Electromagnetic Waves Produced by an Antenna

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Electromagnetic Waves Produced by an Antenna

• When a charged particle undergoes an
  acceleration, it radiates energy
• If currents in an AC circuit change rapidly, some
  energy is lost in the form of EM waves
• EM waves are radiated by any circuit carrying
  alternating current
• An alternating voltage applied to the wires of an
  antenna forces the electric charge in the antenna
  to oscillate

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EM Waves by an Antenna

• Two rods are connected to an ac source, charges
  oscillate between the rods (a)
• As oscillations continue, the rods become less
  charged, the field near the charges decreases and
  the field produced
• at t 0 moves away from the rod (b)
• The charges and field reverse (c)
• The oscillations continue (d)

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EM Waves by an Antenna

• Because the oscillating charges in the rod
  produce a current,
• there is also a magnetic field generated
• As the current changes, the magnetic field
  spreads out from the antenna

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EM Waves by an Antenna
EM waves emitted by a simple vertical antenna are
polarized.The electric field is directed
vertically.The magnetic field is directed
horizontally in circles around the antenna.The
waves propagate horizontally, radially from the
antenna.
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Geometrical optics. Ray approximation.
Light is a kind of electromagnetic wavesAnd
waves are difficult!In many cases, though,
difficulties can be avoided and geometrical
optics can be applied.It is based on the
suggestion that Light travels in straight lines
called rays.(Why do we suggest that btw?) It
is called ray approximation and it reduces optics
to ray tracing and geometry. We do geometrical
optics.
A ray a is a line in the direction along which
light energy is flowing.A laser beam (or a beam
from your cars headlight) is really a bundle of
many parallel rays.
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Question How come light waves can be reduced to
rays? Is it always valid?
Consider an unbounded plain wave of light. All
it takes to characterize it is its direction and
intensity (which can be thought of as density of
rays). So, the ray approximation is OK.
After passing through an aperture the plain wave
becomes a beam and gets bounded. Does it keep
going along a straight line? Depends on the
relation between the size of the aperture and
wave length.
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Ray approximation waves on water surface
Waves propagate in straight lines unless they hit
something (a barrier or an aperture) having a
size comparable with the wave length
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In general all bounded light beams in free space,
including laser beams, are somewhat expanding and
loosing their intensity (density of rays).They
are expanding no matter how hard you try to keep
them narrow, just because of the fact that they
are bounded!
What about spherical waves? Can we apply ray
approximation to them too?Sure thing!
http//www.people.vcu.edu/rgowdy/mod/104/sphraymv
.htm1
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The ray model (continued) Light travels through
a transparent medium in straight lines called
rays, at speeds v c/n, where n is the index of
refraction of the medium. Light rays do not
interact with each other. A light ray
continues forever unless it has an interaction
with matter that causes it to change directions
or be absorbed.
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• Light has four different ways in which it can
  interact with matter. At an interface between
  two media, light can be reflected or
  refracted. Within a medium light can be
  scattered or absorbed.

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Examples of using of the ray approximation1)
A point source, a screen, and a 1D and 2D
apertures between them. What is the shape of the
image on the screen? Image of what is it? What
does it size depend on?
The shape of the image corresponds to the shape
of the aperture
The image is an image of the aperture
Its size is proportional to the size of the
aperture and to the ratio of distances
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Examples of using of the ray approximation1)
How do you get a point source? What does it mean,
rays filling the aperture? What happens if we
close a half of the aperture?2) Two point
sources, an aperture and a screen. Want kind of
pattern on the screen do they produce? What about
a line of point sources?3) A line of point
sources and a very small (pinhole) aperture. What
kind of image will be on the screen? What happens
if we cover a half of the aperture?
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Image of the aperture.. mostly
Image of the aperture
Image of both the aperture and the source.
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Congratulations!We have just discovered the
earliest model of photo camera, Camera obscura.
What are the drawbacks of the camera obscura?