Transcript Chapter 26 Properties of Light

Part VI: Light
Chapter 26
Properties of Light
17-Jul-15
Physics 1 (Garcia) SJSU
Electromagnetic Waves
Moving charges (currents) create magnetic fields.
Oscillating magnetic fields create electric fields.
These effects create electromagnetic waves.
17-Jul-15
Physics 1 (Garcia) SJSU
Demo: Light & Sound
Sound waves can only travel
through a material, such as
air, but light waves can
travel through vacuum.
Can see cell phone
ringing inside vacuum
chamber but don’t hear
the sound.
Sound waves are not electromagnetic waves
17-Jul-15
Physics 1 (Garcia) SJSU
Speed of Light
Speed of light is 300,000,000 m/s
Since
(Wave Speed)
(Wavelength) =
(Frequency)
then radio station at 100 Megahertz has
300,000,000 m/s
(Wavelength) =
= 3 meters
100,000,000 Hz
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Physics 1 (Garcia) SJSU
Electromagnetic Spectrum
Cell phone
Very broad spectrum of
electromagnetic waves
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Physics 1 (Garcia) SJSU
Transparent & Opaque Materials
A material is transparent or
opaque to different
wavelengths depending
on its atomic properties.
Some opaque materials
absorb certain waves
while other materials
reflect those electromagnetic waves.
17-Jul-15
Physics 1 (Garcia) SJSU
For visible light, glass is
transparent, while rubber
and metal are opaque
(rubber absorbs,
metal reflects)
Check Yourself
The forced oscillations of ultraviolet light
happen to match the natural frequency of
electrons in glass. So what happens?
So is glass opaque to ultraviolet light?
17-Jul-15
Physics 1 (Garcia) SJSU
Demo: Ultraviolet Light
Fluorescent (Day-Glo) paint converts
invisible ultraviolet light into visible light.
Test opacity of:
• Glass
• Water
• Sunscreen
Wavelength of ultraviolet light
is about the size of bacteria
(shorter than visible light).
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Ultraviolet
Lamp
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Demo: Microwaves
Test transparency, opacity, & reflectivity of:
• Metal (aluminum foil)
• Wood (book)
Microwave
Microwave
• Glass
Receiver
Transmitter
• Water
Wavelength of microwaves
is fraction of a centimeter
(longer than visible light).
17-Jul-15
Physics 1 (Garcia) SJSU
Perception of Distance
Visually, we experience distance by
• Occultation (objects hide what’s behind them)
• Geometric Perspective (objects look smaller as
they get further away)
• Atmospheric Perspective (distant objects are
hazy and bluish)
• Lighting and shadows
• Stereopsis (different view in each eye)
• Relative motion (as you move, nearby objects
shift more than distant objects)
Occlusion
The simplest way that
we perceive distance
is by the fact that
closer objects
occlude (hide) the
objects behind them.
Even in this surreal painting we
immediately see the boy as
being closer to us than the
woman because he partially
blocks our view of her.
Detail from The Madonna of Port Lligat, Salvador Dali, 1950
Distorted Occlusion
The image is disturbing
but the reason isn’t
immediately apparent.
When occlusion
is incorrect, we
are very
cognizant of the
distortion.
This channel of
water needs to
be behind the
lower part of the
right tower.
Detail from Waterfall, M.C. Escher, 1961
Pre-15th Century Paintings
Occlusion but no sense of distance
Mongol Ruler and consort
enthroned, 14th century
Road to Calvary, Martini, 1315
Renaissance Paintings
Scenes in these paintings look realistic
The Annunciation, Botticelli, 1489
Marriage
of the
Virgin,
Raphael,
1504
Perspective
The difference is the
introduction of visual
perspective by Filippo
Brunelleschi of
Florence.
Objects in the distance
look smaller as
determined by
geometric rules.
Florence, Italy
Perspective Example
The gazelles in this photo appear to be roughly the same physical size.
Thanks to John Clapp for these slides
Perspective Example
Move from here…to here
Let’s move one using cut-and-paste. How big will it be?
Perspective Example
Surprised? Objects appear much smaller with distance!
Your brain adjusts and “sees” the animals as equal size.
Perspective Example
Move down
Even a short distance into the background makes a surprising difference.
Perspective Example
Move from here…to here
Now let’s go the other way and move from foreground to background.
Try to try
visualize
it’s size…
Again,
to visualize
how large the gazelle will be when cut-and-pasted.
Perspective Example
Surprised? Instead of a gazelle it’s now Godzilla.
Try this at home with your own photos.
Drawing with Perspective
From that example we see that it’s not
easy to predict how large or small
objects will be at different distances.
How do artists create
images with realistic
perspective?
By using geometry!
Perspective Demonstration
Now add the rest of the figures and draw this.
Perspective Demonstration
Horizon
We can check that the persons in the foreground and background
are the same height by drawing lines back to the horizon.
Distorted Perspective
Modern painters
sometimes distort
the perspective for
dramatic effect.
The two buildings converge
to two different horizons.
This feels weird and
unnatural, which is what the
artist intended (note the title
of the painting).
Mystery and Melancholy of a
Street, de Chirico, 1914
Shadows
Size and sharpness of a shadow depends
on size and distance of light source and of
object casting the shadow.
17-Jul-15
Physics 1 (Garcia) SJSU
Ray Tracing for Shadows
Trace rays from light source to wall to map out location of
deep shadow (umbra) and fuzzy shadow (penumbra).
Light source
Object
Wall
Penumbra
UMBRA
Penumbra
17-Jul-15
Physics 1 (Garcia) SJSU