Electromagnetic RadiationElectromagnetic Radiation
LightLight
1600’s: 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
note: red light bends the least, violet the most
Newton
Thought light made up of tiny particles with no mass
– explains why shadows have sharp edges
• couldn’t explain how different colored particles were different or why were refracted differently by prism
• couldn’t explain why 2 beams of light didn’t affect each other when crossed– particles of light should collide off each other
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 couldn’t explain shadows with sharp edges
Wave Vocabulary
• Transverse and Longitudinal waves
• Wavelength
• Frequency
• Amplitude
• Velocity
Direction of displacement is at right angle to direction of travel
Transverse Wave
Direction of displacement = direction of travel
Transverse vs Longitudinal WavesTransverse (ocean)
Longitudinal (compression/sound)
Wavelength vs. Frequency
• WAVELENGTH: distance light travels to complete 1 cycle
• FREQUENCY:
number of cycles completed in 1 second
Wave Velocity
• speed of wave in direction is traveling
• velocity = wavelength x frequency
velocity =
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
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
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
Maxwell• 1864-1873 worked out mathematical
equations describing electricity & magnetism
• electric & magnetic fields cannot exist independently
• electromagnetic radiation in Maxwell’s equation moved at same speed as light!– could notnot be coincidence!
Light that can’t be seen?
• light was electromagnetic radiation!
• Maxwell’s equations unified electricity, magnetism, & light
• but visible light only accounts for a fraction of Maxwell’s equations– ? other frequencies and wavelengths
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
Infrared Radiation
• By 1850: – infrared rays were demonstrated to have all
the properties of light– except could not be seen by human eye
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
Discovery of ultraviolet!
After Herschel discovered infrared using thermometers:
– 1801: Ritter repeated Scheele’s experiment with paper soaked in AgNO3 & put strips beyond violet
– strips darkened even quicker than in violet light– discovered ultraviolet light!
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
X-Rays
• 1895: Roentgen discovered that his cathode ray tube
was emitting radiation = X-rays
http://www.chem.uiuc.edu/clcwebsite/video/Cath.wmv
Electromagnetic Spectrum
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 =
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!
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
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
Explanation
• Pre-Planck: – energy considered to be continuous– could be broken into smaller & smaller bits
indefinitely
• Planck: Planck: –energy consists of tiny particles that caenergy consists of tiny particles that ca
nn’’t be divided into anything smallert be divided into anything smaller
