Quizlet on terms from this lecture

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Test 1

  • In this room next Wednesday at recitation time (6:30 pm - 7:50 pm)
  • Covers chapter 1, 2, and however we’ve got in 3
  • This evening’s recitation is a survey to get a sense of each student’s background and how it affects their performance
    • Bring a pencil
    • 5 bonus points
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Clicker question

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  • 4.21ft^3 to Liters

2nd Question

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  • How many atoms are in a sample of copper with volume of 0.475 cm^3 and a density of 8.96 g / cm^3?

Chapter 3

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Properties of Waves

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  • Wavelength (λ) is the distance between identical points on successive waves.
  • Amplitude is the vertical distance from the midline of a wave to the peak or trough.
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  • Frequency (ν) is the number of waves that pass through a particular point in 1 second (Hz = 1 cycle/s).
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  • The speed (v) of the wave = λ x ν

Light

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Amplitude and Wavelength

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  • Wavelength and amplitude are independent properties.
    • The wavelength of light determines its color (intensive physical property).
  • The amplitude, or intensity, determines its brightness (extensive physical property)
    • Brightness dependent on amplitude

Color

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  • The color of light is determined by its wavelength or frequency.
  • White light is a mixture of all the colors of visible light.
    • A spectrum
    • Red Orange Yellow Green Blue Indigo Violet
  • When an object absorbs some of the wavelengths of white light and reflects others, it appears colored; the observed color is predominantly the colors reflected.

Maxwell

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  • Maxwell (1873), proposed that visible light consists of electromagnetic waves.

Electromagnetic Spectrum

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Example problem

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  • An electromagnetic wave has a frequency of 6.0 x 10^4 Hz. Does this frequency fall in the visible region? Convert this frequency into wavelength (nm).

Clicker Question

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  • What is the wave length of a wave with frequency 8.6 * 10^13 Hz?

Einstein and the Photoelectronic Effect

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  • Hertz observed that when (some) light shines on a metal surface, electrons are produced from the surface.
    • The electrons emitted from the metal surface are photoelectrons.
    • This phenomenon is called the photoelectric effect.
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    1. Number of electrons depends upon light intensity
    2. More Kinetic energy at higher frequency of light
    3. Emission has a frequency threshold, below which there are no electrons

Explaining the Photoelectric Effect

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  • Classic theory explanation:
    • The photoelectric effect according to classic wave theory attributed the electrons’ being emitted from the metal surface to the light energy being transferred to the electrons.
    • Classic theory states the following:
      • If the wavelength of light is made shorter or the light wave’s intensity is made brighter, more electrons should be ejected.
      • Energy of a wave is directly proportional to its amplitude and its frequency.
        • Example: If a dim light is used there should be a lag time before electrons are emitted in order to give the electrons time to absorb enough energy.

Explaining the Photoelectric Effect

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  • Einstein’s explanation: Quantum theory
    • Experimental observations indicate the following:
      • A minimum frequency was needed before electrons would be emitted regardless of the intensity called the threshold frequency.
      • High-frequency light from a dim source caused electron emission without any lag time.

Einstein’s idea: “Light Is Quantized.”

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  • Ejection of electrons from a metal surface by light:
    • One photon at the threshold frequency gives the electron just enough energy for it to escape the atom.
    • Binding energy, φ
    • When irradiated with a shorter wavelength photon, the electron absorbs more energy than is necessary to escape.
    • This excess energy becomes kinetic energy of the ejected electron.
    • Where (hυ) is a quantized packet of energy

Explaining the Photoelectric Effect

  • Einstein’s explanation: Quantum theory
    • Einstein proposed that the light energy was delivered to the atoms in packets called quanta or photons.
      • Energy = (hυ)
        • hυ = quanta
    • The energy of a photon of light is directly proportional to its frequency.
      • E = hc/λ
      • Or it is inversely proportional to its wavelength.
    • Symbols:
      • Planck’s Constant, (h) is a proportionality constant with a value of h = 6.626 × 10−34 J · s.
      • Speed of light (c) value is 3.00 × 108 m/s

Black Body Radiation

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  • If you heat something up in the dark, you can see it with infrared radiation
  • “Black Body Problem” Solved by Planck in 1900 “… an act of despair … I was ready to sacrifice any of my previous convictions about physics …” Energy (light) is emitted or absorbed in discrete units (quantum).

Vocab

Term Definition
Wavelength the distance between identical points on successive waves
amplitude the vertical distance from the midline of a wave to the peak or trough
frequency the number of waves that pass through a particular point in 1 second (Hz = 1 cycle/s)
photoelectric effect describes the phenomenon in which light shines on a metal surface and electrons are produced from the surface
threshold frequency minimum frequency needed before electrons are emitted
quanta (photons) the packets of energy that light travels with
planck’s constant (h) 6.626 * 10^-34 J s
speed of light (c) 3 * 10^8 m/s