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Snell's law experiment with glass block

  1. Jan 5, 2016 #1
    1. The problem statement, all variables and given/known data

    A student wants to find the refractive index of a rectangular block of glass. He draws around the block and marks the position of a ray of light that travels through the block. With the block removed, the student can draw in a normal line and then measure the angle of incidence and the angle of refraction. The student repeats this process for different angles of incidence. His measurements are shown in the table.

    Angle of incidence Angle of refraction

    10° ............................... 6.5°
    20° .................................. 13°
    30° ...................................20°
    40° ................................. 25°
    50° ....................................32°
    60° ..................................35°

    2. Relevant equations
    Snells law
    Refractive index = (sin i / sin r)


    3. QUESTIONS & the attempt at a solution

    (i)
    Draw a diagram to show the measurements the student needs to make.

    I'm okay with this part

    (ii) What was the independent variable in this investigation? What was the dependent variable?

    Independent variable is the angle of incidence (because it is the one that can be varied).
    Dependent variable is the angle of refraction (because it is dependent on the angle of incidence).

    (iii) What, if any, safety precautions should be taken if this experiment is carried out in the classroom?

    Ensure that the room is dark for there is no stray light that might make the ray of light difficult to see.

    (iv) Describe two possible reasons the measurements may not be completely accurate.

    The beams of light may be too thick and splayed so measurements of angles will be inaccurate.

    (v) What difference would it make to the results if light of a different color was used in the experiment?

    The wavelength will be different so the angle of refraction will be different.
    What formula links the angle of refraction and the wavelength?


    Thanks
     
  2. jcsd
  3. Jan 5, 2016 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Hi Barclay

    When doing homework problems on the forum or elsewhere ... it is a good thing to show your working diagrams etc
    to make sure the student has the correct understanding


    That's not really a safety issue is it ? ... that's more of an experiment contamination issue :wink:
    when speaking of safety precautions, I would expect them to be referring to student safety as they did the experiment

    so in that light, consider the light sources, that can produce a narrow beam, that you may use for this experiment
    one would require safety eyewear and one wouldn't ... what would they be ?

    you already know that, you listed it near the top of your post :smile:
    so write it out in full, and refer to the wiki page on it if you are not sure


    Dave
     
    Last edited: Jan 5, 2016
  4. Jan 6, 2016 #3
  5. Jan 6, 2016 #4
    A laser light can be used such as from a laser pen or a laser spirit level (used by builders)



    Regarding a formula linking wavelength and refractive index I found this

    snells_law.png
     
    Last edited: Jan 6, 2016
  6. Jan 6, 2016 #5

    gneill

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    Staff: Mentor

    Do a search on the Sellmeier equation.
    Upload a file from your machine. Use the UPLOAD icon at the lower right of the edit panel.
     
  7. Jan 7, 2016 #6
    Have I got the independent and dependent variables mixed up?

    I'm concerned because I read somewhere that : x-axis = independent variable y-axis = dependent variable.

    But later in the question in the book it says: Draw a graph of sin i (y-axis) against sin r (x-axis).



    Have seen it but it's beyond High School level. Way above my needs. Thanks
     
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