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Angle of refraction snells law

  1. Mar 20, 2012 #1
    im having difficulty understanding how light leaves a prism mainly because im not sure about what angles we are talking about when we say angle of refraction/incidence.
    Lets say for example light hits a triangular prism leg at a 90 degree angle (incidence = 90 deg?) and the prism has index of 1.4 compared to surrounding air's 1.0 using snells law would get an angle of refraction. Now Im getting lost trying to find the angle that it strikes the hypotenuse and the angle that it leaves the prism. how would i use snells law again?
  2. jcsd
  3. Mar 21, 2012 #2
    All I know is that when a monochromatic ray of light (let's say) enters from medium 1 to medium 2 then ,
    Refractive index of medium 2 or 1μ2 = angle of incidence in medium 1 / angle of incidence in medium 2

    Representation : 1μ2= μ21
    Yes , we represent refractive index of medium 2 with respect to medium 1 as 1μ2

    Always remember this to avoid confusion.

    angle of refraction/incidence becomes inverse refractive index.

    In your case
    1.4 = sin 90o/sin r

    And please I cannot understand what you mean by leg angle or etc etc ? Are you taking a right angled prism ? Please give me a diagram of your question.
  4. Mar 21, 2012 #3

    Doc Al

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    Staff: Mentor

    I assume you mean that the light hits perpendicular to the first surface. Since angles (in Snell's law) are measured from the normal, that would mean that the angle of incidence is 0°, not 90. To find the angle that the light hits the other side of the prism, you'll need to use a bit of geometry. Figure out the new angle of incidence and apply Snell's law to find the angle the light makes upon leaving the prism.
  5. Mar 21, 2012 #4
    Angle of incidences and refraction(and even reflection) are always measured from the normal of refracting surface(or reflecting surface)

    Remember, angle of incidence, refraction and normal always lie in the same plane.

    As Doc Al pointed, since the ray is perpendicular to surface, it is actually parallel to normal and therfore angle of incidence is 0 degrees.

    For further calculation you will need the angle of prism and some simple applications of geometry trigonometry
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