Demonstrating refractive index

The refractive index of a material governs how much light bends as the light moves into it. You’ve probably seen this bending effect when looking at the surface of a swimming pool: the bottom of the pool looks closer to the surface than it actually is because light rays bend as they travel from water to air.

But if the refractive index of two materials is the same, as is the case for sunflower oil and Pyrex, then light doesn’t bend at all, and you end up with the nice effect demonstrated below.

To say that this demonstration impressed my pupils would be an understatement.

9 thoughts on “Demonstrating refractive index

  1. Ha! Very neat indeed. *polite ripple of applause from the pavilion end*

    Is there a some specific similarity in physical properties that gives sunflower oil and Pyrex their similar refractive indices or is it just that a bunch of property/refractive index curves, when combines, happen to approach the same value?

  2. We turn this into the classic magic trick. Already have a pyrex tube in there. Ask a pupil to smash a separate one in a cloth and sprinkle glass in oil. Say a magic words and you can pull out the tube whole again. For extra magic have two in there and pretend you’ve made one out of thin air.

  3. The same trick of index matching is used everyday to fix dings and small scratches in the windshield of automobiles.

  4. Excellent post. It also uses things that are easy to get. I wonder if we can develop a series of such experiments that rely on every-day objects and occurrences to illustrate science concepts. The crack of a whiplash as an illustration of a sonic boom; a thin woven fabric draped on a clothesline that creates Moire patterns when you see the sun through it … that sort of stuff. Seriously fun pedagogy ( not ‘fun fun’). All Science topics, and just cooking and kitchen science may yield a treasure house! / Nary

  5. Sunflower oil is gooey and messy to deal with. So even though Sonflowers index of refraction of 1.4646 is close to that of Pyrex at 1.470 there are alternatives. For instance you can make your own cheaper, index matching liquid out of sugar and water. For an index to match that of Pyrex you can make, by weight, a 72% sugar to 28% water.

    But rather than having to deal with pyrex you can use other sugar and water mixtures to index match other glass or clear plastic materials such as polymethyl methacrylate, PMMA, with index 1.49. The index range for sugar and water is from 1.33 to 1.47 for % sugar mixtures of 25 to 75 % by weight. I have not experimented with all the possibilities but I think acceptable results can be obtained for index mismatches of +- 0.015 or less.

  6. FAO bweber

    Please could you tell me how you mixed up the solution 72% sugar to 28% water at normal room temperature to give a clear solution?
    Many thanks
    David Ings

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