Electromagnetic Radiation

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Electromagnetic Radiation
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Electromagnetic Spectrum
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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

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note red light bends the least, violet the most
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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

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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

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Wave Vocabulary

• Transverse and Longitudinal waves
• Wavelength
• Frequency
• Amplitude
• Velocity

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Transverse Wave
Direction of displacement is at right angle to
direction of travel
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Transverse vs Longitudinal Waves
Transverse (ocean)
Longitudinal (compression/sound)
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Wavelength vs. Frequency

• WAVELENGTH
• distance light travels to complete 1 cycle
• FREQUENCY
• number of cycles completed in 1 second

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Wave Velocity

• speed of wave in direction is traveling
• velocity wavelength x frequency
• velocity ??

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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

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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

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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

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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!

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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

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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

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Infrared Radiation

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

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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

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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!

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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

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X-Rays

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

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

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Electromagnetic Spectrum
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Comparing Light

• 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 ??
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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!

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Pre-Planck

• 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

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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

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Explanation

• 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