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Chapter 27
Properties of Light
Electromagnetic
Radiation
Photons
Light
These three are the same…
• Light
* pure energy
• Electromagnetic Waves
* energy-carrying waves emitted by vibrating
electrons
• Photons
* particles of light
Electromagnetic Wave
Velocity
• The speed of light is the same for all
seven forms of light.
• It is 300,000,000 meters per second or
186,000 miles per second.
Michelson’s 1878 Rotating Mirror
Experiment
• German American physicist A.A. Michelson
• Using this method, Michelson was able to
calculate c = 299,792 km/s
Picture credit
The Electromagnetic Spectrum
• A range of light waves extending in
wavelength from radio waves to
gamma rays
The Electromagnetic Spectrum
•
•
•
•
•
•
•
Radio Waves - communication
Microwaves - used to cook
Infrared - “heat waves”
Visible Light - detected by your eyes
Ultraviolet - causes sunburns
X-rays - penetrates tissue
Gamma Rays - most energetic
The Visible Spectrum
•A range of light waves extending in
wavelength from about 400 to 700
namometers.
Questions
• Is it correct to say that radio
wave is a low-frequency light
wave?
• Is a radio wave also a sound
wave?
• See question on page 455.
*
Transparent Materials
• Transparent - the term applied to
materials through which light can
pass in straight lines
Opaque Materials
• Opaque - the term applied to
materials that absorb light
Example Questions
• Are clouds transparent or opaque to
visible light?
• Answer: opaque
• Are clouds transparent or opaque to
ultraviolet light?
• Answer: transparent
Shadows
• Umbra - the darker part of a shadow
where all the light is blocked
• Penumbra - a partial shadow
• These terms also apply to Solar
Eclipses and Lunar Eclipses.
Solar Eclipse
Sun
Umbra
Full Shadow
Earth
Moon
Partial Shadow
• A solar eclipse occurs when
the Moon passes in front of
the Sun.
Penumbra
Lunar Eclipse
Sun
Earth
Moon
• A lunar eclipse occurs when
the Moon passes into the
Earth's shadow.
Polarization
Net electric field is zero – Unpolarized light!
Electric field only going
up and down – say it is
linearly polarized.
Light can have other types of polarizations such as circularly polarized
or elliptically polarized. We will only look at linearly polarized light.
How do we make Polarized Light?
I. Polarizers-
Polarizers are made of long
chained molecules which absorb
light with electric fields
perpendicular to the axis.
• Light is totally polarized when an object
with a vertical Polarization axis and
another object with a horizontal
polarization axis are placed next to each
other.
• This causes no light to be able to get
through the lens.
Questions
• Which type of eclipse is
dangerous to view with the
unprotected eye?
• Why are lunar eclipses more
commonly seen than solar
eclipses?
• See question on page 460.
*
Chapter 27 Review Questions
You will observe a total eclipse of
the sun when...
(a) you stand in the penumbra of the moon’s
(b) shadow
you stand in the umbra of the moon’s shad
(b) you stand in the umbra of the moon’s
shadow
(c) sunlight diffracts around the moon
(d) sunlight reflects from the moon to the
earth
The speed of light...
(a) has never been measured
(b) is about the same as that of
sound
(c) is infinitely fast
(d) is very fast, but not infinite
(d) is very fast, but not infinite
In the dark at late evening, no color
is seen because of lack of
stimulation of
a. rods.
b. cones.
b. cones.
c. cornea.
d. crystalline lens.
End of Chapter 25