planck and photoelectric effect

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Light, Energy, and More October 23, 2007 Chemistry

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Page 1: Planck And Photoelectric Effect

Light, Energy, and MoreOctober 23, 2007

Chemistry

Page 2: Planck And Photoelectric Effect

Recap…

• Electromagnetic Spectrum

• High Energy• Low Energy• Wave Nature of Light

Page 3: Planck And Photoelectric Effect
Page 4: Planck And Photoelectric Effect

What’s Going On Here?

Page 5: Planck And Photoelectric Effect

When we heat metal what happens?

Page 6: Planck And Photoelectric Effect

• Does the wave model of light explain these changes?– Does not explain different wavelengths and

frequencies at different temperatures

• What is light?– Radiation….what is radiation?

• Particles or rays of energy

• What is temperature anyways?– The measure of the average kinetic energy of the

particles in an object– Kinetic Energy vs. Potential Energy

• Too many questions….

Page 7: Planck And Photoelectric Effect

Max Planck (1900)

• German Physicist• Began to look for

answers• Matter can only gain

or lose energy in small quantized amounts

• What’s quantized?

Page 8: Planck And Photoelectric Effect

Vocab Word!!!

• QUANTUM– Minimum amount of energy that can be

gained or lost by an atom– The emitted light from a glowing metal is a

ENERGY…this energy is quantized

Page 9: Planck And Photoelectric Effect

If energy is now quantized…how can we determine the amount of

energy of a quantum?

• What is energy measured in?• What are we observing?• What happens to the color when we

increase the temperature (energy)?– Proportional or inversely proportional?

• Now we need a constant…– Planck’s constant, h=6.626 x 10^-34 J*s

Page 10: Planck And Photoelectric Effect
Page 11: Planck And Photoelectric Effect

Time to put these words into action!

• What is the frequency and wavelength electromagnetic radiation that emits 1.68 x 10^-17 J of energy? What type of electromagnetic radiation is this?

• Wavelength= 1.18 x 10^-8 m

• Ultraviolet radiation

Page 12: Planck And Photoelectric Effect

Some questions to answer…

• What is the color we see?

• What happens to the energy of the radiation when we increase the frequency, v, of the radiation emitted?

• Iron at room temp…color and E?

• Iron with a little heat…color and E?

• Iron with lots of heat…color and E?

Page 13: Planck And Photoelectric Effect

According to Planck’s Theory…

• If we have a given v, matter can emit or absorb E only in whole number multiples of hv (1hv, 2hv, 3 hv…)

• Matter can ONLY have specific amounts of energy

• Wall of kids building blocks– We can only add or take away

in increments of whole blocks…we cannot remove half a block

Page 14: Planck And Photoelectric Effect

The Big Mystery of the 1900’s…

• The Photoelectric Effect…– What caused these

color changes in metals???

Page 15: Planck And Photoelectric Effect

Photoelectric Effect

• Electrons (photoelectrons) are emitted from a metal’s surface when a light of a certain frequency shines on the surface

• Certain specific amounts of energy (what’s this called???) needed to knock out electrons from metal atoms.

Page 16: Planck And Photoelectric Effect
Page 17: Planck And Photoelectric Effect

Albert Einstein (1905)

• Added onto Planck’s Theory…

• Called the electron’s emitted, PHOTONS (the little energy packets Planck called quantums)

• Now… Ephoton = hv

Page 18: Planck And Photoelectric Effect

• Planck paved the way for the explanation behind the mystery

• But some one else came into the picture…

Page 19: Planck And Photoelectric Effect

Now light is not just a wave…

• Einstein’s Dual Nature of Light– Particle and wave characteristics– Light is a beam of tiny particles, called

photons, acting like a wave

Page 20: Planck And Photoelectric Effect

NEW WORD!!!

• Photon

• A particle of electromagnetic radiation with no mass that carries a quantum of energy

Page 21: Planck And Photoelectric Effect
Page 22: Planck And Photoelectric Effect

What Einstein added…

• Energy of a photon has a minimum or threshold value to eject photoelectrons

• What must happen for the photoelectric effect to occur?– Energy of a photon (particle of EM radiation)

must have the minimum energy requirement to free the electron from the atom of metal

Page 23: Planck And Photoelectric Effect

Mystery Solved!• No matter how long a

light of a certain frequency is shone on metal (intensity), electrons will not be ejected unless the minimum amount of energy is shone.

• Silver metal– Photoelectrons ejected

when a light with a frequency of at least 1.14 x 10^15 Hz or greater is used

• Sodium metal– Red light– Violet light

Page 24: Planck And Photoelectric Effect

Revised Planck’s Work…

• Einstein piggy-backed off of Planck’s Theory and we now have…..

Photon

Page 25: Planck And Photoelectric Effect

Time to do a little work….

• Tiny water drops in the air disperse the white light of the sun into a rainbow. What is the Energy oa a photon from the violet portion of the rainbow if it has a frequency of 7.23x10^14 Hz?

• E=4.79 x 10^-19 J– Energy in a photon of violet light

Page 26: Planck And Photoelectric Effect

A couple more…

• A photon has an energy of 2.93 x 10^-25 J. What is its frequency? What type of electromagnetic radiation is the photon?

• V=4.42 x 10^8 Hz

• TV or FM waves

Page 27: Planck And Photoelectric Effect

Practice makes perfect… • What is the energy of each photon in the

following types of radiation?– 6.32 x 10^20 Hz– 9.50 x 10^13 Hz– 1.05 x 10^16 Hz

• What types of radiation are each?

• 4.19 x 10^-13 J gamma or x-ray

• 6.29 x 10^20 J infrared

• 6.96 x 10^-18 J ultraviolet