Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Undefined Angle of Incidence?

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A 1.0-cm-thick layer of water stands on a horizontal slab of glass. Light from a source within the glass is incident on the glass-water boundary.
    What is the maximum angle of incidence for which the light ray can emerge into the air above the water?


    2. Relevant equations
    Snell's Law: nisin[tex]\theta[/tex]i = ntsin[tex]\theta[/tex]t


    3. The attempt at a solution
    I drew a diagram like this:

    air (n = 1.00)
    ________________ light ray[tex]\uparrow[/tex]
    water (n = 1.33) light ray[tex]\uparrow[/tex]
    ________________ light ray[tex]\uparrow[/tex]
    glass (n = 1.50) light ray[tex]\uparrow[/tex]

    The ray of light travels from the glass upward.

    nglasssin90 = nwatersin[tex]\theta[/tex]water

    nwatersin[tex]\theta[/tex]water = nairsin[tex]\theta[/tex]air

    Therefore, using equality of alternate angles:

    nglasssin90 = nairsin[tex]\theta[/tex]air


    (1.50)*(sin90) = (1.00)*(sin[tex]\theta[/tex]air)

    Solution: undefined
    What is the maximum angle of incidence if it is undefined?
     
  2. jcsd
  3. Oct 13, 2008 #2
    Think about where Snell's law measures [tex]\theta[/tex] from.
     
  4. Oct 13, 2008 #3
    Sorry buddy, that was no help. After spending about an hour on that problem, I think I deserve a little more than that. I don't expect anyone to DO the work for me but come on now, let's be serious.

    My question is: What is the maximum angle of incidence for which the light ray can emerge into the air above the water?
     
  5. Oct 13, 2008 #4
    Yes I am aware of that, I'm showing you what you've done wrong.
    For a ray of light perpendicular to the surface, you wrote: [tex] n_1sin(90) [/tex]
    Why did you write sin(90)?
    Think of where snell's law measures [tex] \theta [/tex] from.
     
  6. Oct 13, 2008 #5
    If it is incident it's perpendicular and if it's perpendicular it's 90 degrees.
     
  7. Oct 13, 2008 #6
    90 degrees from the tangent.
    doesn't Snell's law measure [tex] \theta [/tex] from the normal?
     
  8. Oct 13, 2008 #7
    Ok, so that makes it 45 degrees?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook