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    Prism, Internal reflection, Rainbow

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    (a) You have a large triangular prism made of silica (ns = 1.45) resting on a table. The internal angles of the triangle are 90 degree and 45 degree. You also have a laser beam (wavelength = 633 nm) set up parallel to the tabletop.

    How would you point the laser beam onto the prism to make the light travel through the prism then leave in a direction

    i. at 90 degree
    ii. at 180 degree

    to the direction the light was traveling when it hit the prism?

    For each case, draw a neat schematic diagram of the prism and laser beam showing all the relevant information.

    (b) To maximize the light leaving the prism, someone suggests a coating of glass (ng = 1.6), but they only have enough material to coat up to two sides of the prism with thin films. For each of the configurations above, choose which side(s) of the prism, if any, you would get coated. Justify your choice.

    Hint: Make use of total internal reflection if you can.

    (c) If you wanted to create a right-angled triangular prism that split sunlight into its different colours, what property would it need to have?

    (d) A rainbow effect can also be seen if sunlight shines on:

    1. a film of oil on water
    2. a prism coating with variation in its thickness

    Explain this phenomenon using your understanding of physics. Make it clear how this is different from the phenomenon described in (c).

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    Solution Preview

    Please see the attached document for the solutions.

    The concept of total internal reflection is involved in the given questions. Let me explain this for a while.

    When any light travels from denser to rarer medium and incidents at an angle (say i) which is larger than a particular value (called critical angle), the light will reflect in the same medium itself. This process is called total internal reflection.
    In a given medium, the value of the critical angle can be determined from the process from,
    Where the n's used are the refractive indices of the respective media.

    In the given question, the prism is made up of a material of n = 1.45. So the ...

    Solution Summary

    Prism, internal reflection and rainbows are examined in the solution.