An Electromagnetic Wave
Physics 10
Properties of Light
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The Visible Spectrum
⇒ An electromagnetic wave is a transverse
wave consisting of oscillating electric and
magnetic fields.
The Electromagnetic Spectrum
⇒ Imagine if you can the world view of little
creatures who could see only a very tiny portion
of the visible spectrum, creatures who are color
blind to all other parts. Their world view would
be very limited.
⇒ Guess what? We are like those little creatures,
in that the spectrum of colors we can see are a tiny
portion of the electromagnetic spectrum!
⇒ The electromagnetic spectrum consists of
electromagnetic waves of all frequencies.
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The Electromagnetic Spectrum
Transmission of Light
⇒ Light is transmitted through a transparent material
(such as glass) through a process of absorption and reemission by electrons within the material.
⇒ Visible light makes up a very small portion (less
than one millionth of 1%) of the electromagnetic
spectrum.
Why would you expect the speed of light
to be slightly less in the atmosphere than
in a vacuum?
⇒ The average speed of light will be less
where it interacts with absorbing and reemitting particles of matter, such as in the
atmosphere. The greater the number of
interactions along the light’s path, the less the
average speed.
⇒ The light that leaves the glass is the same frequency as
the light that enters the glass. However, it is “new light”;
it is not the same light that entered the glass.
Pretend a person can walk only at a certain
pace – no faster, no slower. If you time her
uninterrupted walk across a room of known
length, you can calculate her walking speed.
If, however, she stops momentarily along the
way to greet others in the room, the extra
time spent in her brief interactions gives an
average speed across the room that is less
than her walking speed. How is this like light
passing through glass? In what way is it not?
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Shadows
Answer:
This is like light passing through glass because
there is a slight delay when light is absorbed and
then re-emitted by the atoms or molecules.
Between the atoms or molecules, light travels at a
constant speed, c.
It is not like light passing through glass because
the light that entered the glass is not the same as
the light that emerges from the glass. Every time
an atom absorbs light, “new light” is emitted in its
place.
Shadows
⇒ Shadows can either be sharp or blurry.
⇒ A shadow is just a region of space where
light rays cannot reach because they are
blocked.
umbra ⇒ total shadow
penumbra ⇒ partial shadow
⇒ A penumbra results when
some of the light is blocked
but where other light fills in.
Phases of the Moon
⇒ The different phases of
the moon are a result of the
relative positions of the sun,
Earth, and moon.
⇒ A sharp shadow is produced by a large far-away
light source or a small nearby light source.
⇒ A blurry shadow is produced by a large nearby
light source.
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Lunar Eclipse
Lunar Eclipse
⇒ A lunar eclipse occurs when the moon is
completely in the Earth’s shadow.
Solar Eclipse
Solar Eclipse
⇒ A solar eclipse occurs when the moon’s
shadow falls on part of the Earth.
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Solar Eclipse
The Human Eye
⇒ The fovea is the spot on the center of our field of
view where our vision is most distinct. Greater detail can
be seen at the fovea that at the side parts of the eye.
Rods and Cones
⇒ There are two kind of “antennae” in the retina for
detecting light : rods and cones (named because of their
different shapes).
Rods and Cones
⇒ In the human eye, there are about 6-7 million cones
and about 75-150 million rods.
rods ⇒ sensitive to
lightness or darkness
(but not color)
⇒ Cones are concentrated in the
fovea. The number of cones
decreases as you move away from
the fovea.
cones ⇒ sensitive to color
(there are three different
types of cones)
⇒ Rods are concentrated toward
the periphery of the retina.
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Rods and Cones
⇒ It takes more energy (or more light) to
“activate” the color-sensitive cones than it does
the rods.
⇒ Because of this, if the intensity of light is very
low, the cones will not respond and the things we
see will have no color. (This is why stars appear
white)
⇒ The rods on the periphery of our retina can not
see color, but they are very sensitive to motion.
Our peripheral vision is poor, but we are sensitive
to anything moving in our periphery.
Solutions to Ch. 26 Exercises
14) Again you should agree with your friend. Electromagnetic waves
(and therefore radio waves) travel about one million times faster
than sound waves.
21) Radio waves and light (i.e. visible light) are both
electromagnetic waves and travel at the same speed through a
vacuum. Radio waves have a longer wavelength and a smaller
frequency than visible light.
Solutions to Ch. 26 Exercises
3) The fundamental source of electromagnetic radiation is oscillating
electric charges, which emit oscillating electric and magnetic fields.
9) Between us and celestial bodies is the vacuum of space. The fact
that we can see the sun and stars from Earth is convincing evidence
that light travels through a vacuum.
13) You should agree with your friend. All electromagnetic waves
travel through space at 3.0 × 108 m/s. Microwaves and ultraviolet
have different wavelengths and different frequencies but the same
speed.
Solutions to Ch. 26 Exercises
28) Transparency or opaqueness is determined by the
match between incident light frequencies and the resonant
frequency of the material. A substance that is transparent
to a range of light frequencies will be opaque to those
frequencies that match its own resonant frequency.
23) The speed of light in the atmosphere is less than in a vacuum
because when light travels through a transparent material, it is
constantly absorbed and re-emitted by atoms in the material.
There is a slight time delay between absorption and re-emission
so the average speed of light is less through the atmosphere.
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Solutions to Ch. 26 Exercises
36) (a) Moon observers would see the Earth in the path of the
sunlight and see a solar eclipse. (b) Moon observers would see a
small shadow of the Moon slowly move across the full Earth. The
shadow would consist of a dark spot (the umbra) surrounded by a
not-as-dark circle (the penumbra).
Solutions to Ch. 26 Exercises
48) You're seeing the galaxies as they "were" when light
left them, long long ago.
Good luck on the Exam!!!
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