Undefined Angle of Incidence?

  • Thread starter Suavez
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  • #1
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Homework Statement


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?


Homework Equations


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


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?
 

Answers and Replies

  • #2
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Think about where Snell's law measures [tex]\theta[/tex] from.
 
  • #3
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Think about where Snell's law measures [tex]\theta[/tex] from.
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?
 
  • #4
325
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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.
 
  • #5
10
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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.
If it is incident it's perpendicular and if it's perpendicular it's 90 degrees.
 
  • #6
325
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90 degrees from the tangent.
doesn't Snell's law measure [tex] \theta [/tex] from the normal?
 
  • #7
10
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90 degrees from the tangent.
doesn't Snell's law measure [tex] \theta [/tex] from the normal?
Ok, so that makes it 45 degrees?
 

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