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B Refraction problem (temperature dependence)

  1. Jun 26, 2018 #1
    Hi Everybody
    I am doing an experiment for a school project where I have chosen to see is the index of refraction changes in a body of water when the temperture changes. So I am using the angle of incidence and refraction to measure using snells law.
    The Problem:
    Obviously I need a container to hold the water but it will effect my results and how can I resolve this.
    Thanks for the help
     
    Last edited: Jun 26, 2018
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  3. Jun 26, 2018 #2

    sophiecentaur

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    If the light enters the water at the top surface (air / water) the container will have no effect. Not as convenient as doing it with a glass block on a bench but, if you can get hold of a container with flat sides, you could put the block against a screen with graph paper on it. Problem is that the RI doesn't change much over a convenient range of temperatures. (Look this up and make sure).
    Did you consider using the 'pin' method, rather than just shining a ray? Higher accuracy.
     
  4. Jun 26, 2018 #3

    ZapperZ

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  5. Jun 26, 2018 #4
  6. Jun 26, 2018 #5
    [QUOTE=" 'pin' method, rather than just shining a ray? Higher accuracy.[/QUOTE]
    Hi
    what is the pin method?
    Is this
     
    Last edited: Jun 26, 2018
  7. Jun 26, 2018 #6

    ZapperZ

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    That one was really difficult and this one isn’t? I think it is the other way around! You are trying to find a variation that is rather small which can easily be beyond what you might be able to detect without expensive, accurate equipment.

    Zz.
     
  8. Jun 26, 2018 #7
    My teacher told me that it wasn't really an experiment and that I was proving a the theroy and that I needed first hand evidence but he said this one was better.
     
  9. Jun 26, 2018 #8

    ZapperZ

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    What do you, and your teacher, think is “first hand evidence” to verify a theory in physics?

    Zz.
     
  10. Jun 27, 2018 #9

    sophiecentaur

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    Hmm. I think your teacher may just be being 'controversial' about this. Look up some facts about the RI of water variations with temperature and ask him if he seriously thinks a simple experiment will show the variation over a convenient and safe range of temperatures that can be measured accurately and has he a method to ensure that the water temperature is uniform.
    How many hours of uninterrupted experimental time (and lab space) will he make available to you?

    Pin method: I was at school long before laser pointers were available and this was far better than the incredibly naff light boxes that were available but I still think the pin method is preferable for a bench top experiment. Your YouTube video is one example and it actually does the job much better than a laser pointer could. Google Images "Optical pin methods' for many other examples. They are mostly done by sticking (vertical!!) pins into a horizontal board with paper on it and using your eye to line them up so that all the pins are hidden by (behind) the one nearest you. The pins are a great, semi permanent record of the experiment and you can draw ray lines and trace the outline of your mirrors / glass blocks etc. and measure angles very accurately after the equipment has been put away.
     
  11. Jun 27, 2018 #10

    sophiecentaur

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    @Elbraido
    This isn't really on the School syllabus, I think, but we did a really clever optics practical in Uni (1966) which was based on the Abbe Refractometer. It is based on the Critical Angle (below which Total Internal Reflection stops) and a small amount of fluid, sitting the back of a glass D is all you need (and you can use the pin method to see the angle where the ray stops escaping from a surface. Look it up. It is potentially quite simple and the temperature of the block / water drop would be easy to measure and maintain.
    This refractometer is good for analysing tiny samples of liquids and can tell you the concentrations of solutions etc.
     
  12. Jun 27, 2018 #11
    @sophiecentaur
    Thanks for your help. Firstly this a do at home experiment with our 3 week holiday coming up and I am also in Australia which I beleive is differernt to the syllabus in the UK. Plus your second comment about the Critical Angle, I really like it and i might consider it but could you please send like a diagram of how you would be seting it up. I will add a the assignment notifaction for clarification.
    THANKS for the great help so far
     

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  13. Jun 28, 2018 #12

    sophiecentaur

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    That mark scheme looks scary - but they're all like that. The requirement for a log book is a very healthy one and good practice. I must say, I never actually ran one except when required to when I was on courses. Colleagues used them very effectively when they looked back on previous jobs and I was always a bit envious of that resource.
    The actual Abbe refractometer is very sophisticated and I'm not suggesting you do the same thing! You can see the effect when you hold a glass of water with water drops on the outside and you can see the pads of your fingers where they actually touch the glass but the edges disappear where light from inside the glass is 'TIR'ed.
    If you can get hold of a glass D from college / school, you can firstly use the pin method to see the reflection as with a mirror, with two pins a line on the eye side and you line them up with the images of another two. This link shows the method with a mirror and you can do the same with a D by making sure the pins are positioned radially (normal incidence on the curved face).
    That will allow you to find the critical angle for glass by rotating the D until the image disappears. That gives you the RI of the glass. Everything now needs to be done on a vertical plane so that a blob of water will sit at the reflection point. Repeat the same basic operation to find the new critical angle. This will give you the RI of glass into water. The critical angle is much greater because the two speeds are more similar.
    The RI of air / water can be calculated by RIaw = RIag/RIgw
    I have to go now but will return to this when you get back to me with questions. (You may need to check my formula, btw - it's been a long time . . . .)
     
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