ChemistryChemistry

Chapter 5Chapter 5

Wave Nature of LightWave Nature of Light

�� Visible light is a type of electromagnetic Visible light is a type of electromagnetic radiationradiation

�� It is a form of energy which exhibits It is a form of energy which exhibits wavelike behavior as it travels through wavelike behavior as it travels through spacespace

�� Other examples of electromagnetic Other examples of electromagnetic radiation include microwaves, xradiation include microwaves, x--rays and rays and radio wavesradio waves

Electromagnetic RadiationElectromagnetic Radiation

�� Made of photonsMade of photons

�� A photon is a discrete packet of A photon is a discrete packet of

electromagnetic energyelectromagnetic energy

�� The Energy can be calculated as eitherThe Energy can be calculated as either

�� E=hv orE=hv or

�� E=hc/E=hc/λλ

Characteristics of WavesCharacteristics of Waves

�� Wavelength (Wavelength (λλ): Lambda is the shortest distance ): Lambda is the shortest distance

between equivalent points on a continuous between equivalent points on a continuous

wave, it is measured from crest to crest and is wave, it is measured from crest to crest and is

usually expressed in metersusually expressed in meters

�� Frequency (v): Nu is the number of waves that Frequency (v): Nu is the number of waves that

pass a given point per second. One Hertz (Hz: pass a given point per second. One Hertz (Hz:

The SI unit of frequency) equals one wave per The SI unit of frequency) equals one wave per

second.second.

Wave CharacteristicsWave Characteristics

�� Amplitude is the waves height from origin Amplitude is the waves height from origin

to crestto crest

�� Wavelength and frequency do not affect Wavelength and frequency do not affect

the amplitudethe amplitude

�� Wavelength and frequency are inversely Wavelength and frequency are inversely

proportional (as one increases the other proportional (as one increases the other

decreases)decreases)

Electromagnetic Wave RelationshipElectromagnetic Wave Relationship

�� c = c = λλvv

�� c = the speed of light in a vacuumc = the speed of light in a vacuum

�� λλ = the wavelength= the wavelength

�� v = the frequencyv = the frequency

�� All electromagnetic waves, including All electromagnetic waves, including

visible light, travel at a speed of visible light, travel at a speed of

3.00 x 103.00 x 1088 m/s in a vacuumm/s in a vacuum

RelationshipsRelationships

�� c = c = λνλν

�� v = c/v = c/λλ

�� λλ = c/v= c/v

�� MHz = 10MHz = 1066HzHz

�� Hz = sHz = s--11

�� nm = 10nm = 10--99mm

�� c = 3.00 x 10c = 3.00 x 1088m/sm/s

WavelengthWavelength

Electromagnetic WavesElectromagnetic Waves

�� The visible light The visible light

spectrum is from spectrum is from

about 350nm about 350nm

(violet) to 800nm (violet) to 800nm

(red)(red)

�� Blue is around Blue is around

450nm450nm

�� Below 350nm is Below 350nm is

ultraviolet above ultraviolet above

800nm is infra800nm is infra--redred

CalculationsCalculations

�� A radio station broadcasts at 122.0 MHz. A radio station broadcasts at 122.0 MHz.

Calculate the wavelength of this Calculate the wavelength of this

frequency.frequency.

�� Calculate the frequency of a radiation Calculate the frequency of a radiation

which is 614 nm in length.which is 614 nm in length.

�� What color is this radiation?What color is this radiation?

Particle Nature of LightParticle Nature of Light

�� In addition to exhibiting wavelike behaviors, In addition to exhibiting wavelike behaviors,

light also behaves as a particlelight also behaves as a particle

�� As a result, when objects are heated they will As a result, when objects are heated they will

only emit electrons when light of a specific only emit electrons when light of a specific

frequency shines on themfrequency shines on them

�� Example: Iron is gray at room temperature, Example: Iron is gray at room temperature,

glow red when heated, then orange and finally glow red when heated, then orange and finally

blue at excessively high temperatureblue at excessively high temperature

Particle NatureParticle Nature

�� The different frequencies of the colors cannot be The different frequencies of the colors cannot be

explained by the wave nature of light, they are explained by the wave nature of light, they are

explained by the gain or loss of energyexplained by the gain or loss of energy

�� This energy is in the form of a QuantumThis energy is in the form of a Quantum

�� A Photon is a particle which carries a quantum A Photon is a particle which carries a quantum

of energyof energy

�� The energy of the photon depends on the The energy of the photon depends on the

frequencyfrequency

Atomic Emission SpectraAtomic Emission Spectra

�� Neon lights are explained by the wave model of Neon lights are explained by the wave model of lightlight

�� The light is produced by passing electricity The light is produced by passing electricity through the gas through the gas

�� If the light neon emits is passed through a prism If the light neon emits is passed through a prism we don’t get the full range of colors like we do we don’t get the full range of colors like we do with visible lightwith visible light

�� Instead we observe discrete lines which Instead we observe discrete lines which correspond to the radiation emitted by neoncorrespond to the radiation emitted by neon

Separation of light by a prism Separation of light by a prism

according to wavelengthaccording to wavelength

Continuous, emission, and Continuous, emission, and

absorption spectraabsorption spectra