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Friday, 23 March 2012

Reflection of Waves

(Updated on 13/1/2015, Tues):

1. Reflection is a wave phenomenon: All waves - water waves, sound wave, electromagnetic waves (including light) - can be reflected. Reflection occurs when all or part of the waves are returned when the waves encounter or hit an obstacle.

2. When waves are reflected, their direction of travel changes.
    (SPM Physics: 2010 Paper 1, Q32 at pg. 238)

3. Since, upon reflection, waves direction changes, velocity of the waves therefore changes too.

(Thus, for the ensuing SPM Physics 2011 Paper 1, Q32: Which statement is correct when water waves are reflected by a reflector? The answer should not be A "The velocity of water waves before and after reflection are the same" as erroneously given in the sole booklet of Past Year Questions sold in bookshops throughout the country. It should be C "The amplitude of the water waves becomes smaller after reflection (due to imperfect return of wave energy as some wave energy is transformed to other forms of energy - e.g. heat and/or sound and as the reflected waves travel, damping occurs resulting in the amplitude eventually becoming zero - which is why all reflected waves eventually subside and all echos get fainter and finally disappear over time)

4. When reflection occurs: The waves speed v, frequency f and wavelength λ remains the same (v = λf).

5.  The amplitude and energy of the reflected waves will be lower because when reflection occurs, not all the wave energy are returned - some will be refracted or diffracted and some will be transformed into heat and/or other forms of energy (sound for water waves) and, over time, damping (overcoming resistance) occurs too.

6. Reflection of waves obeys the Law of Reflection:
  • 1) The angle of incidence i` = The angle of reflection r`; and 
  • 2) The incident waves, the reflected waves and the normal lie in the same plane. 

 7.  Crest and Trough of Water Waves Act as Lenses: In ripple tank experiments, you will note that there is a lamp placed above the tank shining light down through the water waves creating on the white paper placed below the tank a water waves pattern consisting of bright bands (representing the crests) and darker bands (representing the trough) . This happens because:
  • Crests act as convex lens - converging light - creating bright bands on the paper; whereas,
  • Troughs act as concave lens - diverging light - creating darker bands.
8. Mechanical stroboscope:
  • This device enables us to view moving water waves as stationary waves.
  • Water waves appear stationary when the water waves frequency, f is equal to the frequency of the stroboscope fs multiplied by the number of slits n on the stroboscope:  f = n x fs  
9. Experiments Relating to Reflection:
  • Water Waves - “Ripple Tank & Plane Reflector” Experiment 
  • Sound Waves - “Cardboard Tubes & Stopwatch” Experiment 
  • Light Waves - “Ray Box & Plane Mirror” Experiment 
10.Application of Reflection of Sound:

Echoes are caused by the reflection of sound. A sound wave will continue to bounce around or reverberate until it has lost all its energy. A wave loses some of its energy when it hits an objects before being reflected. The energy can be lost as heat - the amplitude of reflected sound wave and thus loudness of the sound get progressively lower over time.

The phenomenon of the reflection of sound is used to determine the distance between the two objects, for example depth of seabed, depth of cave or width of a valley. The type of sound used depend on the media in which the sound wave is to travel: For examples, for echoes through air, normal audible sound can be used; whereas, to penetrate human body, sea bed and the likes until reflection occurs, ultrasound of higher frequency/energy is used.

Sonar (Sound Navigation and Ranging) is used to detect underwater objects (corals / fishes) or to determine the depth of the water by means of an echo. Sonar equipment emits a high frequency sound signal (ultrasonic frequency) which is reflected by the object in the water. The reflected sound wave is received by the sonar receiver. The time taken for the echo to return is used to determine the distance of the object below the water surface. 

Sonar wave of high frequency (ultra-sonic sound) is used because it possesses more energy, high penetration power and can travel further through water.

(Edited by on 13/1/2015 - some graphics belong to Cikgu Wong of Penang, thanks!)

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