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Light, Energy, and MoreOctober 23, 2007
Chemistry
Recap…
• Electromagnetic Spectrum
• High Energy• Low Energy• Wave Nature of Light
What’s Going On Here?
When we heat metal what happens?
• 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….
Max Planck (1900)
• German Physicist• Began to look for
answers• Matter can only gain
or lose energy in small quantized amounts
• What’s quantized?
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
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
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
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?
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
The Big Mystery of the 1900’s…
• The Photoelectric Effect…– What caused these
color changes in metals???
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.
Albert Einstein (1905)
• Added onto Planck’s Theory…
• Called the electron’s emitted, PHOTONS (the little energy packets Planck called quantums)
• Now… Ephoton = hv
• Planck paved the way for the explanation behind the mystery
• But some one else came into the picture…
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
NEW WORD!!!
• Photon
• A particle of electromagnetic radiation with no mass that carries a quantum of energy
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
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
Revised Planck’s Work…
• Einstein piggy-backed off of Planck’s Theory and we now have…..
Photon
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
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
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