Light and
Electromagnetic Waves
Mark Lesmeister
Dawson High School
PART I
ELECTROMAGNETIC WAVES
Introduction to Light & Other
EM Waves.
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Visible light is one type of
electromagnetic wave. Radio waves are
another.
We have a lot of experience with these
types of waves.
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What do you feel as you move your hand
closer to a bright light source?
What happens to radio stations when you
get farther away?
Lab: Light Intensity
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Work in groups of 2-3.
Make sure the source lines up with the
sensor, and that you know where the front
of the sensor is on the scale.
Start with sensor 5 cm from the light
source and measure the illumination.
Move the sensor 2 cm farther away, and
measure the illumination again. Repeat
this step until you have 10 readings.
Analyzing the Brightness Lab
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Decide what equation you think fits the
graph. (In addition to the ones we
already studied, you may want to
consider 1/d2 ).
Confirm your hypothesis by linearizing
your data and drawing a new graph.
Do you get a straight line?
Discuss the results with the other
groups in your section.
Brightness
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The brightness of a light wave is the
power transmitted by the wave per unit
of area.
The brightness decreases by the square
of the distance from the source.
Electromagnetic Radiation
Electromagnetic Radiation
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Electromagnetic radiation consists of
oscillating electric and magnetic fields
with different wavelengths.
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Source: Wikipedia
Electromagnetic Spectrum
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Each group will examine one portion of
the electromagnetic spectrum. For your
part, make a whiteboard showing
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Frequency range
Wavelength range
Uses of the radiation.
The Electromagnetic Spectrum
The Electromagnetic Spectrum
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Radio waves
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l > 0.1 m
Communications; MRI’s
Microwaves
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0.1m>l > 0.0001 m
Communications;
navigation; ovens
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0.0001m>l > 7 x 10-7m
Night-vision, thermometers,
therapy
4 x 10-7 m>l > 10-9 m
Sterilization; “blacklight”
X-rays
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7 x 10-7m>l > 4 x 10-7 m
Human sight
Ultraviolet light
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Infrared waves
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Visible light
10-7 m >l > 10-11 m
Medical diagnostics and
treatments
Gamma rays 10-11 m >l
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Cancer treatments; food
irradiation
The speed of light
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All electromagnetic waves move at the
speed of light.
The speed of light is approximately 3.00
x 108 m/s, or 186,000 mi/s.
The wave speed equation for light is
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c=fl
Speed of light = frequency x wavelength
PART II
CHARACTERISTICS OF
VISIBLE LIGHT
Color
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Electromagnetic waves with
wavelengths between 700 nm and 400
nm fall within the visible spectrum.
The wavelength (or frequency) of a
light determine the color of the light.
Color
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The colors of the visible light spectrum vary
continuously in the visible range, but are
usually named as follows, from longest to
shortest.
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Red
Orange
Yellow
Green
Blue
Indigo
Violet
Additive Primary Colors
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Light from a source such as
the sun contains a variety of
colors.
The additive primary colors
of light are red, green and
blue.
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When these colors of light are
combined in equal amounts, they
produce white light.
When they are added together in
various proportions, they can
produce all the colors of the
visible spectrum.
Red
Green
Blue
Pigments
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When an object is
illuminated by a light source,
it may absorb some colors of
light and reflect others.
The color of light that an
object appears to have is
determined by the colors of
light that it reflects.
The primary pigments, or
primary subtractive colors,
reflect one color of light and
absorb all others.
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These pigments are cyan,
magenta, and yellow.
When these three are
combined, they filter out all
colors, and the mixture is
black.
© Holt Rinehart & Winston
Additive and subtractive
primary colors
Red
Yellow
Green
Magenta
Cyan
Blue
Polarization
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The electric and
magnetic fields in an
electromagnetic wave
are always
perpendicular to each
other and the direction
of the wave.
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The direction of
oscillations is usually
random.
The wave can be made
to oscillate in one
direction only. This is
known as a linearly
polarized wave.
Source: Wikipedia
Polarizing Visible Light
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A polarizing filter polarizes light waves.
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When light is filtered through two filters
aligned at right angles, then the
transmitted light wave is sharply reduces.
Light that reflects off a flat surface at
certain angles can be polarized.
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This effect is used in polarized sunglasses
in order to reduce glare.