# Incident and refracted angles

#### steppenwolf

ok so this was in my end of unit test today and i have been stressing as to what i should have done, so as it's not really homework you could always just tell me the answer

pretty basic stuff, a ray of light is incident from a substance upon the surface of another substance with n=1. i can't remember the exact refractive index of the first material but it was about 1.4 or something, hmm. anyway, i worked out the incident abgle to be 47 degrees and to find the refractive angle used
sin incident angle x n1 = sin refractive angle x n2
seeing as n2 = 1 then you get
sin refracted angle = sin incident angle x n1
which ended up being about 1.09 or something, ok, a sine>1 ???

well i just pretended it was 1 and so worked out the refracted angle to be total, ie 90 degrees. was this the right thing to do? was this some measurment discrepancy i was supposed to disregard? help, i don't want to wait a month for results to understand this problem!

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#### HallsofIvy

Homework Helper
Wait a month? Why not just go to your teacher and ask?

I cringed at "i just pretended it was 1" but basically, that is what happens. If the angle of incidence and index of refraction
are such that n*sin(theta)> 1 then the light is completely reflected- there is no angle of refraction.

#### steppenwolf

thanks, that's what i thought. i would ask my teacher but i don't know where he lives, in his lab maybe but i doubt it, i will go stake out the science department until he surfaces...

#### arcnets

Originally posted by steppenwolf
i will go stake out the science department until he surfaces...
You could look at it as a part of the test... apart from learning physics, they want you to get familiar with the institute...

#### HallsofIvy

Homework Helper
Going to class might be a good way of finding your instructor!

Failing that, checking the syllabus he gave you at the beginning of the year, the schedule posted on his office door or even asking the department secretary just might be ways of determining when he is in.

#### steppenwolf

i don't have classes for a few weeks actually!
but thanks anyway, i am now stumped by a certain geometry problem, is it worth a new thread? hmm, alright then!
*goes off to post new thread on particularily nasty geometry shenanigans*

#### dav2008

Gold Member
Originally posted by HallsofIvy
Wait a month? Why not just go to your teacher and ask?

I cringed at "i just pretended it was 1" but basically, that is what happens. If the angle of incidence and index of refraction
are such that n*sin(theta)> 1 then the light is completely reflected- there is no angle of refraction.
Yep..When doing these problems, when lights going form more dense to less dense, you have to check for the critical angle...and like Halls said, if your angle of incidence is greater than that critical angle, you get reflection...and when it reflects its just incidence=reflection

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