P5g Refraction Of Waves Investigation Notes

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  • 1. P5g - Refraction of waves Refraction is…
  • 2. Refraction
  • 3. Refraction
    • When waves slow down, the wavelength gets shorter
    • When waves speed up, the wavelength gets longer
    • This causes the wave to change direction unless it is traveling along the normal.
  • 4. Refraction investigation
    • Intro
    • You are going to calculate the refractive indices and critical angles for glass and perspex blocks
  • 5. Refraction investigation
    • Method
    • Watch the brief intro
    • Draw up 2 results tables, one for glass and one for perspex
    • Collect your apparatus
    • Repeat each experiment at least 5 times using the same angle i.
  • 6. Refractive index (n)
    • n = speed of light in a vacuum
    • speed of light in the medium
    • n = 300 000 000 m/s
    • speed of light in the medium
  • 7. Refractive index (n) n = speed of light in a vacuum speed of light in the medium Q ~ What is n for a) air b) water 225 000 000 Water 299 900 000 Air 300 000 000 Vacuum Speed of light (m/s) Medium
  • 8. Snell’s law i r n = sin i sin r medium i medium r “ Angle of refraction depends on the refractive index”
  • 9. Snell’s law n = sin i sin r Q ~ If the medium is water (n = 1.33) and angle i is 30  , what is angle r ? r = 22  n = sin i sin r 1.33 = sin 30 sin r sin r = sin 30 1.33 sin r = 0.5 = 0.38 1.33
  • 10. Snell’s law n = sin i sin r Q ~ In another medium, if i is 30  and r is 12  , what is n? Q ~ If the medium is water (n = 1.33) and angle i is 40  , what is angle r ?
  • 11. Refraction investigation
    • Analysis:
    • Calculate n for air/glass and air/perspex interfaces using your experimental results
    • Pick one result from each material and use angles i and r
    • Use Snell’s law (n = sin i / sin r) to work out your values for n.
  • 12. Refractive index between 2 media i r medium i medium r n = n r n i
  • 13. Refractive index between 2 media i r water (n = 1.33) air (n = 1.0) = 0.75 n = n r n i n = n air = 1.0 n water 1.33
  • 14. Refractive index between 2 media i r water (n = 1.33) glass (n = 1.5) = 1.12 n = n r n i n = n glass = 1.5 n water 1.33
  • 15. Refraction investigation
    • Analysis
    • You can now calculate the refractive indices (n) for glass and perspex
    • Use your results n air/glass and n air/perspex
    • Assume that n air is 1.0
  • 16. Refraction investigation i r air (n = 1.0) glass (n = ?) n = n r n i n glass = ? n = n glass = n glass n air 1.0
  • 17. Refraction investigation i r air (n = 1.0) perspex (n = ?) n = n r n i n perspex = ? n = n perspex = n perpex n air 1.0
  • 18. Refractive index and critical angle “ Total internal reflection”
  • 19. Refractive index and critical angle n = n r n i = sin i sin r = sin c sin 90 n r n i = sin c n r n i sin 90 = 1
  • 20. Refractive index and critical angle = sin c n r n i
    • Refraction investigation
    • Now, using your results you can calculate:
    • The critical angle for glass
    • 2) The critical angle for perspex
    • We will check your answers with a practical next lesson