Reflection - PowerPoint PPT Presentation

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Reflection

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Reflection When light is incident on a surface, it can be reflected An interesting result is that the angle of incidence (incoming angle) equals the angle of ... – PowerPoint PPT presentation

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Title: Reflection


1
Reflection
  • When light is incident on a surface, it can be
    reflected
  • An interesting result is that the angle of
    incidence (incoming angle) equals the angle of
    reflection (outgoing angle.

2
Reflection from a curved surface
  • When the surface doing the reflecting is curved,
    the light can be brought to a focus.
  • The curved surface can be parabolic or spherical.
  • Spherical surfaces are cheaper and easier to
    construct.

3
Power towers
  • Use many collectors and focus the light to a
    central point.
  • Achieves high temperatures and high power
    density.
  • Each individual collector is called a heliostat
  • Must be able to track the sun and focus light on
    the main tower

4
How they work
  • Light is collected at the central tower, which is
    about 300 feet tall. There are on the order of
    2000 heliostats.
  • Used to heat water and generate steam
  • Steam drives a turbine which generates
    electricity
  • Often include auxiliary energy storage to
    continue to produce electricity in the absence of
    sunlight
  • More costly to construct and operate than coal
    fired plants.
  • Good candidates for cogeneration-waste steam
    could be used for space heating

5
Solar troughs
  • A parabolic shaped trough collects the light and
    focuses it onto a receiver.
  • The receiver has a fluid running through it which
    carries the heat to a central location where it
    drives a steam turbine
  • May have more than a hundred separate troughs at
    such a facility

6
Trough Pictures
7
Direct Conversion of sunlight to
energyPhoto-voltaics
  • Photoelectric effect
  • When electromagnetic energy impinges upon a
    metal surface, electrons are emitted from the
    surface.
  • Hertz is often credited with
  • first noticing it (because he
  • published his findings) in 1887
  • but it was seen by Becquerel
  • In 1839 and Smith in 1873.

8
Photoelectric effect
  • The effect was a puzzle
  • The theory of light as a wave did not explain the
    photoelectric effect
  • Great example of the scientific method in action.
  • Up until this point, all the observations of
    light were consistent with the hypothesis that
    light was a wave.
  • Now there were new observations could not be
    explained by this hypothesis
  • The challenge became how to refine the existing
    theory of light as a wave to account for the
    photoelectric effect

9
Photoelectric effect explained
  • Einstein in 1905 explained the photoelectric
    effect by assuming light was made of discrete
    packets of energy, called photons.
  • Not a new idea, he was building upon an idea
    proposed by Planck, that light came in discrete
    packets. (in fact, Newton proposed a particle
    like explanation of light centuries earlier).
    The problem for Planck was his discrete packets
    were in conflict with the wave like behavior of
    light.

10
Photoelectric effect explained
  • But now, a behavior of light was observed that
    fit Plancks energy packet idea.
  • So electromagnetic radiation appears to behave as
    if it is both a wave and a particle.
  • In fact, you can think of light as discrete wave
    packets-packets of waves which, depending upon
    the measurement you make, sometimes exhibit
    particle behavior and sometimes exhibit wave
    behavior.
  • Einstein won the Nobel prize for his explanation
    of the photoelectric effect.

11
Semi conductors
  • Devices which have conductive properties in
    between a conductor and an insulator.
  • Normally, the outer (valence) electrons are
    tightly bound to the nucleus and cannot move.
  • If one or all of them could be freed up, then the
    material can conduct electricity
  • Silicon is an example of a semi-conductor.

12
Silicon
  • Element 14 in the periodic table
  • Very common element (sand, glass composed of it)
  • 8th most common element in the universe
  • Its 4 outer valence electrons are normal tightly
    bound in the crystal structure.
  • However, when exposed to light, the outer
    electrons can break free via the photoelectric
    effect and conduct electricity.
  • For silicon, the maximum wavelength to produce
    the photoelectric effect is 1.12 microns. 77 of
    sunlight is at wavelengths lower than this.
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