Friday 23 March 2012

Diffraction of Waves


1. Diffraction refers to the spreading out of waves as they move through a gap or bend around an obstacle about the size of the wavelength or smaller. If the size (of the gap, aperture or obstacle) is significantly larger than the wavelength, the effects of diffraction would not be obvious.

2. Effects of Diffraction:
  • No change in: frequency (f), wavelength (λ) and speed (v) of the waves; and
  • Change in: direction and amplitude a of the waves upon diffraction. (Amplitude of diffracted waves < amplitude of incident waves; and direction of propagation changes and the waves spread out)
3. Experimental Evidence of Diffraction:
  • Water waves:
    • Through Slit - Ripple Tank & Metal Bars of Different Slit Sizes
    • Through Obstacle - Ripple Tank & Metal Bars of Different Sizes as Obstacles.
  • Light waves (wavelength about 10^-6 or 1 micrometer):
    • Narrow Slit Experiment and the Diffraction Pattern
  • Audible sound waves (wavelength about 1.5cm ~ 15 m):
    • Sound Heard from Hidden Corner Experiment
4. Application of Diffraction of Waves:
  • In transmission and reception of radio waves because diffraction enables waves to spread to areas behind obstacles, buildings, mountains or through tunnels
    • AM (Amplitude-Modulated) waves of long wavelength (186 m ~ 560 m) can spread around large obstacles like big buildings and hills
    • FM (Frequency-Modulated) waves of shorter wavelength (2.6 m ~ 3.4 m) can pass through smaller obstacles like tunnels and bridges.
  •  Operation of infra-red remote controls - infra- red light of even smaller wavelength compared to radio waves can be diffracted through even smaller obstacles found at home like the furniture, etc. - to turn on the TV and so on.
  • Study of Atomic Structure: Diffraction of X-ray (wavelength in picometers) through atoms (atomic radius in picometers) enables scientist to study the atomic structure of a substance or chemical.
5. SPM Past Year Questions:
  • 2010 P1 Q32 pg. 238
  • 2008 P1 Q31 pg. 147
  • 2005 P1 Q29 pg. 9 (on AM waves form (Answer: A). What if the the question asks about FM waves form? (The answer would be: C). Click here to look at both the AM and FM wave forms)
6. Diffraction of Waves (In Very Simple Terms)
  1. Diffraction is the spreading of a wave as it goes through a narrow gap or passes round a small obstacle.
  2. The effect of diffraction increases when the width of the gap is decreased.

  3. The effect of diffraction increases when the wavelength is increased.

Application of diffraction in sea waves


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