Transcript Chapter 22

Chapter 22
Reflection and Refraction
of Light
Conceptual questions: 9,11,16,17,18
Quick quizzes: 2,3,4
Problems: 21,28,49,56
Dual Nature of Light
Experiments display either the wave nature or
the particle nature of light
 Nature prevents testing both qualities at the
same time
 “Particles” of light are called photons
 Each photon has a particular energy
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E=hƒ
h is Planck’s constant
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h = 6.63 x 10-34 J s
Geometric Optics – Using a
Ray Approximation
Light travels in a straight-line path in a
homogeneous medium
 The ray approximation is used to
represent beams of light
 A ray of light is an imaginary line drawn
along the direction of travel of the light
beams

Ray Approximation
A wave front is a
surface passing
through points of a
wave that have the
same phase and
amplitude
 The rays,
corresponding to the
direction of the wave
motion, are
perpendicular to the
wave fronts

Reflection of Light
A ray of light, the incident ray, travels in
a medium
 When it encounters a boundary with a
second medium, part of the incident ray
is reflected back into the first medium
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This means it is directed backward into the
first medium
Specular Reflection

Specular reflection is
reflection from a
smooth surface
 The reflected rays
are parallel to each
other
Diffuse Reflection

Diffuse reflection is
reflection from a
rough surface
 The reflected rays
travel in a variety of
directions
 Diffuse reflection
makes the road easy
to see at night
Law of Reflection
The normal is a line
perpendicular to the
surface
 The incident ray makes an
angle of θ1 with the
normal
 The reflected ray makes
an angle of θ1’ with the
normal
 The angle of reflection is
equal to the angle of
incidence
θ1= θ1’

Reflection of light,
quiz
Refraction of Light
The incident ray, the
reflected ray, the
refracted ray, and
the normal all lie on
the same plane
 The angle of
refraction, θ2,
depends on the
properties of the
medium

sinθ1 v 2
= =constant
sinθ 2 v1
v – speed of light
QUICK QUIZ 22.2
If beam 1 is the incoming beam in the figure
below, which of the other four beams are
reflected and which are refracted?
Refraction Details
v1>v2
 The angle of
refraction is less
than the angle of
incidence
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The ray bends
toward the normal
Refraction Details
v1<v2
 The angle of
refraction is greater
than the angle of
incidence
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The ray bends away
from the normal
The Index of Refraction
Refraction occurs because the speed of
light is different in the two media
 The index of refraction, n, of a medium
can be defined

speed of light in a vacuum c
n

speed of light in a medium v
Frequency Between Media

As light travels from
one medium to
another, its
frequency does not
change

Both the wave speed
and the wavelength
do change
Index of Refraction Extended
f = f1 = f2
 v1 = ƒ λ1
v2 = ƒ λ2
 The ratio of the indices of refraction of the
two media can be expressed as

c
1 v 1
n1 n2



2 v 2 c
n1
n2
Snell’s Law of Refraction

n1 sin θ1 = n2 sin θ2

θ1 is the angle of
incidence

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30.0° in this diagram
θ2 is the angle of
refraction
Table 22.1
Snell’s law,
illustration
QUICK QUIZ 22.3
A material has an index of refraction that increases
continuously from top to bottom. Of the three paths
shown in the figure below, which path will a light ray
follow as it passes through the material?
QUICK QUIZ 22.4
As light travels from vacuum
(n = 1) to a medium such as glass (n
> 1), which of the following
properties remains the same:
(a) wavelength,
(b) wave speed, or
(c) frequency?
Problem 22.21
The light shown in the
figure makes an angle
of 20o with the normal
NN’ in the linseed oil
(n=1.48). Determine
angles q and q’.
Problem 22.28
A cylindrical cistern, constructed below ground level, is
3.0 m in diameter and 2.0 m deep and is filled to the
brim with a liquid whose index of refraction is 1.5. A
small object rests on the bottom of the cistern at its
center. How far from the edge of the cistern can a girl
whose eyes are 1.2 m from the ground stand and still
see the object?
Dispersion

The dependence of
the index of
refraction on λ is
called dispersion
Refraction in a Prism
δ the angle of deviation
 n()
 Since all the colors
have different angles of
deviation, they will
spread out into a

spectrum

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Violet deviates the most
Red deviates the least
Using Spectra to Identify
Gases
All hot, low pressure gases emit their
own characteristic spectra
 The particular wavelengths emitted by a
gas serve as “fingerprints” of that gas
 Some uses of spectral analysis

Identification of molecules
 Identification of elements in distant stars
 Identification of minerals

The Rainbow
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At the back surface the
light is reflected
It is refracted again as
it returns to the front
surface and moves into
the air
The rays leave the drop
at various angles
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The angle between the
white light and the violet
ray is 40°
The angle between the
white light and the red
ray is 42°
Observing the Rainbow
Double rainbow
Huygen’s Principle
Huygen assumed that light is a
form of wave motion rather
than a stream of particles
 All points on a wave front
produce spherical secondary
waves, which propagate in the
forward direction with speeds
characteristic of waves in that
medium

Huygen’s Principle and the
Law of Refraction
In time Δt, ray 1
moves from A to B
and ray 2 moves
from A’ to C
 From triangles AA’C
and ACB, all the
ratios in the Law of
Refraction can be
found


n1 sin θ1 = n2 sin θ2
Total Internal Reflection

Total internal
reflection can occur
when light attempts
to move from a
medium with a high
index of refraction to
one with a lower
index of refraction

Ray 5 shows internal
reflection
Critical Angle

The critical angle is
an angle of
incidence that will
result in an angle of
refraction of 90°
n2
sin q c 
for n1  n2
n1
Fiber Optics
Conceptual questions
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18. If a beam of light with a given cross-section enter
a new medium, the cross section of the refracted
beam is
a) larger
b) smaller
c) not changed
9. In dispersive materials, the angle of refraction for a
light ray depends on the wavelength of light. Does
the angle of reflection depend on the wavelength?
11. Explain why a diamond loses most of its sparkle
when submerged in carbon disulfide.
16. Explain why an oar partially in water appears to
be bent.
17. Why do astronomers looking at distant galaxies
talk about looking backward in time?
Problem 22.56
A laser beam strikes one end of a slab of material, as in
Figure P22.56. The index of refraction of the slab is
1.48. Determine the number of internal reflections of
the beam before it emerges from the opposite end of the
slab.
Solution
Problem 21-49
As shown in Figure P22.49, a light ray is incident
normally on one face of a 30°-60°-90° block of
dense flint glass (a prism) that is immersed in water.
(a) Determine the exit angle θ4 of the ray. (b) A
substance is dissolved in the water to increase the
index of refraction. At what value of n2 does total
internal reflection cease at point P?
Review questions
1. Visible light of which color bends the
most when changing mediums?
 A. yellow
 B. green
 C. violet
 D. red
2. Water has an index of refraction of 1.3. Approximately
how fast does light move through the water?
a. 1.0 x 108 m/s
b.
2.3 x 108 m/s
c. 3.0 x 108 m/s
d.
4.9 x 108 m/s
3. The sky is blue because it
a. Absorbs light at 390 nm
b. Reflects light at 390 nm
c. Absorbs light at 700 nm
d. Reflects light at 700 nm