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Calculate Critical Angle

  1. Mar 11, 2014 #1
    Light takes "t1" seconds to travel a distance "x" cm in vacuum and same light takes "t2" seconds to travel "10x" cm in a medium.The critical angle for the corresponding medium is .. ??
  2. jcsd
  3. Mar 11, 2014 #2


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    This sounds like homework.

    What is the definition of critical angle? What do you know about the relationship between the critical angle and the index of refraction? Do you have enough information to try to determine the index of refraction in this case?
  4. Mar 11, 2014 #3
    the angle of incidence at which angle of refraction becomes 90 degress is critical angle.(C)
    relation is
    angle of refraction = 1/sinC
    Another thing that i know is refractive index for vacuum is "1"

    This is not homework..i am preparing for a competetive exam.
    I could not figure out how to solve this problem.
  5. Mar 11, 2014 #4


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    Staff: Mentor

    Even though this is not literally "homework", it is "homework-like", that is, a specific exercise that you want to find the solution for. By our rules, it belongs in the "Homework and Coursework Help" forums."

    You've already received some help with this, so I will move this to the appropriate forum instead of deleting it and asking you to re-post it. Please use the homework forums for similar questions in the future.

    (Questions for clarifying concepts are OK in the main forums.)
  6. Mar 11, 2014 #5


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    It's just Snell's Law with the appropriate angles inserted.
    Doesn't that equation give you what you want?
    If you are still doubtful, it is almost certainly to be found if you Google "Critical Angle Hyperphysics"
    If in doubt, always try Hyperphysics first. It has most of the facts and formulae you are likely to need - but it doesn't do your thinking for you. haha
  7. Mar 11, 2014 #6
    i think i have the Solution

    when light travels from denser to rarer medium then,
    sin r / sin i = v1 / v2
    so from given data,

    →sin 90 / sin C = [x/t1]/[10x/t2]
    →1/sin C = t2/10t1
    → sin C = 10t1/t2
    → C = arcsin(10t1/t2)

    I think this is the right answer,
    Please correct me if i am wrong.
  8. Mar 11, 2014 #7


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    Go for it.
    This is a confidence thing, I think.
    What numerical answer do you get for glass?
    Does it look reasonable? (It's unlikely to be 'a bit right', isn't it? If it looks OK then it is ok)
    Google critical angle values. The exercise will do you good. Get used to self checking.
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