The Nature of Light

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Chapter 11

• The Nature of Light

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Dual Nature of Light
Light behaves both as a wave and a particle.
Early experiments showed that light exhibits
interference, a wave characteristic. Fig.
11.3 Approximately 100 yrs. later, a series of
experiments indicated that light also has
particle characteristics. This occurred when
light of certain wavelengths shown on metal
caused electrons (e-) to be ejected from the
metals surface. Fig. 11.5
3
Photoelectric Effect

• The photoelectric effect led to results that
  could not be
• explained by assuming that light acts as a wave.
• E- are released by the metal when high-frequency
• light (violet) strikes the metal but not when
• low-frequency light (red) strikes.
• Dim high-frequency can produce the effect, but
• even very bright low-frequency light cannot.
• The e- begin to leave the surface as soon as the
  light
• strikes the surface, even with dim light.
• The higher the frequency of the incoming light,
  the
• greater are the speed and kinetic energy of the
  ejected e-.

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Max Planck suggested that light is emitted in
tiny bundles of energy, quanta. The present-day
accepted name for this packet of energy is a
photon. The amount of energy carried by an
individual photon can have only a certain value
and no others. energy h(frequency of
light) E
hf h Plancks constant 6.626 x 10-34 J
s Einstein proposed a theory to explain the
photoelectric effect. Fig. 11.6 Light acts as
though it has a split personality sometimes it
acts like a wave, sometimes like a particle.
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Source of Light
Bohr was successful in explaining how light is
emitted by an atom. An e- in the ground state
receives energy and is elevated to a higher
energy level. Fig. 11.8 An e- in a higher energy
level is said to be in an excited state. The
excited state is unstable and the e- releases
its excess energy and falls back to ground
state. It does this by emitting a photon of light
energy.
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Diffraction Grating
When there is a need to separate light of
different wavelengths with high resolution, then
a diffraction grating is most often the tool of
choice. A large number of parallel, closely
spaced slits constitutes a diffraction grating.
The diffraction grating is an extremely
important tool in the study of astronomy.
Astronomers have a good idea of the elements
present in a star, the abundance of each element,
the stars temperature, etc. This information
is derived by studying the light from that star
with a diffraction grating.
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Spectral Analysis

• The colors falling on a screen after white light
  passes
• through a diffraction grating is called a
  spectrum.
• Three types of spectra can be produced
• continuous spectrum If a hot solid or hot,
  high-pressure
• gas, passes through a diffraction grating, all
  the hues and
• shades of color show. (Fig. 11.15)
• emission (or line) spectrum If a hot,
  low-pressure gas
• is used, only a few distinct colors are produced.
  (Fig. 11.16)
• Every chemical element produces its own
  characteristic
• pattern of spectral lines. It serves as a
  fingerprint of the
• element.

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3. absorption spectrum The light from a hot
solid or high-pressure gas is first passed
through a cool gas, and then the light is passed
through a diffraction grating. (Fig. 11.17) The
cool vapor removes those photons that it would
emit if it were already excited. Most of our
knowledge about the stars and our Solar
System comes from spectra these objects
produce. The Doppler effect applies to all types
of waves, including light waves. (Fig. 11.19)
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Electromagnetic Spectrum
The electromagnetic spectrum shows the entire
range of electromagnetic radiation. The spectrum
ranges from long wavelength, low frequency,
radio waves to short wavelength, high frequency,
gamma rays. Visible light makes up a very small
portion of the electromagnetic spectrum. 1.
radio waves are low frequency and long
wavelength The frequency of the wave that a
station is allowed to transmit is called the
carrier wave. Electronic equipment converts the
sound signal to an electrical signal, and this is
superimposed on the carrier wave. (Fig. 11.21)
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2. microwaves are short-wavelength radio waves
If there are no water or oil molecules in a
substance, microwaves either reflect off or pass
through without absorption. 3. infrared
radiation commonly called heat radiation This
radiation is the type emitted by warm objects,
and when it is absorbed by materials, it
increases the kinetic energy of the molecules of
the absorber, thus increasing its
temperature. 4. visible light waves from low
frequency to high frequency includes the colors
ROY G. BIV red, orange, yellow, green, blue,
indigo, violet 5. ultraviolet light can excite
e- in most molecules and cause a multitude of
chemical reactions causes sunburn
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6. X-rays high frequency, high-energy photons
have great penetrating power can penetrate skin
and flesh but not bone 7. gamma rays very high
frequencies and energies emitted by nuclei
during nuclear disintegrations
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Chapter 12

• Inside the Atom

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Atomic Theory

• Democritus believed that the smallest part of
  matter was
• indivisible and called it atomos.
• John Dalton provided scientific evidence that
  matter is
• composed of atoms.
• each element is composed of small particles
  called atoms
• all atoms of a given element are identical, but
  they differ
• from those of any other element
• atoms are neither created nor destroyed in any
  chemical
• reaction
• a given compound always has the same relative
  numbers
• and kinds of atoms

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cathode ray tube
The phenomenon of electrical discharge can be
duplicated in the laboratory with a device called
a cathode ray tube. Two pieces of metal in an
evacuated tube are connected to a power source.
The positive plate is called the anode, and the
negative plate is the cathode. (Fig. 12.1) When
the high-voltage source is connected to these
plates, a green glow is seen on the
phosphorescent coating at the end of the glass
tube. It was determined that this stream of
particles originated at the cathode and had a
negative charge. They were called cathode rays.
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J.J. Thomson calculated the charge-to-mass ratio
of these particles and found it to be 1.76 x 108
C/g. He renamed these cathode rays,
electrons. Robert Millikan performed an oil
droplet experiment and determined the charge of
a single electron. Using Thomsons charge-to-mass
ratio he calculated an e- mass. charge of an e-
1.60219 x 10-19 C mass of an e- 9.10953 x
10-28 g Since atoms are electrically neutral,
there had to be a particle of positive charge.
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Early Models of the Atom
The earliest model of the atom was that of a
tiny, hard, indestructible sphere. J.J. Thomson
proposed the plum pudding model of the atom.
According to this model, negative charges are
distributed evenly throughout an atoms
positively charged interior. Ernest Rutherford
performed the gold foil experiment. From this
experiment he concluded that the atom is composed
of a small, dense region of positive charge,
called the nucleus. The e- circle about the
nucleus in orbits like planets about the Sun. The
atom is mostly empty space.
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Scientists determined that, since atoms are
electrically neutral, they must have the same
number of positive and negative charges. The e-
have a negative charge. The positive particles in
the nucleus were called protons (p). Protons
and electrons alone could not account for the
mass of the atom. A third particle was
discovered in the nucleus. Since it was
electrically neutral, it was called a neutron
(n0). atomic number the number of protons In a
neutral atom the number of electrons is equal to
the number of protons. mass number the number of
protons and neutrons The mass of an atom is
located in the nucleus. isotopes atoms of the
same element that differ in mass due to a
difference in number of neutrons
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Just as light has a dual nature, so does matter.
The wave nature of particles is a necessary
feature in understanding the subatomic world.
Electrons do not travel in fixed orbits but
occupy a region around the nucleus. This is
referred to as an electron cloud. Regions where
the cloud is most dense represent those
locations where the electron is most likely to
be found. (Fig. 12.10)