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Homework Help: Incidence angle - tilted face

  1. Jul 18, 2017 #1
    1. The problem statement, all variables and given/known data
    A plate is subjected to rays from a light source located 35 degrees above the horizon. The plate itself is tilted 12.5 degrees around its vertical axis reducing the exposure to the light source.
    Find the incidence angle between the light source and the front face of the plate.
    2. Relevant equations

    3. The attempt at a solution
    I believe the solution is based on trigonometry mostly, but could not sort it out...
    Any help or even direction would be much appreciated!
  2. jcsd
  3. Jul 18, 2017 #2
    Just to clarify, the plate is vertical and then tilted around its vertical axis.
  4. Jul 18, 2017 #3


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    Try making a 3D drawing of the mirror and rays and upload it so that we have something tangible to talk about .

    If you can't easily visualise what the problem looks like in 3D try doing some 2D drawings first :

    One drawing showing a view broadside on so that the 35 degree angle of the rays is shown correctly and another drawing showing a view looking down from above so that the angle of the mirror is shown correctly .

    Doesn't matter if your first tries are not quite correct - we can soon put them right .
  5. Jul 18, 2017 #4
    I have made two 2D skeches for the above problem, I hope it helps.
    Basically the light source is at 35 degrees of elevation to the plate if it was just perpendicular to the horizontal plane. However, the plate is tilted by 12.5 degrees around its vertical axis which changes the angle of incudence.

    It is also similar to the sketch below but much simpler I assume because we only take into account the vertical plate.

    Thank you in advance!

    Attached Files:

    Last edited by a moderator: Jul 18, 2017
  6. Jul 18, 2017 #5


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    Take an origin in the plate, with the X axis parallel to the plate and the Y axis normal to it, both horizontal.
    Consider a ray incident at the origin.
    If you move a distance y along the Y axis from the origin, how far would you have to move in the X direction to be back under the ray?
    How far are you from the origin now?
    How far would you have to move in the Z direction to reach the ray?
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