Electron As a Particle and Electron As a Particle and WaveWave

Electrons get excited when energy is Electrons get excited when energy is absorbed by using heat or electrical absorbed by using heat or electrical energyenergy

Excited electrons move back to their Excited electrons move back to their ground state when they release the ground state when they release the same amount of energy that was same amount of energy that was absorbed (conservation).absorbed (conservation).

Light energy (electromagnetic Light energy (electromagnetic radiation) is released when an radiation) is released when an electron returns to a lower energy electron returns to a lower energy levellevel

Light EnergyLight Energy Light travels as a wave and is made of Light travels as a wave and is made of

photons.photons. Light contains quanta energy called Light contains quanta energy called

photons (Einstein)photons (Einstein) Speed of light in a vacuum 3.0 x 10Speed of light in a vacuum 3.0 x 1088 m/s m/s

How fast is that??How fast is that?? It would take 2.5 seconds for radar It would take 2.5 seconds for radar

waves traveling at the speed of light to waves traveling at the speed of light to travel 478,000 miles -- leave the earth, travel 478,000 miles -- leave the earth, bounce off the moon and return!!bounce off the moon and return!!

Speed of LightSpeed of Light

c = the speed c = the speed of light of light

c = c = λλvv λλ = =

wavelengthwavelength v = frequencyv = frequency

Wavelength: one Wavelength: one wave, the distance wave, the distance of one crest and of one crest and one trough (Unit: one trough (Unit: meters)meters)

Frequency: # of Frequency: # of waves per second waves per second (Unit: s(Unit: s-1-1 or Hertz, or Hertz, Hz)Hz)

The Electromagnetic The Electromagnetic SpectrumSpectrum

Radio Waves Radio Waves MicrowavesMicrowaves Infrared RadiationInfrared Radiation VISIBLE LIGHTVISIBLE LIGHT

ROY G BVROY G BV UltravioletUltraviolet X-RaysX-Rays Gamma RaysGamma Rays

Low Frequency, Long Low Frequency, Long WavelengthsWavelengths

v = 10v = 101010 Hz Hz λλ = 10 = 10-1-1 m m

VISIBLE LIGHTVISIBLE LIGHT 400 – 800 nanometers400 – 800 nanometers Nanometer = 10Nanometer = 10-9-9 m m

High Frequency, High Frequency, Short WavelengthsShort Wavelengths

v = 10v = 102020 Hz Hz λλ = 10 = 10--

1212mm

http://en.wikipedia.org/wiki/Electromagnetic_spectrum

http://en.wikipedia.org/wiki/Visible_light

Max PlanckMax Planck Energy is absorbed and emitted in Energy is absorbed and emitted in

discrete units of light energy called discrete units of light energy called quanta (also known as photons)quanta (also known as photons)

The energy of a light quantum The energy of a light quantum corresponds to the energy difference corresponds to the energy difference between two energy levels.between two energy levels.

E = hE = hυυ Energy is equal to Planck’s constant Energy is equal to Planck’s constant

multiplied by frequencymultiplied by frequency h = 6.63 x 10h = 6.63 x 10-34 -34 JJ••ss

Photoelectric EffectPhotoelectric Effect

Absorption of light striking a metal Absorption of light striking a metal with a certain frequency, has enough with a certain frequency, has enough energy to release an electron energy to release an electron (photon) of light(photon) of light

Sample ProblemsSample Problems Yellow light with a frequency of Yellow light with a frequency of

approximately 6.0 x 10approximately 6.0 x 101414 Hz is the Hz is the predominant frequency in sunlight. predominant frequency in sunlight. What is the energy of this light?What is the energy of this light?

4.0 x 104.0 x 10-19-19 J J A quantum of a certain color of A quantum of a certain color of

visible light is found to have an visible light is found to have an energy of 5.0 x 10energy of 5.0 x 10-19-19 J. What is the J. What is the color of this light?color of this light?

Violet because the frequency is 7.5 x Violet because the frequency is 7.5 x 10101414 Hz which is equivalent to 400 nm Hz which is equivalent to 400 nm (the shortest wavelength for light)(the shortest wavelength for light)

Changing Energy LevelsChanging Energy Levels Excited State:Excited State:

When an eWhen an e- - has gained energy to jump to a has gained energy to jump to a higher energy levelhigher energy level

Ground State:Ground State: Most stable (original location of the electron)Most stable (original location of the electron) Excited electrons lose energy to return to Excited electrons lose energy to return to

stabilitystability This releases energy as This releases energy as

electromagnetic radiation (a photon of electromagnetic radiation (a photon of light)light)

Conservation of EnergyConservation of Energy The energy absorbed always The energy absorbed always

equals energy emitted.equals energy emitted. Recall that electrons can not be in Recall that electrons can not be in

between energy levels; however between energy levels; however electrons can skip and move up electrons can skip and move up 2,3,4… levels at a time.2,3,4… levels at a time.

The energy is released similarly. The energy is released similarly. Either in large amounts (drop back Either in large amounts (drop back several energy levels) or the electrons several energy levels) or the electrons can jump back one energy level at a can jump back one energy level at a time.time.

Photoelectric EffectPhotoelectric Effect

Electron as a particle is ejected from Electron as a particle is ejected from the surface of a metal exposed to a the surface of a metal exposed to a specific frequency of lightspecific frequency of light

Photon “a packet of light” Photon “a packet of light” E = hE = h

http://www.bing.com/videos/search?q=photoelectric+effect+video&FORM=VIRE1#view=detail&mid=63FE23E8E374EEB1237C63FE23E8E374EEB1237C