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Index of refraction
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Index of refraction

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  • 1. Index of Refraction
    What factors affect the speed of light?
  • 2. Light Review
    Light travels through air at approximately 3.0 x 108m/s.
    The relationship between speed of light, wavelength, and frequency is expressed in the following equation:
    λ = wavelength (m) c = speed of light (m/s)
    ν= frequency (s-1) or (Hz)
    λ = c
    ν
  • 3. Light Review
    Let’s look at the equation another way:
    The relationship between frequency (ν) and wavelength (λ) is constant. If frequency increases the wavelength decreases.
    One way to change the speed of light is to change the medium or “thing” it is traveling through.
    c = λν
  • 4. For example…
    Pretend like you are skating on a pond.
    You skate to the edge, into a snowbank, and what happens?
  • 5. You slow down!
    You travel faster on the pond because it is icy and there is less friction.
    You travel slower and stop in the snow because it has more friction.
  • 6. Speed of light
    Light waves travel quickly through a vacuum and through gases. Gas has a very low density so light travels through it quickly.
  • 7. Speed of Light
    Liquids are much more dense than gases. Light slows down when it enters a liquid from a gas. This causes the light to bend or “refract.”
  • 8. Speed of Light
    Solids are even more dense than liquids. When light enters a solid it slows down even more.
    We can determine the identity of a solid by how fast light is traveling through it!
  • 9. Terms to Know
    The angle at which a beam of light changes mediums (gas to liquid, liquid to solid, solid to gas, etc.) is called the angle of incidence (θ1).
    Air
    θ1
    Water
  • 10. Terms to Know
    The angle at which a beam of light bends in the new medium is called the angle of refraction (θ2).
    Air
    θ1
    Water
    θ2
  • 11. Snell’s Law
    Snell’s Law describes the relationship between the angle of incidence and the angle of refraction. This ratio is called the index of refraction (n).
    (n1) (sin θ1) = (n2) (sin θ2)
    Air
    θ1
    Water
    θ2
  • 12. Let’s try an example:
    It is common for scientists to measure the amount of sugar or sucrose in a beverage using the index of refraction. A certain clear lemon-lime drink is analyzed to find the index of refraction. The angle of incidence is __30°__ and the angle of refraction is ___21°___. What is the index of refraction for the liquid?
    Air (n = 1.000)
    θ1
    Lemon-Lime Drink (n=?)
    θ2
  • 13. Example
    AirLemon Lime Drink
    (n1) (sin θ1) = (n2) (sin θ2)
    (1.000) (sin 30°) = (n2) (sin 21° )
    (1.000) (sin 30°)= (n2)
    (sin 21° )
    1.375 = (n2)
  • 14. In the Lab
    You will measure the index of refraction of three liquid compounds:
    Water (H2O)
    Methanol (CH3OH)
    Glycerol (C3H8O3)
  • 15. To measure the angles
    Use the protractor paper provided.
    Place the semi circle dish on the top “half” of the protractor.
    Line up the laser with the angle of incidence.
    Record the angle of refraction.
  • 16. Closure Questions
    How does the index of refraction of corn oil compare to air?
    How does the index of refraction of corn oil compare to water?
  • 17. Closure Questions
    How does the index of refraction of corn oil compare to Pyrex® (n=1.474)?
  • 18. Closure Questions
    What factors might change the index of refraction of a solution of 95% methanol in water?
  • 19. Closure Questions
    What are some ways scientists might use the index of refraction to identify materials?
  • 20. Index of Refraction Simulator
    http://phet.colorado.edu/en/simulation/bending-light