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• Electromagnetic Spectrum

• Light 1600s: sunlight considered purest form of light

1665 Isaac Newtonpassed beam of sunlight through a prismbeam spread outband of colors (Roy G. Biv)rejoined the colors with 2nd prism & got white light again

• note: red light bends the least, violet the most

• NewtonThought 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 crossedparticles of light should collide off each other

• Christian Huygens1678: 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

• Wave VocabularyTransverse and Longitudinal wavesWavelengthFrequencyAmplitudeVelocity

• Direction of displacement is at right angle to direction of travelTransverse Wave

• Transverse vs Longitudinal WavesTransverse (ocean)Longitudinal (compression/sound)

• Wavelength vs. FrequencyWAVELENGTH: distance light travels to complete 1 cycle

FREQUENCY: number of cycles completed in 1 second

• Wave Velocityspeed of wave in direction is traveling

velocity = wavelength x frequency velocity =

• Light: Particles vs. WavesHuygens 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 astronomerspeed of light = 3.0 X 108 m/sec

• Calculating Wavelengths of Light

visible light waves have lengths ~ 1/20,000 cmred a little longer, violet a little shorter

short wavelength explains why light cast sharp shadows despite being waveswaves can only bend around obstacles that are about same lengthcannot bend around anything substantially longer than itself

• The Etherwater waves move in watersound waves move in airlight waves move through vacuumgravity, electricity, & magnetism also felt across vacuum

people couldn't accept this: postulated subtle form of matter called ethernot easily detectable

Ether idea held until 1900

• Maxwell1864-1873 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!

• 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

• 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 RadiationBy 1850: infrared rays were demonstrated to have all the properties of lightexcept could not be seen by human eye

• Silver Nitrate as detector1614: 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 spectrumdarkened least quickly in red and fastest in violet? the first photographs

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

• Radio Waves1888: Hertz used oscillating electric current to emit electromagnetic radiationhad detector that could move around to map the electromagnetic wave & determine its length

found radio waves far beyond infrared radiationhave wavelengths from cm to km

• X-Rays1895: Roentgen discovered that his cathode ray tube was emitting radiation = X-rays

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

• Electromagnetic Spectrum

• Comparing LightLONG wavelengthred endslow frequencylow energySHORT wavelengthviolet endrapid frequencyhigh energyAll electromagnetic radiation has same velocity: 3.00 x 108 m/sec

3.00 x 108 m/sec =

• Heat Flowobjects 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-Planckclassical 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

• ExplanationPre-Planck: energy considered to be continuouscould be broken into smaller & smaller bits indefinitely

Planck: energy consists of tiny particles that cant be divided into anything smaller