Light and Electromagnetic Waves Mark Lesmeister Dawson High
School
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ELECTROMAGNETIC WAVES PART I
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Introduction to Light & Other EM Waves. Visible light is
one type of electromagnetic wave. Radio waves are another. We have
a lot of experience with these types of waves. 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?
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Lab: Light Intensity 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.
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Analyzing the Brightness Lab Decide what equation you think
fits the graph. (In addition to the ones we already studied, you
may want to consider 1/d 2 ). 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.
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Brightness 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.
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Electromagnetic Radiation
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Electromagnetic radiation consists of oscillating electric and
magnetic fields with different wavelengths. Source: Wikipedia
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Electromagnetic Spectrum Each group will examine one portion of
the electromagnetic spectrum. For your part, make a whiteboard
showing Frequency range Wavelength range Uses of the radiation.
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The Electromagnetic Spectrum
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Radio waves > 0.1 m Communications; MRIs Microwaves 0.1m>
> 0.0001 m Communications; navigation; ovens Infrared waves
0.0001m> > 7 x 10 -7 m Night-vision, thermometers, therapy
Visible light 7 x 10 -7 m> > 4 x 10 -7 m Human sight
Ultraviolet light 4 x 10 -7 m> > 10 -9 m Sterilization;
blacklight X-rays 10 -7 m > > 10 -11 m Medical diagnostics
and treatments Gamma rays10 -11 m > Cancer treatments; food
irradiation
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The speed of light All electromagnetic waves move at the speed
of light. The speed of light is approximately 3.00 x 10 8 m/s, or
186,000 mi/s. The wave speed equation for light is c = f Speed of
light = frequency x wavelength
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Huygens Principle A continuous line of wave crests is called a
wave front. The motion of a wave can be determined by treating each
point of a wave front as a new wave source. Holt Rinehart &
Winston
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CHARACTERISTICS OF VISIBLE LIGHT PART II
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Color 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.
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Color The colors of the visible light spectrum vary
continuously in the visible range, but are usually named as
follows, from longest to shortest. Red Orange Yellow Green Blue
Indigo Violet
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Additive Primary Colors Light from a source such as the sun
contains a variety of colors. The additive primary colors of light
are red, green and blue. 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 BlueGreen
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Pigments 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. 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
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Additive and subtractive primary colors Red BlueGreen
YellowMagenta Cyan
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Polarization The electric and magnetic fields in an
electromagnetic wave are always perpendicular to each other and the
direction of the wave. 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
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Polarizing Visible Light A polarizing filter polarizes light
waves. 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. This effect is used in polarized sunglasses in order to
reduce glare.