[Physics Form 5] The Superpose

Situation 1:
Two circular water waves of the same frequency that are in phase, or of a constant phase difference, are produced adjacent to each other.

These two waves, such as S₁ and S₂ in the below diagram, are known as coherent waves.

Due to their close proximity, the wavefronts of the waves overlap. This sets off the phenomenon of interference.

Shown in the diagram is the plan view, or view from the top, of the overlapping waterfronts. The full circle represents the crests or peaks while the dotted lines represent the troughs. The crests and troughs form circles of points that are in phase.

Question & Answer

1. What do line P and point L represent?
   Due to interference:
   All crests of S₁ and S₂ that are in phase overlap each other. The same goes to all troughs of S₁ and S₂. This reinforces the amplitude of the waves and is known as constructive interference, represented by line P.
   
   The meeting of a crest of S₁ and trough of S₂, and vice versa, causes the cancellation of the amplitude of the waves. This is known as destructive interference, which is represented by point L.
   
   
2. What can be done to analyse the phenomena occurring at P and L?
   By placing a floating object on the surface of water along line P and point L, the movement of object can be observed and the phenomena analysed.
   
   
3. How are the sources, represented by S₁ and S₂, produced using;
   
   • Water waves
     By using two circular dippers fixed to a motor - both the dippers will vibrate at the same frequency and be in phase, as dictated by the motor.
     
     
   • Light waves
     Light from a single bulb is incident on a double slit. Each slit acts as a secondary source of light that has the same frequency and phase.