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Do now!

Do now!. You have 10 minutes to complete your Electromagnetic spectrum “mini-book”. Can you also hand in the Refraction practical to Mr Porter?. Wave Properties. Refraction, diffraction and superposition. Today’s lesson. Refraction of light. The mug trick!. The mug trick!. Refraction.

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Do now!

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  1. Do now! You have 10 minutes to complete your Electromagnetic spectrum “mini-book”. Can you also hand in the Refraction practical to Mr Porter?

  2. Wave Properties Refraction, diffraction and superposition

  3. Today’s lesson • Refraction of light

  4. The mug trick!

  5. The mug trick!

  6. Refraction When a wave changes speed (normally when entering another medium) it may refract (change direction)

  7. Water waves Water waves travel slower in shallow water

  8. Sound waves • Sound travels faster in warmer air

  9. Light waves Light slows down as it goes from air to glass/water

  10. Snell’s law There is a relationship between the speed of the wave in the two media and the angles of incidence and refraction Ray, NOT wavefronts i r

  11. Snell’s law speed in substance 1 sinθ1 speed in substance 2 sinθ2 =

  12. Snell’s law In the case of light only, we usually define a quantity called the index of refraction for a given medium as n = c cm where c is the speed of light in a vacuum and cm is the speed of light in the medium c vacuum cm

  13. Snell’s law Thus for two different media sinθ1/sinθ2 = c1/c2 = n2/n1

  14. Refraction – a few notes The wavelength changes, the speed changes, but the frequency stays the same

  15. Refraction – a few notes When the wave enters at 90°, no change of direction takes place.

  16. A practical!

  17. Let’s try some refraction questions! Page 237 questions 5, 6, 7, 8 and 10 Homework due Wednesday 10th Feb

  18. Diffraction

  19. Diffraction Waves spread as they pass an obstacle or through an opening

  20. Diffraction Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

  21. Diffraction Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

  22. Diffraction That’s why we can hear people around a wall but not see them!

  23. Diffraction of radio waves

  24. Superposition

  25. Principle of superposition When two or more waves meet, the resultant displacement is the sum of the individual displacements

  26. Constructive and destructive interference When two waves of the same frequency superimpose, we can get constructive interference or destructive interference. + = = +

  27. Superposition In general, the displacements of two (or more) waves can be added to produce a resultant wave. (Note, displacements can be negative)

  28. Superposition Let’s try adding some waves!

  29. Interference patterns

  30. If we pass a wave through a pair of slits, an interference pattern is produced

  31. Path difference Whether there is constructive or destructive interference observed at a particular point depends on the path difference of the two waves

  32. Constructiveinterference if path difference is a whole number of wavelengths

  33. Constructiveinterference if path difference is a whole number of wavelengths antinode

  34. Destructive interference if path difference is a half number of wavelengths

  35. Destructive interference if path difference is a half number of wavelengths node

  36. Let’s try some questions! Page 243 Questions 4, 5, 6, 7.

  37. Tonight You need to read pages 195 to 209

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