Transcript visible light

Electron As a Particle and Wave
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Electrons get excited when energy is
absorbed by using heat or electrical
energy
Excited electrons move back to their
ground state when they release the same
amount of energy that was absorbed
(conservation).
Light energy (electromagnetic radiation) is
released when an electron returns to a
lower energy level
Light Energy
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Light travels as a wave and is made of
photons.
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Light contains quanta energy called photons
(Einstein)
Speed of light in a vacuum 3.0 x 108 m/s
 How fast is that??
 It would take 2.5 seconds for radar waves
traveling at the speed of light to travel
478,000 miles -- leave the earth, bounce
off the moon and return!!
Speed of Light
c
= the speed
of light
 c = λv
 λ = wavelength
 v = frequency
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Wavelength: one
wave, the distance
of one crest and
one trough (Unit:
meters)
Frequency: # of
waves per second
(Unit: s-1 or Hertz,
Hz)
The Electromagnetic Spectrum
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Radio Waves
Microwaves
Infrared Radiation
VISIBLE LIGHT
 ROY G BV
Ultraviolet
X-Rays
Gamma Rays
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Low Frequency, Long
Wavelengths
v = 1010 Hz λ = 10-1 m
VISIBLE LIGHT
400 – 800 nanometers
Nanometer = 10-9 m
High Frequency, Short
Wavelengths
v = 1020 Hz
λ = 10-12m
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http://en.wikipedia.org/wiki/Electromagne
tic_spectrum
http://en.wikipedia.org/wiki/Visible_light
Max Planck
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Energy is absorbed and emitted in discrete
units of light energy called quanta (also
known as photons)
The energy of a light quantum
corresponds to the energy difference
between two energy levels.
E = hυ
Energy is equal to Planck’s constant
multiplied by frequency
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h = 6.63 x 10-34 J•s
Photoelectric Effect
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Absorption of light striking a metal with a
certain frequency, has enough energy to
release an electron (photon) of light
Sample Problems
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Yellow light with a frequency of
approximately 6.0 x 1014 Hz is the
predominant frequency in sunlight. What
is the energy of this light?
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4.0 x 10-19 J
A quantum of a certain color of visible
light is found to have an energy of 5.0 x
10-19 J. What is the color of this light?
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Violet because the frequency is 7.5 x 1014
Hz which is equivalent to 400 nm (the
shortest wavelength for light)
Changing Energy Levels
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Excited State:
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When an e- has gained energy to jump to a
higher energy level
Ground State:
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Most stable (original location of the electron)
Excited electrons lose energy to return to
stability
 This releases energy as electromagnetic
radiation (a photon of light)
Conservation of Energy
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The energy absorbed always equals
energy emitted.
Recall that electrons can not be in
between energy levels; however electrons
can skip and move up 2,3,4… levels at a
time.
The energy is released similarly. Either in
large amounts (drop back several energy
levels) or the electrons can jump back one
energy level at a time.
Photoelectric Effect
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Electron as a particle is ejected from the
surface of a metal exposed to a specific
frequency of light
Photon “a packet of light”
E = h
http://www.bing.com/videos/search?q=ph
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