
from a region of higher n to a region of lower n
strikes the boundary at an angle greater than the
critical angle such that all light reflects back into the
region of higher n.
Total Internal Reflection
The light traveling through a transparent optical
fiber always hits the internal boundary of the
optical fiber at an angle greater than the critical
angle, so all of the light is reflected and none of the
light is transmitted through the boundary.
Quiz
1. Which determines the angle of refraction between
two media?
A
index of refraction
C
index of reflection
B
index of dispersion
D
index of rotation
Quiz
2. Which is the correct formula for the index of
refraction?
A
n=v2/v1
C
n=v/c
B
n=c/v
D
n=cv
Quiz
3. Which is the certain angle of incidence when the
refracted light ray lies along the boundary of two
mediums?
A
refraction angle
C
critical angle
B
reflection angle
D
dispersion angle
Quiz
4. Which occurs when light traveling from a region of
higher n to a region of lower n strikes the boundary at an
angle greater than the critical angle such that all light
reflects back into the region of higher n?
A
total internal dispersion
C
total internal refraction
B
total optical refraction
D
total internal reflection
Quiz
5. Which occurs when white light separates into a spectrum
of colors when it passes through a glass prism?
A
reflection
C
refraction
B
dispersion
D
magnification
A, B, C, C, B