Transcript Waves and Light Notes

You just got finished with your 20
“Nuggets” of knowledge about
people who helped shape atomic
theory…
 We will go back and talk about the early
days of atomic theory soon.
 Today, let’s focus on the development of
atomic theory in the 1900s and WAVE
MECHANICS.
In the late 1800’s, scientists noticed that objects
glowed at certain colors when heated to high
temperatures…
Low temps  Warmer  Higher temps Even hotter
Infrared
Yellow glow
White glow
(not visible) (molten metal) (light bulb)
Bluish glow
Waves and Light Notes
 Add a new scientist to your list:
Scientist #21:
1900 Max Plank
– Studied the relationship between heat and
the color of light that an object gave off. He
noticed that only certain colors were
produced.
-- Classical physics could not explain it.
Max Plank
Conclusion:
Energy does not exist as a continuous stream.
Energy exists in small, discrete, definite packets of
energy called quanta (1 quantum = 1 “chunk”
of energy) This is the beginning of quantum
mechanics!
Energy = Plank’s Constant (h) x frequency
E = h∙√
h = 6.626 x 10-34 Joule-sec
√ = frequency in 1/sec or Hertz
Scientist 22
 1905 Albert Einstein
 Was doing an experiment with
light “sometimes” causing a
piece of cesium or sodium
metal to produce an electric
current.
 Conclusion: The energy of the
light had to be at a particular
level in order for an electron to
be released. (All or None).
 The “Photoelectric Effect”
Albert Einstein
 Won a Nobel Prize in Physics for his work on the
photoelectric effect.
 Discovered a relationship between: electrons,
light, frequency, energy, and electricity.
 Remember E = h∙√
of light is high.
If frequency is high, then energy
 Discovered that light also acts like it is a particle:
photons. This supported Plank’s quantum theory
of light.
Group of Scientists for #23
 Johann Balmer
 Theodore (Teddy) Lyman
 Fredrick Paschen
 These three studied light as it was given off by
hydrogen atoms that were heated.
 The light contains a pattern of light called an
Atomic Emission Spectrum or Bright Line
Spectrum.
Emission Spectra
 The pattern for Helium is below.
 A non-colored pattern for Hydrogen is below
and at the bottom of page 143 in text.
Waves
 A wave is a method of transmitting energy
through a medium.
 There are two types of waves
 Longitudinal waves
 Transverse waves
Longitudinal Waves
 Examples: sound, slinky, traffic
Transverse Waves
Examples: water, light, sine
General Properties of Waves
 Amplitude – a measure of the intensity of the
wave
 For sound, amplitude is volume
 For light, amplitude is brightness
 Wavelength λ (lambda) – the distance between
one part of a wave and the exact same part on
the next wave
 Units can be miles, feet, yards, meters, cm, km, nm,
Angstroms (1 x 1010 Å = 1 m)
General Properties of Waves
 Frequency – √ (nu) – the number of waves
generated in a specific amount of time
 For sound, frequency is pitch
 For light, frequency is color or type of energy
Units are in /sec, sec-1, waves/sec, cycles/sec,
Hertz (Hz)
Speed
 In general, speed = distance/time
 For any wave, there is another formula for speed
 Speed = wavelength x frequency
 For a light wave, the product of frequency and
wavelength always equals a constant (c), the
speed of light
c = λ∙√
λ=c/√
√ =c/λ
The Speed of Light
(use the one to match your units!)
 186,000 miles/sec
 3.00 x 108 m/sec = 300,000,000 m/s
 3.00 x 1010 cm/sec = 30,000,000,000 cm/s
 3.00 x 1017 nm/s
 3.00 x 1018 Å/sec
Relationships between variables
 As the wavelength of light increases, its
frequency decreases
 As the frequency of light increases, its
wavelength decreases
 As the frequency of light increases, the energy
increases
The Electromagnetic Spectrum
 Light is just one kind of Electromagnetic
Radiation.
 Electromagnetic Radiation includes these kinds
of waves: radio waves, microwaves, infrared,
visible light, ultraviolet, x-rays, and gamma rays.
Electromagnetic Waves
 All of these are forms of energy.
 All of these travel in transverse waves.
 All of these travel at the same speed….
THE SPEED OF LIGHT
(c)
Louis de Broglie #25
 Asked the question: If waves can have particle
properties, can particles have wave properties?
 He proposed that ALL MATTER could be viewed as
having wave properties: electrons, protons, atoms,
marbles, elephants, humans!
de Broglie wavelength = Plank’s constant
momentum
(we will come back to
this later)
 = h
mv
Scientist #24
 1912-1914 Niels Bohr
 (Draw the picture that is on the board)
Atomic Spectra
 Passing electric current through a gas in a neon
tube energizes the electrons of the atoms of the
gas and causes them to emit light.
Atomic Spectra
 When atoms absorb energy, electrons move into
an excited state.
 These electrons then lose energy by emitting a
photon when they return to lower energy levels.
Atomic Spectra
 Each specific frequency of visible light emitted corresponds
to a particular color.
 Each specific element has a unique emission spectrum that
consists of discrete lines. No two elements have the same
emission spectrum, so it is kind of like a fingerprint for that
element!
 This is the fingerprint
for Mercury!
Atomic Spectra
 Much of our knowledge of the composition of
the universe comes from studying the atomic
spectra of stars, which are hot glowing bodies of
gases.
Three Differences Between
Ground State and Excited State
Electrons
 Ground State electrons…are closer to the
nucleus than excited state electrons.
 Ground State electrons…have less energy than
excited state electrons.
 Ground State electrons…are more stable than
excited state electrons.
Why could electrons only exist in
certain orbits?
 No one could explain why until de Broglie (1924).
 De Broglie saw a connection between his theory
of wave characteristics of matter and the stable
orbits of the Bohr model.
 An orbit was only stable if it contained an
integral (whole) number of electron
wavelengths!
1st orbit: 1
2nd orbit: 2 
3rd orbit: 3 