Light I Chapter 2627

1

• Light I (Chapter 26-27)
• Light Traveling Through Materials
• Most of the objects that we see are made visible
  by the light that they __________ from sources
  such as the sun, flames or white hot filaments
  from light bulbs.
• Objects which are seen due to light being
  reflected from them are said to be
  ________________.
• Objects which emit light from them are said to be
  _____________.
• 2. Materials such as air, water and glass allow
  light to travel through them in _________ line
  rays (however, light will travel more slowly
  through these materials than it will in a
  vacuum).
• Objects which allow light to travel through them
  in straight lines are said to be _____________
  and objects may be seen through them. Not all
  wavelengths of light travel through glass (UV and
  IR are absorbed by the glass, visible light
  passes through glass). Our ozone layer absorbs
  much of the UV radiation from the sun. (pg. 501)
• Objects which allow light to travel through them
  in diffuse directions are said to be ___________.
  Objects cannot be seen through such materials
  (paper would be an example of a translucent
  material).
• Objects which allow no light to travel through
  them are said to be _____________. Opaque
  materials cast shadows (see page 503-5). The
  outline of a shadow conveys that light travels in
  straight line rays. 2 shadow regions can be
  observed when the source of light is larger than
  the object being illuminated (the dark shadow
  region is called the ___________ the lighter
  shadow region is called the _____________).

reflect
illuminated
luminous
straight
transparent
translucent
opaque
umbra
penumbra
2

• Electromagnetic Waves (pg 497-498)
• Visible light is part of a family of wave regions
  called the electromagnetic spectrum. These waves
  are partially electric and partially magnetic and
  require no ___________ in which to propagate.
• Visible light has a range of wavelengths of 400
  700 nm (700 nm for red 400 nm for violet)
• You need to know the relative positions of the
  different regions (radio, radar, micro, infrared,
  visible, ultraviolet, x-rays and gamma).
• James Maxwell determined the speed of
  electromagnetic radiation and found them to
  travel the speed of ___________light, so he
  deduced that visible light was an
  _________________ wave.
• c ?f (c 3.00.108 m/s)
• Lights Finite Speed and Measurements
• 1. Until the end of the 17th century,
  light was believed to travel at an _____________
  speed.
• The first to observe lights _________ speed was
  Danish astronomer Roemer who observed that the
  time between eclipses of Jupiters moon Io
  changed as the earth went around the sun (there
  was a larger interval between eclipses as the
  earth moved away from the Jupiter and a smaller
  interval as the earth moved towards Jupiter).
• The ________________ of the earths orbit around
  the sun was not known at the time so Roemer could
  not determine a value for the finite speed of
  light (the time discrepancy was about 1000 s).
• Christian Huygens used Roemers data with the
  known diameter of the earths orbit about the sun
  
• (3.1011 m) to determine a value for the
  finite speed of light ( _ ___________
  m/s).
• c d / t 3.1011 m / 1000 s 3 .108 m/s
• http//www.colorado.edu/physics/2000/waves_particl
  es/lightspeed_evidence.html

medium
visible
electromagnetic
infinite
finite
diameter
300,000,000
3

• Michelsons Determination of the speed of light.
• 1. In 1880, Albert Michelson devised the most
  famous experiment for the determination of the
  speed of light ( ).
• Light from an intense source was shown at an
  ____________ mirror. The mirror was adjusted so
  that it would reflect the source light from one
  mountain (Mt. San Antonio) to another mountain
  (Mt. Wilson) ___ km away and back to the other
  side of the mirror (see fig. 27.3 pg. 406).
• Since the distance to the mountain and back was
  known, all that was needed was to determine the
  _____ for the light to travel that distance.
• (t 70,000 m /3.108 m/s
  ).
• In order to determine such a small time, he made
  the octagonal mirror ________.
• If the mirror did not spin at the proper rate,
  the light would not reflect into Michelsons
  ___________.
• The light would reach the eyepiece when it made
  ___ of a turn. So he merely took 1/8th of the
  period of the spin rate of the mirror. As a
  result he won the Nobel Prize.
• Newton, Planck and Einstein (light as a particle)
• Newtons Corpuscular Theory
• The first advocate for light acting as a
  __________ was Isaac Newton. He stated that
  light was composed of _______________. He
  believed that light was a particle that traveled
  in _________ line rays because of its very high
  speeds.
• He stated light could not be a wave like sound
  for it did see evidence of the wave property of
  _____________ (sound can be heard around a
  corner, light cannot be seen around a corner
  without the assistance of a mirror). Actually
  light does exhibit the property of diffraction
  (and interference) as was shown by Christian
  Huygens.
• Newton easily explained the fact that light
  exhibited the wave properties of ____________ and
  _______________. Reflection by the analogy of a
  high speed ball __________ off a surface. His
  explanation of refraction required for light to
  travel faster in ________than in air. 130 years
  later this was found to be ________.

c
octagonal
35
time
0.0002 s
spin
eyepiece
1/8th
particle
corpuscles
straight
diffraction
reflection
refraction
bouncing
water
false
4

• Max Planck (relationship of energy and frequency)
  pg. 601-602
• 1, Max found that metals became different
  _________ when heated.
• 2. He realized the color (each color is a
  certain __ or ___ ) was related to the
  ___________ .
• 3. Planck found that light was absorbed or
  emitted by atoms in discreet quantities (little
  chunks) called ____________ (a packet or little
  chunk of energy).
• Einstein showed light acts as a particle through
  explaining the Photoelectric Effect (pg. 603-4)
• 1. Using Plancks ideas, Einstein explained the
  photoelectric effect through light acting as a
  ____________. This effect could not be explained
  by __________ theory.
• 2. In the photoelectric effect, light is shown on
  a photoactive metal (an alkali metal). What
  puzzled scientists was that very _________ red
  light could be shown on the metal and no
  electrons would be ejected. But, very ____
  violet light shown on the metal would eject
  electrons. By wave theory the more intense, the
  more energy, and thus the more electrons that
  should be ejected.
• 3. Einstein stated that light is composed of
  ___________ he called photons. The shorter the
  __ of the light (higher the frequency) the more
  _______ the photon contained. A certain energy
  (called _____ function (W)) was required to eject
  an electron. Only frequencies that contained
  this energy (called _____________ frequency) or
  greater could eject the electrons.
• 4. As light, above the threshold frequency,
  becomes more ____________ (brighter), the
  ________ the number of electrons ejected from
  the metal surface. The higher the frequency of
  the light, above the threshold frequency, the
  _________ the electrons will be ejected.
• 5. Light below the threshold frequency does not
  contain the required ______ function to eject an
  electron no matter how bright.

colors
energy
? f
quanta
particle
wave
intense
dim
?
particles
energy
work
threshold
intense
greater
faster
work
5
If above Threshold frequency
The higher the frequency (shorter the
wavelength) The faster the electron will be
ejected.
The more intense (brighter) the light The more
electrons that will be ejected
Threshold frequency
6

• E mc2
• E hf
• since f c/?
• We get E h c/?
• Therefore, mc2 h c/ ?
• m h / c?
• Apparent mass of a photon depends upon the
  wavelength of the photon.
• All Photons have same velocity.
• Thereby photons with short wavelengths will have
  more mass (energy)
• DeBroglie (Particles can act as Waves) pg.
  608-609
• Louis deBroglie showed that not only do waves act
  as particles (as with light) but particles can
  act as __________.
• using ? h/mc
• (for a particle c v because particles dont
  travel at light speeds)

Apparent mass of photons
5. The energy of a photon may be determined by
the following E h f (h 6.63.10-34 J.s) or
E hc/? (since f c/?) 6. The combination of
Plancks and Einsteins work helped to prove that
light had a dual nature it acted as a
____________ (when interacting with matter) as
well as a _______ (when traversing through
space).
particle
wave
waves
small
?
7

• These problems arent very heavy, in fact,
  theyre pretty Light
• Quantum A packet of _________ that exhibits
  properties of particles and waves.
• A quantum of light is referred to as a
  __________.
• Energy of a Photon E hf or E hc/?
  h 6.63.10-34 J.s
• Electron Volt (eV) 1 eV 1.60.10-19 J
• De Broglie Wavelength ? h/mv
• Mass Constants mp 1.67.10-27 kg me
  9.11.10-31 kg
• Photoelectric Effect Ephoton W KE KE
  ½ mv2
• Threshold Frequency minimum frequency to eject
  an ____________ from a photoactive metal.
• Wavelengths of Colors
• ultraviolet light - below 400 nm Infrared
  above 700 nm
• Example
• 1. The frequency from WZPL is 99.5 MHz. How much
  energy does a photon of this frequency contain in
  Joules? in electron volts?
• E hf
• E (6.63.10-34 J.s) (99.5 .106 hz)
• E 6.60.10-26 J

energy
photon
electron
(eV / 1.60.10-19 J )
?eV 6.5969.10-26 J
?ev 4.12.10-7 eV
8

• 2. A neon helium laser has a wavelength 633 nm.
  What would be the energy of a photon of this
  light in electron volts?
• E hf f c / ?
• E hc / ?
• E (6.63.10-34 J.s)(3.00.108 m/s) / 633.10-9 m
• E 3.1422 .10-19 J
• E 1.96 eV
• 3. What is the de Broglie Wavelength for a proton
  traveling 1.0 the speed of light?
• h / mv
• (6.63.10-34 J.s) / (1.67.10-27 kg
  (0.010)(3.00.108 m/s))
• 1.3.10-13 m
• 4. A photocell from an electric door has a work
  function of 2.70 eV.
• a. What is the threshold frequency?
• E ph W KE
• Eph W
• hf W

(eV / 1.60.10-19 J )
(at threshold f, KE 0 J)
9

• b. What is the de Broglie wavelength of the
  escaping electron when 200. nm light is shown
  upon the metal?
• h/mv
• (we know the work function and mass of the
  electron that would escape)
• (we can obtain the velocity of
  the electron from the KE)
• KE Eph W
• KE (hc / ? ) W
• KE ((6.63.10-34 Js)(3.00.108 m/s) / 200..10-9
  m) (2.70 eV (1.60.10-19 J / eV))
• KE 9.945.10-19 J 4.320.10-19 J
• KE 5.625.10-19 J
• KE ½ mv2
• v (2KE / m)1/2
• v (2(5.625.10-19 J) / 9.11.10-31 kg))1/2
• v 1.1112.106 m/s
• ? h / mv
• (6.63.10-34 Js) / (9.11.10-31 kg (1.1112.106
  m/s))
• 6.55.10-10 m

10

• Light III
• To this point we have learned that light has a
  dual nature in that it acts as a _________ when
  interacting with matter and as a _________ when
  propagating through space. We have also seen how
  _____________ showed that particles have a wave
  nature with measurable wavelengths. ? h / mv
• Bohr Model of the Atom (Quantum Mechanics) pg.
  624-625
• 1. Physicist Niels Bohr studied the emission
  spectra of elements to devise an atomic model.
• An emission spectrum arises when an element is
  heated and light (spectral lines) of different
  _______ can be observed when viewed through a
  prism. Chpt 30 interactive figures 1-5
• b. Bohr proposed that the electrons in an atom
  could exist only in certain energy levels. The
  heat caused electrons to ________ between these
  energy levels, from the ground state (lowest
  energy levels) to an excited state (electrons in
  higher energy levels).
• c. When the electron transitions from the higher
  energy level back to a lower energy level, energy
  is released in the form of ________. The energy
  of the photon of light emitted is equal to the
  ____________ in energy between the 2 energy
  levels involved in the jump (?E). From this
  energy difference, the wavelength of light may be
  obtained.
• ? hc/?E
• 2. See the Bohr Model (fig. 32.8 pg. 624)
• a. In the Bohr model, electrons of the atom
  orbit the nucleus like planets orbiting the sun.
  
• However, only certain energy levels are
  allowed for electrons, where there are no
  forbidden ________ for planets.
• b. In this model the electrons ________ nature
  is explored.
• c. Each ________ level contains an integer
  number of deBroglie wavelengths. See pg. 628.
• d. Destructive interference occurs when there
  are not an integer number of deBroglie
  wavelengths so no energy level is allowed for
  that wavelength of the electron. Constructive
  interference occurs to create a ____________ wave
  when there are an integer number of deBroglie
  wavelengths

particle
wave
deBroglie
? or f
transition
light
difference
orbits
wave
energy
standing
11

• In this model, the 1st energy level contains ___,
  the 2nd energy level contains ___ while the 6th
  energy level contains ____ deBroglie wavelengths.
• Atomic Spectra of Elements (pg. 582-589)
• 1. Emission Spectra pg. 585 Hewitt interactive
  fig. 30.5
• Above we discussed how light is emitted when
  electrons jump from higher to lower energy
  levels. The result of these jumps is a series of
  _________ spectral lines called an emission
  spectrum. See pg. 585 to observe the emission
  spectra of many elements
• b. All elements have different arrangements
  of electrons and therefore have different
  emission spectra. Therefore, emission spectra
  can be used to ____________ an element like a
  fingerprint.
• c. Emission spectra are utilized in the
  determination of the unknown ___________ or the
  composition of distant stars. While emissions
  are observed in the production of light from any
  source, they are also responsible for the
  ________ seen in fireworks displays, LASERS and
  neon signs.
• 2. Absorption Spectra interactive figure 30.8
• a. When an atom absorbs a quantity of energy
  equal to the difference in energy between __
  allowed energy levels in an atom, the electron
  will jump to the higher energy level.
• b. When many _________ of light are shown on a
  material, 1 or several of the colors of light may
  match the difference in energy between 2 levels.
  
• c. The result is that the absorbed wavelength of
  light will be _____________ from the light that
  originally struck the material and the material
  will have a characteristic color.
• d. White materials absorb ____ wavelengths in
  the visible spectrum. White light can be
  produced by the mixing of red, green and blue
  light. A yellow material absorbs _______ light
  and allows red and green to pass through.
• e. An absorption spectrum is the reverse of an
  emission spectrum here a series of _____ lines
  appear on the colored ROYGBIV colored background.
  The dark lines are the result of light being
  absorbed (therefore it is removed from the
  incident light).

2
1
6
bright
identify
elements
colors
2
?s or fs
removed
no
blue
black
12

• Example 1 Wavelength of emission of light for
  electron transition of n 2 to n1
• n 2 to n 1 and n 4 to n 3 in hydrogen
• ?E Ef - Eo
• ?E 0.00 eV - 10.20 eV
• ?E -10.20 eV
• hc / ?E
• (6.63.10-34 Js (3.00.108 m/s))
• -10.20 eV ( 1.60.10-19 J / eV)
• -1.22.10-7 m (109 nm / m)
• 122 nm emitted (UV light)
• Example 2 Wavelength to ionize from
• n 3 in Hydrogen
• ?E Ef Eo (ionization energy at n
  infinity)
• ?E 13.60 eV 12.09 eV
• ?E 1.51 eV
• ? hc / ?E
• ? (6.63.10-34 Js (3.00.108 m/s)) 109 nm

?E Ef - Eo ?E 12.09eV - 12.75 eV ?E -0.66
eV ? hc / ?E ? (6.63.10-34 Js (3.00.108
m/s)) -0.66 eV ( 1.60.10-19 J / eV) ?
-1.88 .10-6 m (109 nm / m) ? 1900 nm
emitted (IR light)

• Example 3 What 2 energy levels for a
• 434.7 nm emission in hydrogen?
• ?E hc / ?
• ?E (6.63.10-34 Js (3.00.108 m/s)) eV
• 434.7.10-9 m 1.60.10-19J
• ?E 2.86 eV
• Emission is from n 5 to n 2

13

• Hydrogen Spectrum Mercury Spectrum
• n ? - - - - - - - - - - - - 13.60 eV
  (ionization energy) n ? 10.44 eV (ionization
  energy)
• n5 ______________ 13.06 eV n9 8.85
  eV
• n4 _______________ 12.75 eV n8 8.84 eV
• n7 7.93 eV
• n3 ______________ 12.09 eV n6 7.72 eV
• n5 6.70 eV
• n4 5.46 eV
• n 2 ______________ 10.20eV n3 4.89 eV
• n2 4.67 eV
• n1 0.00eV
• n1 _______________ 0.00 eV

14
Nice web pages

• (dancing tooth emission absorbance simulation)
• http//einstein.byu.edu/masong/HTMstuff/Absorb2.h
  tml
• (Bohr model)
• http//www.wwnorton.com/chemistry/overview/ch3.htm
  Bohr
• (flame tests)
• http//scidiv.bcc.ctc.edu/wv/spect/emission-flame-
  exp.htmlAnchor-barium
• (emission absorbance/ speed of light others)
• http//www.colorado.edu/physics/2000/index.pl