Can someone help me with a refraction WAVE question PLEASE (PHYSICS) Watch

MrToodles4
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Its just this very last part on question 6 d that is bothering me. Here's a link to the question:

http://pmt.physicsandmathstutor.com/.../Refracton.pdf

We worked out the critical angle for the boundary between the core and cladding which was 80.4 degrees. But in this next bit of the question is says:

"Ray B enters the end of the fibre and refracts along the core-cladding boundary. Calculate the angle of incidence, of this ray, at the point of the entry to the fibre."

In the markscheme they did 180 - 90 - 80.4 which = 9.6 degrees for the angle of refraction. And then use snell's law n1sin=n2sin (and i understand this bit) but i just don't understand why they did 180 - 90 -80.4. I tried drawing a triangle but i don't know where this angle of 80.4 degrees goes because its a critical angle from the previous part?? Any help is much appreciated thank you.

MS here: http://pmt.physicsandmathstutor.com/...acton%20MS.pdf
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univ4464
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You may be getting confused with what the angle of refraction is - as the angle between the normal and the refracted ray. I attached a diagram. I assume the 180 - 90 - 80.4 is just the angles in a triangle - the right angle - the known angleName:  IMG_20170503_183323.jpg
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MrToodles4
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(Original post by alfredholmes)
You may be getting confused with what the angle of refraction is - as the angle between the normal and the refracted ray. I attached a diagram. I assume the 180 - 90 - 80.4 is just the angles in a triangle - the right angle - the known angleName:  IMG_20170503_183323.jpg
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Size:  271.0 KB
Thats exactly what my diagram looks like. Thanks a bunch i guess the bit thats confusing me was from the previous bit on "showing the critical angle at the boundary between the core and the cladding is 80.4." I guess its just to the vertical right? Thank youuuu
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univ4464
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(Original post by MrToodles4)
Thats exactly what my diagram looks like. Thanks a bunch i guess the bit thats confusing me was from the previous bit on "showing the critical angle at the boundary between the core and the cladding is 80.4." I guess its just to the vertical right? Thank youuuu
I think there are two ways to interpret that sentence, as in the angle is between the boundary and the core or the critical angle (which is the angle between the ray and the vertical) at the point where the material changes. The latter is the correct interpretation.
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MrToodles4
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(Original post by alfredholmes)
I think there are two ways to interpret that sentence, as in the angle is between the boundary and the core or the critical angle (which is the angle between the ray and the vertical) at the point where the material changes. The latter is the correct interpretation.
I kinda do see it. Thanks a bunch bro. The calculations are fine it was just the picturing
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