Download Electromagnetic Waves • Suppose we have an alternating current

Electromagnetic Waves | MCAT 2015
Electromagnetic Waves
• Suppose we have an alternating
current running through a wire
• If we graphed the electric field
generated by this current, it’d
peak where the current is moving
fastest, and it’d be 0 when the
current stops to change direction
•
•
Together, the two waves make up
an electromagnetic wave
Light travels via this type of
wave
E
o c=
B
o c = speed of light
E = electric field
B = magnetic field
The Electromagnetic Spectrum
• Visible light makes up a small
segment of the spectrum of
electromagnetic waves
o ~ 400-700 nm
Electric Field of an AC Current over Time
At maximum velocity, the current generates its
maximum electric field. When the current halts,
the electric field becomes 0.
•
•
We know that an electric field
generates a magnetic field
o Thus, when the electric
field is at a maximum, so
is the magnetic field
o And, when the electric
field is 0, so is the
magnetic field
We could plot the magnetic field
on the z-axis of the graph
Electric and Magnetic Fields of AC Current
over Time
The electric and magnetic field peak and bottom
out at the same points.
The Electromagnetic Spectrum
Energy and frequency increase to the right of
visible light. Wavelength increases to the left.
You can remember this if you know that the
sun’s UV rays have so much energy that they
burn your skin.
Speed of Electromagnetic Waves
• As with other waves, we can use
v = λf
• In a vacuum, light’s speed is a
constant, c, 3x108 m/s
• Like other waves, light changes
velocity in different media
o To help predict this, we
assign an index of
refraction, n, to media
c
o n=
v
o n = index of refraction
c = speed of light in
vacuum
1
© 2017 J Co Review, Inc., Accessed by Guest on 05-12-2017
Electromagnetic Waves | MCAT 2015
•
v = speed of light in
medium
o Thus, the index of
refraction is a ratio of
light’s speed in a vacuum
to light’s speed in the
given medium
In any medium, the medium’s
atoms will absorb some of the
light’s energy, thereby slowing it
Light Being Slowed by Water Molecules
•
Since a vacuum is atom-free,
light will travel quickest through
a vacuum
o Since c will always
exceed v, n must always
be greater than 1
o The greater n is, the
slower light travels in that
medium
2
© 2017 J Co Review, Inc., Accessed by Guest on 05-12-2017