Transcript ch16_LecturePPT

Chapter 16
Light Waves and Color
Lecture PowerPoint
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What causes color?
What causes reflection?
What causes color?
What causes reflection?
Why does a soap
film display
different
colors?
How do we see
color?
Why is the sky
blue?
What do light, radio waves,
microwaves, and X rays have in
common?
a)
b)
c)
d)
e)
They all can travel through empty space.
They all travel at the same speed.
They all have no mass.
All the above are true.
Only answers a and b are true.
d) These are all forms of electromagnetic
waves. Although seemingly quite different,
they share many properties, including a, b, and
c.
Electromagnetic Waves
 An electromagnetic wave consists of time-
varying electric and magnetic fields, in
directions perpendicular to each other as well
as to the direction the wave is traveling.
 The electric and the magnetic fields can be
produced by charged particles.


An electric field surrounds any charged particle.
A magnetic field surrounds moving charged
particles.
A rapidly alternating
electric current in a wire
generates magnetic
fields whose direction
and magnitude change
with time.
This changing
magnetic field in turn
produces a changing
electric field.
 Likewise, a changing electric field produces a
magnetic field.
 Maxwell realized a wave involving these
fields could propagate through space:


A changing magnetic field produces a changing
electric field, which produces a changing magnetic
field, etc...
Thus a transverse wave of associated changing
electric and magnetic fields is produced.
 Maxwell predicted the speed of electromagnetic
waves in a vacuum using the Coulomb constant k
in Coulomb’s law and the magnetic force constant
k in Ampere’s law:
v  k k  3 108 m s
 This was equal to the known value for the speed
of light!
c  3 10 8 m s

Fizeau’s wheel for measuring the speed of light

There is a wide spectrum of
frequencies and wavelengths of
electromagnetic waves.

Different types of electromagnetic waves have
different wavelengths and frequencies.
There is a wide spectrum of
frequencies and wavelengths of
electromagnetic waves.

Together they form the electromagnetic spectrum.
There is a wide spectrum of
frequencies and wavelengths of
electromagnetic waves.

Since they all travel at the speed of light c in a
vacuum, their frequencies and wavelengths are
related by: v = c = f 
What is the frequency of radio waves with a
wavelength of 10 m?
 = 10 m
v = c = 3 x 108 m/s
v=f
f=v/
= (3 x 108 m/s) / 10 m
= 3 x 107 Hz
What is the frequency of light waves with a
wavelength of 6 x 10-7 m?
 = 6 x 10-7 m
v = c = 3 x 108 m/s
v=f
f=v/
= (3 x 108 m/s) / 6 x 10-7 m
= 5 x 1014 Hz

Waves in different parts of the electromagnetic
spectrum differ not only in wavelength and
frequency but also in how they are generated and
what materials they will travel through.
Radio waves are generated by accelerated charges in an
oscillating electrical circuit.
 X rays come from energy transitions of atomic electrons.
 Gamma rays originate inside an atomic nucleus.


Waves in different parts of the electromagnetic
spectrum differ not only in wavelength and
frequency but also in how they are generated and
what materials they will travel through.
Infrared light is radiated by all warm bodies.
 Oscillating atoms within the molecules of the warm body
serve as the antennas.


Waves in different parts of the electromagnetic
spectrum differ not only in wavelength and
frequency but also in how they are generated and
what materials they will travel through.
X rays will pass through materials that are opaque to
visible light.
 Radio waves will pass through walls that light cannot
penetrate.


Different wavelengths of visible light are associated
with different colors.
Violet is about 3.8 x 10-7 m.
 Wavelengths shorter than the violet comprise ultraviolet
light.
 Red is about 7.5 x 10-7 m.
 Wavelengths longer than the red comprise infrared light.
 In between, the colors are red, orange, yellow, green,
blue, indigo, and violet.

Wavelength and Color
?
 How do we perceive
 What causes different objects
to have
?
 Why is the sky
?
Newton demonstrated that white light
is a mixture of colors.

He showed that white light from the sun, after being split into
different colors by one prism, can be recombined by a
second prism to form white light again.
How do our eyes distinguish color?



Light is focused by the cornea and lens onto the retina.
The retina is made up of light-sensitive cells called rods and
cones.
Three types of cones are sensitive to light in different parts
of the spectrum.
S cones are most sensitive to shorter wavelengths.
 M cones are most sensitive to medium
wavelengths.
 L cones are most sensitive to longer wavelengths.
 The sensitivity ranges overlap, so that light near the
middle of the visible spectrum will stimulate all three
cone types.

Light of 650 nm
wavelength stimulates L
cones strongest and S
cones weakest; the brain
identifies the color red.
S cones are most sensitive to shorter wavelengths.
 M cones are most sensitive to medium
wavelengths.
 L cones are most sensitive to longer wavelengths.
 The sensitivity ranges overlap, so that light near the
middle of the visible spectrum will stimulate all three
cone types.

Light of 450 nm
stimulates the S cones
most strongly, and the
brain interprets that as the
color blue.
S cones are most sensitive to shorter wavelengths.
 M cones are most sensitive to medium
wavelengths.
 L cones are most sensitive to longer wavelengths.
 The sensitivity ranges overlap, so that light near the
middle of the visible spectrum will stimulate all three
cone types.

Light of 580 nm
stimulates both the M and
L cones strongly, and the
brain identifies that as the
color yellow.
A mixture of red and
green light will produce a
similar response.
Color Mixing
 The process of mixing two different
wavelengths of light, such as red and green, to
produce a response interpreted as another
color, such as yellow, is additive color mixing.
Combining the three primary colors
blue, green, and red in different
amounts can produce responses in
our brains corresponding to all the
colors we are used to identifying.
Red and green make yellow, blue
and green make cyan, and blue and
red make magenta.
Combining all three colors
produces white.
Color Mixing
 The pigments used in paints or dyes work by
selective color mixing.

They absorb some wavelengths of light more than
others.
When light strikes an
object, some of the light
undergoes specular
reflection: all the light is
reflected as if by a
mirror.
Color Mixing
 The pigments used in paints or dyes work by
selective color mixing.

They absorb some wavelengths of light more than
others.
The rest of the light
undergoes diffuse
reflection: it is reflected in
all directions.
Some of the light may be
selectively absorbed,
affecting the color we see.
If red light is absorbed,
we see blue-green.
Color Mixing
 The selective absorption of light is a form of
subtractive color mixing.

In color printing, the three primary pigments are
cyan, yellow, and magenta.
Cyan absorbs red but transmits and
reflects blue and green.
Yellow absorbs blue but transmits and
reflects green and red.
Magenta absorbs green but transmits
and reflects blue and red.
Color Mixing
 The selective absorption of light is a form of
subtractive color mixing.

In color printing, the three primary pigments are
cyan, yellow, and magenta.
Cyan mixed with yellow absorb blue
and red, resulting in green.
Cyan and magenta produce blue.
Yellow and magenta produce red.
These resulting colors are the primary
colors for additive color mixing.
Why is the sky blue?
The white light coming from the sun is actually a mixture of light of
different wavelengths (colors).
The longer wavelengths of blue light are scattered by gas molecules
in the atmosphere more than shorter wavelengths such as red light.
The blue light enters our eyes after being scattered multiple times,
so appears to come from all parts of the sky.
Why is the sunset red?
The shorter wavelengths of blue light are scattered by gas molecules
in the atmosphere more than longer wavelengths such as red light.
When the sun is low on the horizon, the light must pass through more
atmosphere than when the sun is directly above.
By the time the sun’s light reaches our eyes, the shorter
wavelengths such as blue and yellow have been removed by scattering,
leaving only orange and red light coming straight from the sun.
Interference of Light
Waves
 Is light a wave or a particle?
 If it is a wave, it should exhibit interference effects:
Recall that
two waves can
interfere
constructively
or
destructively
depending on
their phase.
Light from a single slit is split by passing through two slits,
resulting in two light waves in phase with each other.
The two waves will interfere constructively or destructively,
depending on a difference in the path length.
If the two waves travel equal distances to the screen, they
interfere constructively and a bright spot or line is seen.
If the distances traveled differ by half a wavelength, the two
waves interfere destructively and a dark spot or line appears
on the screen.
If the distances traveled differ by a full wavelength, the two
waves interfere constructively again resulting in another
bright spot or line.
The resulting interference pattern of alternating bright and
dark lines is a fringe pattern.
y
path difference  d
x

Red light with a wavelength of 630 nm strikes a double slit
with a spacing of 0.5 mm. If the interference pattern is
observed on a screen located 1 m from the double slit, how far
from the center of the screen is the second bright line from
the central (zenith) bright line?
= 630 nm = 6.3 x 10-7 m
d = 0.5 mm = 5 x 10-4 m
x=1m

path difference  2  d
y
x
7
2x 26.3 10 m1 m
y

 0.0025 m  2.5 mm
-4
d
5 10 m
 Similarly, interference can occur when light waves
are reflected from the top and bottom surfaces of a
soap film or oil slick.
 The difference in the path length of the two waves
can produce an interference pattern.
 This is called
thin-film
interference.
 Different wavelengths of light interfere constructively
or destructively as the thickness of the film varies.
 This results in the many different colors seen.
 The thin film may also be air between two glass
plates.
 Each band represents a different thickness of film.
Diffraction and Gratings
 The bright fringes in a double-slit interference
pattern are not all equally bright.


They become less bright farther from the center.
They seem to fade in and out.
 This effect, called diffraction, is due to
interference of light coming from different
parts of the same slit or opening.
When the path difference between light coming from the top
half of the slit and that coming from the bottom half is 1/2 of a
wavelength, a dark line appears on the single-slit diffraction
pattern.
x
The position of the first dark fringe is:
y
w

Light with a wavelength of 550 nm strikes a single slit that is
0.4 mm wide. The diffraction pattern produced is observed on
a wall a distance of 3.0 m from the slit. What is the distance
from the center of the pattern to the first dark fringe?
= 550 nm = 5.5 x 10-7 m
w = 0.4 mm = 4 x 10-4 m
x = 3.0 m
y

5.5 10 m3.0 m

 0.0041 m  4.1 mm
0.4 10 m
w
x
7
-3
How wide is the central bright fringe of this diffraction
pattern?
The central bright fringe extends out to the first dark fringe
on either side, so its width is just twice the distance y:
= 550 nm = 5.5 x 10-7 m
w = 0.4 mm = 4 x 10-4 m
x = 3.0 m
2y  24.1 mm   8.2 mm
 The diffraction pattern produced by a square opening
has an array of bright spots.
 Looking at a star or distant street light through a
window screen can produce a similar diffraction
pattern.
 A diffraction grating has a very large number of slits very
closely spaced.
 Whenever the path difference is equal to an integer multiple of
the light wavelength,
y
we get a strong bright fringe for
d  m, m  0,1,2,...
x
that wavelength.
 Different wavelengths will appear at different points on the
screen, spreading the light into its spectrum.

 Diffraction gratings in spectrometers are used to separate and
measure the wavelengths of light.
 Gratings also produce the effects seen in novelty glasses,
reflective gift wrappings, and in the colors seen on a CD.
Polarized Light
 How do polarizing sunglasses and camera
filters work?
 What is polarized light?
Recall that light is an electromagnetic wave consisting of
oscillating electric and magnetic fields:
 The oscillating electric field vector shown is in the
vertical plane, and the magnetic field is horizontal.
 Actually, the electric field could oscillate in the
horizontal with the magnetic field in the vertical plane,
or the electric field could oscillate at some angle to the
horizontal.
 As long as the electric field is always pointing in the
same direction for all the waves, the light is polarized.
 The electric field vector oscillates in a single
direction for polarized light.
 Unpolarized light has random directions of
orientation.
Light is usually
produced
unpolarized.
To make light
polarized,
something must
occur to select just
one direction of
field oscillation.
 Polarizing filters allows only that component
of each electric field vector that is aligned with
the filter’s axis of transmission to pass
through.
The component
perpendicular to
this axis is
absorbed.
 Reflection from a smooth surface of a
transparent material such as glass or water
can also polarize light.
Incoming sunlight is
unpolarized.
When the angle
between the reflected
wave and the
transmitted wave is a
right angle, the reflected
wave is polarized.
Polarizing sunglasses
can help reduce glare
from reflected sunlight.
 Many interesting and colorful effects are
related to the phenomenon of birefringence.



Birefringence is also called double refraction.
Light with different polarizations travels with
different velocities when passing through a
birefringent material.
This causes colorful displays when the birefringent
material is viewed through crossed polarizers.
Calcite crystals are
a good example of
birefringent material.
Lines are doubled
when viewed
through a calcite
crystal.
 Many interesting and colorful effects are
related to the phenomenon of birefringence.



Birefringence is also called double refraction.
Light with different polarizations travels with
different velocities when passing through a
birefringent material.
This causes colorful displays when the birefringent
material is viewed through crossed polarizers.
A plastic lens under
compression shows
stress birefringence
when viewed
between crossed
polarizers.