Major Concepts of Physics PHY102 Spring 2005

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Major Concepts of Physics PHY102 - 2006Lecture
6 February 2nd
Electromagnetic waves
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Update
Tues., January 17th Lecture 1 Introduction to
Course. Thurs., January 19th Lecture 2 Waves
and the Concepts Describing Them Tues., January
24th Lecture 3 Interferences of Waves Youngs
Experiment Thurs., January 26th Lecture 4
Traveling Waves Longitudinal vs. Transverse
Waves Tues., January 31st Lecture 5
Diffraction a close look Quiz 1 Thurs.,
February 2nd, Lecture 6 Electromagnetic
Waves Tues., February 7th Lecture 7 Review
Meeting, Part I Thurs., February 9th Exam 1

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Announcement
Next week Review Meeting 1 Exam 1 is scheduled
on February 9th Materials from Lectures 1-7
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Outline Lecture Objectives 5
  1)         Review - Diffraction through a
single slit.   2)       Review Huygens
principle 3) Electromagnetic waves and
Maxwells work   4) Electromagnetic
spectrum 5) Demonstrations with various EM
waves
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 1) RecallAn important result of our study of
diffraction Let any wave go through a narrow
slit. Then, the light spreads out after leaving
the slit. Its spread can be characterized by the
result  Sin ? ?/a.Here,
a is the width of the slit-opening, and ? is the
wavelength of the wave. Also, ? is the angle
for the first minimum for wave-intensity. Hence,
objects of angular size smaller than ? cannot be
resolved.That is, ? gives the diffraction
limit.  .
Review Diffraction

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Review - Diffraction
                       
 2) Questions to test your understanding   a) A
biologist wishes to see in detail a small object
in a cell whose size is close to the diffraction
limit for light waves.   Should red light or blue
light be used?   Hint ?(blue) 400 nm
(roughly). ?(red) 700 nm
(roughly).   b)         A biologist wishes
to see a structure in a cell whose size is
smaller than the diffraction limit for light
waves. She uses an electron microscope
(Electrons are waves, as well as particles!) Is
the wavelength of the electrons smaller or larger
than that of light?   Remark We will understand
later, how electrons can behave as waves. This
is a subject known as quantum mechanics.
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Diffraction is based on Huygenss principle
                           
 
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 We know that light is a wave.  
But What is it that is
waving?Mystery until 1861.Then A
breakthrough.James Clerk Maxwell found the
answer.He put together all the basic laws known
about a different subject  Electricity
and Magnetism
Electromagnetic Waves

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The results of Maxwell

•  
• Whenever a charge accelerates, it sends waves
• outward in all directions.
•  
• The waving quantity
• An electric field and a magnetic field.
•  
• He found
• The wave-speed v in empty space has one value
  only.
• Its value is 300,000 km/sec.
•  
• We call this value c.

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The Electromagnetic Spectrum
The entire set of frequencies for EM waves, is
called the electromagnetic spectrum. For
example, EM waves with frequency f just below red
are called infrared waves. EM waves with f just
above violet are called ultraviolet.
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Surprise of Maxwell
The surprise of Maxwell   The value of c is the
same as the known speed of light!   The only
reasonable conclusion Light must be an EM
wave.   He answered the question What is waving
in a light wave?   Before Maxwell Light was one
subject. Electricity and magnetism was another
subject.   After Maxwell Both subjects are the
same phenomena.
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Recall
Three laws you studied last semester 1)
Coulombs Law It gives the electric field
produced by a static charge. 2) Oersted and
Amperes experiments Gives the magnetic field
produced by a moving charge. 3) Faradays Law A
changing magnetic field produces an electric
field. Maxwell combined these three results, and
wrote a consistent formulation for electricity
and magnetism.  
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Maxwell work was theoretical
But His ideas were later confirmed by the
experiments of Hertz (1880)   Hertz showed
Accelerating charges produce EM waves.   The
Frequencies of Light Waves   Suppose a charge
performs simple harmonic motion.   Let f be its
frequency.   Then The EM wave it produces must
have the same frequency f.   Find its value for
light.
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Recall
     
  v f? This is valid for any wave.   But
Maxwell showed v c for all EM waves in empty
space.   Conclude c f? for any EM wave in
empty space.   Now We already know the values
for ? for light waves.   From the double-slit
Young experiment or from the diffraction
experiments   ? for light is between 400 nm
(violet or blue) and 700 nm (red).   Use this to
solve for the frequency f, using v f ?.
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Results
f 400 x 1012 Hertz for red light. f 750
x 1012 Hertz for violet light. 1 Hertz 1
Hz 1 full cycle per second. Important note
This is a very narrow range. It is less than a
factor of two. The human eye is color-blind to
EM waves not in this range. But Hertz and
others showed Other f than this small range
also produce EM waves. The eye, happens to not
have sensitivity to anything other than the
range from 400 to 750 trillion Hertz.
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Homework/Textbook
Homework (next week) http//wps.prenhall.com/esm_g
iancoli_physicsppa_6 Go to Chapter 22 Go to
Practice Questions 1, 2, 3, 4, 5, 6, 7, 8, 9,
10 Click submit answers for grading Send the
results to your TA (by E-mail).
Textbook, Chapter 22nd, pp. 616-620, 626-628

• Supplementary materials (Destinations)
• Electromagnetic spectrum
• Electromagnetic radiation

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