Light Waves     
Chapter 11

 Joseph F. Alward, PhD
 Department of Physics
 University of the Pacific


 Thin film interference in soap film.

 
Applets used in this eLecture:

Young's Double Slit 1
Young's Double Slit 2

 

 

 

 

  Oscillating Charges Emit Electromagnetic Waves
1.  Stationary charges set up static electric fields;
     oscillating charges charges set up oscillating
     electric fields.

2.  Charges moving with constant speed, as in
     electrical currents, set up static magnetic fields;
     oscillating charges set up oscillating magnetic
     fields.

Conclusion:  an oscillating charged wand
sets up oscillating electric and magnetic
fields.

 

       

 Generating Electromagnetic Waves         

 

 

 Electromagnetic Waves                         

c = 186, 000 miles/second

   = 3 x 108 meters/second

  

 
Click here to see animation of the
creation of electromagnetic wave

from an oscillating charge. 

 

 

  Unmodulated Carrier Wave     

 

 

 

   

 

  

  Light is a Wave-Like Disturbance                                               

 

 

 

 

 

  The Electromagnetic Spectrum                                               

 

 

 

 

 

   Wavelengths                                                                                            

 

 

 

 

 

 

 

 

 

 

   Wavelengths of Visible Light                                                   

 

 

  

 

 

  

                                                                                                     

 

 

 

   Transparent and Opaque Materials  

  One tuning fork stimulates the vibration of another.  The second tuning fork absorbs the
  sound energy from the first and begins to vibrate, sending out its own sound energy.

 

 

 

  Atomic Tuning Forks
Electrons are light enough
to respond quickly to
oscillating electric field.

They receive the energy
from the electromagnetic
wave, then re-radiate it.

 

 

 

  Absorption and Emission in Solids
Light energy is
absorbed by
the atomic
oscillators and
is re-emitted
by the
oscillating
electron.

 

 

 

   Transmission of Light through Glass and Air

  High frequency ultraviolet (UV) is blocked by atmosphere.  Lower-frequency UV
  gets through atmosphere.  Clouds will allow some UV to pass through.
Ultraviolet light
oscillates at
too high a
frequency for
electrons in
glass molecules,
while infrared is
too low.

Visble light is
just right.

 

 

 

  Light Reflection from Metals
Free electrons in metals
absorb and re-emit
light at the surface.

Not all light incident on
metals is reflected; the
light that enters the
metal is absorbed and
re-emitted, but the
energy is mainly used
to warm the metal;
none of the light passes
through the metal.

 

 

 

 

 

 The Spectrum of White Light                                           

 

 

 

 

 

 

  Reflection and Absorption
White reflects all
colors equally well.

Black absorbs all
light.

Grass absorbs
which range
of colors?

 

 

 

  

   Reflection  

Lipstick reflects differently in different lighting.

 

 

 

  Transmission of Light

When red light shines on a red rose, which becomes warmer, the rose, or the petals?

 

 

 

    Color Spectrum of Sunlight
Through evolution, our
eyes have become most
sensitive to yellow-green
light.  Why?

Yellow light from sodium-
vapor lamps illuminates
better at night than common
tungsten-filament lamps of
the same brightness.

 

 

   Rods and Cones 
Rods in the retina distinguish
brightness.

Cones in retina distinguish
between colors, and require
more energy to respond
than do rods.

Twinkling starlight consists
of separated colors.  Why
do we usually just see
white?

 

 

  

    The Additive Primary Colors
Sunlight may arbitrarily divided
into three man color groups
spanning different ranges of
wavelengths:

Red     Green     Blue

 

 

 

 

 

    The Additive Primary Colors
They're called primary
because any color
can be made from a
suitable combination
of red, green, and blue.

 

 

 

   Complementary Colors
(Red + Green) + Blue = White
 
Red + Green = Yellow

Colors which  combine to
produce white are said to be
complementary.  Therefore,
yellow and blue are
complementary colors.

Red + Blue = Magenta

Of what color is magenta
complementary?

 

 

 

 

   Complementary Colors
Red, green, and blue light sources
are in the plane of the picture at
the bottom, not shown.

A shadow corresponding to each
light source is formed behind the
golf ball.

The yellow shadow is white
light, minus blue.

What color is the middle shadow?

 

 

 

  Mixing Color Pigments (Paints)

              Red paint

In red paint, what primary
colors are absorbed from
white light?

What primary color is
reflected?


         Green Paint

In green paint, what primary
colors are absorbed from
white light?

What primary color is
reflected?

 



 

 

 

   

  Mixing Colored Pigments
1.  Magenta:  reflects red and blue;
     absorbs green.

2.  Yellow:  reflects red and green;
      absorbs blue

3.  Cyan:  reflects green and blue;
     absorbs red.

 Consider bright red triangle of
 color to the right of the boy's head.

Absorbs green (1) and blue (2),
reflects red.   

 

 

 

    Mixing Color Pigments (Paints)  
Magenta reflects red and blue, absorbs green.
Cyan reflects blue and green, absorbs red.
Yellow reflects red and green, absorbs blue.


1.  Which colors must be subtracted (absorbed)
      from white light to give reflected green light?

2.  Which pigments should be added to give
      the color green?


3.  Which pigments should be added to give
      red?

 

 

 

  Light Scattering
Electromagnetic
energy is
absorbed by
electron in atom,
then re-emitted
in all directions
(only one
direction at a
time, for any
particular atom).

 

 

 

 

 Blue Sky
Small-wavelength light (blue-green)
from the sun is scattered by very small
particles of dust, and especially oxygen
nitrogen in the atmosphere.


If no light were scattered by the
atmosphere, would any light from
the sun reach our eyes (assuming
we don't look directly at the sun)?

What color is moon sky?



  

 

 

 

 

    White Sky
Particles in the air
which are larger than
oxygen and nitrogen
scatter red light.

Mixed with the blue
light scattered by N2
and O2, the result is
a nearly white sky.

 

 


 

  Red Sunsets 
Atmosphere at sunset provides a
much longer path through which
white light from the sun must
travel.

Atmosphere scatters a large
percentage of the blue part of the
white light in all directions,
leaving mainly the red part to
reach the eyes of the person
viewing the setting sun.

 

 

 

  White Clouds
Small water-droplet cluster
scatter red light, while larger
clusters scatter blue.  Together,
this scattered light is white.

 

 

 

 Water is Blue-Green
This wave absorbs red
light, and reflects blue-green
(cyan).

Froth is white because tiny
water-droplet clusters
absorb and then re-radiate
red light.  The red mixes
with the blue-green to
produce white.  

 

 

     Water Waves Bend around Obstacles  
Oscillating board
up and down
creates plane
waves
.

If the wavelength
is comparable
to the width of
opening,
significant
changes in
direction occur.

 

 

 

  Diffraction by Edges of Holes

This shows plane wave waves being bent into the "shadow"
regions. Bending is greatest when the opening is small
compared to the wavelength.  
The smaller the wavelenth
is relative to the obstacle
or opening, the sharper is
the shadow.

 

 

  Diffraction by the Edges of Obstacles 

           Wavelength is comparable                    Wavelength is very small
            to size of obstacle.                               compared to object
Smaller wavelengths
create sharper shadows.


Foghorns operate at low
frequency (long wavelength)
to fill shadow regions.

FM radio at wavelengths of
2.8-3.4 meters casts sharper shadows than AM radio at 180-550 meters

 

 

 

 

 

   Inteference of Waves from Two Sources                                             
In some places the water
wavefronts are in phase
(bright spots).

In other places the fronts
overlap with peak and
valley and interfere
destructively (darker
spots).

 

 

    Interference

 This is a "snapshot" at some instant in time of a plane wave from a single source.
If the EM
waves from
two or more
sources
overlap, they
will "interfere".

 

  

  Interference

  Electric field vectors point         Electric field vectors point            Incomplete cancellation.
  in the same direction at all         in opposite directions at all 
  points at same time. This is        points.  This is destructive           
  constructive interference.        interference.

 

 

 

  Young's Double Slit Experiment
Waves from two different
sources overlap.  In some
places the interference is
constructive
, in other
places it is destructive.

 

 

  Applets
Young's Double Slit 1
Young's Double Slit 2

 

 

 

 

  Interference of Two Sources                                                       

 

 

 

 

  Interference from Thin Films
If wave reflected
from water is
out of step--out
of phase
--with
the wave reflected
from gasoline,
there will be
destructive
inteference and
the eye will see
darkness.

 

 

 

   Interference

Iridescence of peacock feathers is caused by
light reflected from complex layered surface.

Different thickness lead to constructive
inteferences for different wavelengths.

   

 

 

     Inteference Projects                                                                                          

Thin film of air between slides produces
 interference pattern.

 

 

 

  Polarization
Illustrating vertical and
horizontal polarized
waves.

 

 

  Plane Polarized Electromagnetic Waves
Plane-polarized
electromagnetic
waves caused
by charge
oscillating in
different planes.

 

 

 

 

   Unpolarized Light

     3D view of planes of oscillations             End view                              Simple
     of just a few of the EM waves                                                            representation
     from an incandescent bulb.

 

 

  Polarizing Materials
Horizontal oscillations are
absorbed; only the vertical
oscillations are transmitted.

Polaroid filters transmit
50 % of unpolarized light.

 

 

  Polaroid Filters

 

 

  Crossed Polaroid Filters

  The first filter absorbs and re-radiates only
  vertically oscillating electric fields; the second
  will pass only horizontally oscillating fields.

 No energy is transmitted out of the second
 filter.

 

 

  Polaroid Lenses
Glasses block horizontally
polarized light.

 

 

  

  Polaroid Filters

  Filters are parallel                          Crossed filters                        

 

 

 

  Polarization by Reflection
"Glare" is sunlight reflected
off smooth surfaces into
the eye.

Most glare is from
horizontal surfaces, so
glasses are worn with
polarizing direction
vertical.

 

Polarization Applets
Polarizing Lenses
Two rotatable lenses,
camera; excellent
Polarization
Shows rotatable filter
on lake; very nice