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30 Cards in this Set
- Front
- Back
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Polarisation
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Phenomenon associated with transverse waves. Using a polarising filter, waves are limited to a single plane. Draw a sketch
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Examples of polarisation
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Reflected light
Light from LCD displays Microwaves can be polarised |
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Malus's law
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Intensity of light passing through a polarising filter at angle theta to the polarisation axis:
I(trans) = I (incident) (cos (theta^2 |
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Coherence
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2 waves are coherent if they have a constant phase difference.
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Phase difference
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A measure of whether waves are "in step" - phase difference of 0. Or out of step - phase difference of 180 degrees.
Also measured in radians. |
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Path difference
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Difference in distance travelled by wave A and wave B.
Measure of length, often written in terms of wavelength (lambda) |
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When is two-source interference constructive? destructive?
You may assume sources are coherent. |
Constructive - 2 sources have a path difference of a multiple of wavelengths.
Destructive - 2 sources have a path difference of an odd # of 1/2 wavelengths. |
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How is a wave's intensity related to its amplitude?
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Intensity = power / cross-sectional area
Intensity is proportional to (amplitude)^2 |
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Describe an experiment to determine the wavelength of monochromatic light using a laser and a double slit.
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Use lambda = (ax)/ D
Draw a sketch and define terms. |
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Describe an experiment to determine the wavelength of light using a diffraction grating.
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Use dsin(theta)=n lambda
Draw a sketch and define terms. |
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What is the separation between adjacent nodes in a stationary wave?
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1/2 wavelength.
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Draw the fundamental mode of vibration for a stretched string
Indicate the wavelength if string is L. |
Draw sketch
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Draw the fundamental mode of vibration for an air column, one end closed.
Indicate wavelength if tube is L |
Draw sketch
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Draw the fundamental mode of vibration for an air column, both ends open.
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Draw sketch
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Describe an experiment using LEDs to estimate the Planck constant using
eV = hf (e is the charge on an electron) |
1. Measure and record V just to get LED to glow for known wavelengths.
2. find f using c=f * wavelength. 3. Plot eV vs. F 4. Gradient is h, Planck's constant. |
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Describe some evidence to support the wave nature of light.
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Diffraction, Interference patterns (i.e. Young's 2-slit interference), polarisation.
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Describe evidence to support the particulate nature of light
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The photoelectric effect. Specifically, energy is proportional to frequency of light, not intensity.
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Work Function
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The amount of energy needed to free an electron from the surface of a metal
phi in hf = phi + KE max |
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Threshold Frequency
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The minimum frequency of a photon needed to release an electron from the surface of a metal.
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What is the relationship between work function (phi) and threshold frequency, fo ?
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phi = hfo
(KE of the electron is zero) |
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What is electron diffraction?
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When a beam of electrons passes through a small enough slit such as the atomic spacing in graphite, interference patterns are observed.
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What is a use of electron diffraction?
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Electron Microscopy - we can use electron diffraction to probe and characterize complex molecules, such as DNA.
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What is the de Broglie equation?
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lambda = h / (mv)
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Why don't we notice our wave nature?
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It is too small. For humans, it is on the order of 10^-34 m. The hydrogen radius is about 10^-10. So, we don't notice our wave nature.
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Why are spectral lines (i.e. in an emission spectrum) evidence for discrete energy levels in atoms?
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Each line represents a specific wavelength, which corresponds to a specific energy. This would be the energy difference between two levels.
hf = E1 - E2 |
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What is the difference between absorption and emission spectra?
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Absorption - full spectrum is there with characteristic lines missing.
Emission - only characteristic lines appear in the spectrum. |
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Describe how an emission spectrum can be produced
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1. put a high voltage (5kV) across a gas discharge tube.
2. Electrons in the gas are excited and jump to higher energy levels. 3. As they relax back to lower levels, they emit photons of wavelength given by hf = E1 - E2. 4. Put the light through a diffraction grating to separate it and determine wavelengths. |
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Describe how an absorption spectrum can be produced.
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White light is shone on a cool gas. Electrons in the gas absorb photons whose energy matches a transition to a higher state. When the light then goes through a diffraction grating, black lines are missing in the spectrum where photons were absorbed.
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Frauenhofer Lines
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Missing wavelengths in the solar spectrum due to absorption by hydrogen, helium, and other elements in the Sun's atmosphere.
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What is the electron volt?
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It is a measure of energy. It is the energy gained by 1 electron being accelerated across 1 volt.
1 eV = 1.6 x 10^-19 Joules. |
