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Electromagnetic Radiation

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Electromagnetic Radiation Electromagnetic Spectrum Light 1600 s: sunlight considered purest form of light 1665 Isaac Newton passed beam of sunlight through a ... – PowerPoint PPT presentation

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Title: Electromagnetic Radiation


1
Electromagnetic Radiation
2
Electromagnetic Spectrum
3
Light
  • 1600s sunlight considered purest form of
    light
  • 1665 Isaac Newton
  • passed beam of sunlight through a prism
  • beam spread out
  • band of colors (Roy G. Biv)
  • rejoined the colors with 2nd prism got white
    light again

4
Note red light bent the least, violet the most
5
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6
Newton
  • Thought light made up of tiny particles with no
    mass
  • explains why shadows have sharp edges
  • couldnt explain how different colored particles
    were different or why were refracted differently
    by prism
  • couldnt explain why 2 beams of light didnt
    affect each other when crossed
  • particles of light should collide off each other

7
Christian Huygens
  • 1678 suggested light composed of waves
  • explained why 2 beams of light could cross each
    other without being disturbed
  • explained refraction in prism
  • - different colors have different
    wavelengths
  • people were used to waves in water
  • water waves move around an obstruction
  • waves couldnt explain shadows with sharp edges

8
Wave Vocabulary
  • Transverse and Longitudinal waves
  • Wavelength
  • Frequency
  • Amplitude
  • Velocity

9
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10
Transverse Wave
Direction of displacement is at right angle to
direction of travel
11
Wavelength vs. Frequency
  • WAVELENGTH
  • distance light travels to complete 1 cycle
  • FREQUENCY
  • number of cycles completed in 1 second

12
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13
Wave Velocity
  • speed of wave in direction is traveling
  • Velocity wavelength x frequency
  • Velocity ??

14
Light Particles vs. Waves
  • Huygens had better argument, but Newton was more
    famous
  • people went with Newton's theory Light was
    particles!
  • Speed of light
  • 1st determined about 1676 by Danish astronomer
  • speed of light 3.0 X 108 m/sec

15
Calculating Wavelengths of Light
  • visible light waves have lengths 1/20,000 cm
  • Red a little longer, violet a little shorter
  • short wavelength explains why light cast sharp
    shadows despite being waves
  • waves can only bend around obstacles that are
    about same length
  • cannot bend around anything substantially longer
    than itself

16
The Ether
  • Water waves move in water
  • Sound waves move in air
  • Light waves move through vacuum
  • gravity, electricity, magnetism also felt
    across vacuum
  • people couldn't accept this
  • postulated subtle form of matter called ether
  • not easily detectable
  • Ether idea held until 1900

17
Maxwell
  • 1864-1873 worked out mathematical equations
    describing electricity magnetism
  • electric magnetic fields cannot exist
    independently
  • electromagnetic radiation in Maxwells equation
    moved at same speed as light!
  • could not be coincidence!

18
Light that cant be seen?
  • Light was electromagnetic radiation!
  • Maxwells equations unified electricity,
    magnetism, light
  • But visible light only accounts for a fraction of
    Maxwells equations
  • ? other frequencies and wavelengths

19
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20
Heat Light Separate or same?
  • 1800 Herschel studied spectrum with thermometer
  • - found highest temperature at red end
    coolest at
  • violet end
  • placed thermometer beyond red temp was higher
    there than anywhere in visible spectrum
  • discovered Infared rays, which we cannot see

21
Infrared Radiation
  • By 1850
  • infrared rays were demonstrated to have all the
    properties of light
  • except could not be seen by human eye

22
Silver Nitrate as detector
  • 1614
  • known that silver nitrate (AgNO3 - white cmpd)
    darkens on exposure to sunlight
  • 1770
  • Scheele soaked strips white paper in AgNO3
    solutions and placed them in different parts of
    spectrum
  • darkened least quickly in red and fastest in
    violet
  • ? the first photographs

23
Discovery of ultraviolet!
  • After Herschel discovered infrared using
    thermometers
  • 1801 Ritter repeated Scheeles experiment with
    paper soaked in AgNO3 put strips beyond violet
  • strips darkened even quicker than in violet light
  • discovered ultraviolet light!

24
Radio Waves
  • 1888
  • Hertz used oscillating electric current to emit
    electromagnetic radiation
  • had detector that could move around to map the
    electromagnetic wave determine its length
  • found radio waves far beyond infrared radiation
  • have wavelengths from cm to km

25
X-Rays
  • 1895
  • Roentgen discovered that his cathode ray tube was
    emitting radiation X-rays

http//www.chem.uiuc.edu/clcwebsite/video/Cath.wmv

26
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27
Electromagnetic Spectrum
28
Comparing
  • Long wavelength
  • red end
  • slow frequency
  • low energy
  • Short wavelength
  • violet end
  • rapid frequency
  • high energy

All electromagnetic radiation has same velocity
3.00 x 108 m/sec
3.00 x 108 m/sec ??
29
Heat Flow
  • Objects hotter than surroundings lose heat as
    electromagnetic radiation
  • higher the temp, the more intense the radiation
  • Hot objects glow!
  • glow different colors at different temperatures!

30
Ultraviolet Catastrophe
  • classical physics
  • assume every wavelength has equal chance of being
    radiated
  • classical wave model
  • could not explain why different colors were
    emitted at different temperatures

31
Planck
  • shorter the wavelength, the less chance it has to
    be emitted!
  • matter can gain or release energy only in very
    small increments called quanta

32
What physical explanation goes with Plancks
assumption?
  • Pre-Planck
  • energy considered to be continuous
  • could be broken into smaller smaller bits
    indefinitely
  • Planck
  • energy consists of tiny particles that cant be
    divided into anything smaller
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