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Homework Help: Some wavelength questions

  1. Jun 10, 2007 #1
    1. The problem statement, all variables and given/known data
    I've got a couple so I'm going to keep them all within this thread.

    1) A ray of green light is incident in air on a flat glass surface, at an angle of 45 degrees measured from the normal. The glass has a refractive index of 1.4 for this green light.

    (a) if the color of th eincident light is changed from green to red, the incident angle staying the same, will the refracted angle change? Explain
    (b) What is the speed of the green light in the glass?

    2) A certain metal has a work function of 3eV.
    (a) What is the threshold wavelength to cause photoelectric emission in this metal?
    (b) For a light of wavelength longer/shorter than the threshold, incident on this metal, what happens?

    3) Sodium has a work function of 2.28eV. What is the kinetic energy of an electron ejected from sodium where it is illuminated by light of wavelength
    (i) 410nm
    (ii) 550nm

    3. The attempt at a solution

    Physics is far from my forte and I have a feeling 1b, 2b and 3 are all a matter of using a simple formula. However I was unable to find any relevant equations. I'm also unsure of the theory behind 1a and 2a so if somebody could explain that too it would be much appreciated! :blushing:
  2. jcsd
  3. Jun 10, 2007 #2


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    For the first question look up Snell's law. For the others, start by looking up the Photoelectric Effect.
  4. Jun 10, 2007 #3
    The only version of Snell's I am familiar with is this one:


    But I can't see how that's related to speed of light...
    Last edited by a moderator: Apr 22, 2017
  5. Jun 10, 2007 #4


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    For the speed part consider

    n = c/v

    where n is the index of refraction, c is the speed of light in a vacuum, and v is the speed of light in a medium.
  6. Jun 10, 2007 #5
    I knew it was something basic...:rofl:

    n=1.4, c=3*10^8

    hence v=214285714.3m/s
  7. Jun 11, 2007 #6
    for question 3 (ii) my value of hf is less than that of W, thus resulting in a negative value for Kinetic energy. Does this mean that the energy of the wavelength is not enough to overcome the work function of the metal and hence the electron is unable to leave the surface?
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