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    First calculate the angle of reflection to help you calculate i

    Angle of incidence = angle of reflection.

    Angle of reflection = 85

    Angles in a triangle

    i = 180 - (30 + angle of reflection)
    i = 180 - (30 + 85)
    i = 180 - 115
    i = 75o

    Then use refractive index = (sin(angle of incidence))/(sin(angle of refraction)) to calculate r

    use 1.41 for refractive index as that is the substance the light is entering.

    1.41 = (sin(75))/(sin(r))

    rearrange and solve for r

    sin(r) = (sin(75))/1.41
    r = sin-1 (sin(75))/1.41
    r = 43.24o

    I would have put solution in spoiler if I knew how. Sorry.

    I hope I was able to help.
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    (Original post by Linked)
    thank you for your help.

    But for question 2 the mark scheme says you are wrong.

    It is using this formula n1sini=n2sinr

    How will I be able to distinguish which formula to use?

    Because I too did use the same formula as you and was hoping whether someone can tell me why.

    Ah. I think we have to use the formula that they stated because the light ray is present in the core with refractive index which is not equal to the one of air, 1. the formula I used might only work when light is going from air into another substance.

    I've never seen that formula before.
 
 
 
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