Quintessentially Physics

Sunday, 29 July 2012

Interference is the effect of superposition of two waves from two coherent sources.
Coherent waves are the waves that can produce same frequency, same amplitude and same phase ( or with constant phase difference ).

The principle of superposition states that when two waves meet at the same point, the resultant displacement at that point is equal to the vector sum of the displacements produced by the individual waves at that point.

The constructive interference occurs when two crests or two troughs meet each other.

Diffraction is the spreading of a wave as it goes through a narrow gap or passes round a small obstacle.
The effect of diffraction increases when the width of the gap is decreased.

During the day, the layers of air near the ground is warmer than the layers of air higher up.
Air layer near the ground is less dense.
Therefore, sound wave is bent toward the normal.

At night, the layers of air near the ground is cool faster than the layers of air higher up.
Air layer near the ground is denser.
Sound travels faster in warm air than in cool air resulting increasing of wavelength.
When the angle of incidence is larger than the critical angle, total internal reflection occurs.
Therefore, the path of the sound curves reflects downwards and you can hear the sound easily at night.

Click on the diagram below to play!

In the centre of the ocean, the wavefronts are straight and parallel to each other. This is because the water waves travel at uniform speed as the depth of the sea water is uniform.
When the waves reach the coast, the water is shallower.
Wave speed is reduced and refraction occurs. The wavefronts are refracted and become closer to each other. Wavelength decreases.
Refraction causes the wavefronts to be bent towards the normal.

Wave refraction causes wavefronts to parallel the shape of the coastline as they approach shore.
Wave refraction also causes wave energy to converge at cape and resulting erosion.
Wave energy diverges at bay and spreads out to a wider region, resulting deposition of sand. Therefore, amplitude of waves at the bay is smaller than at the cape.

Refraction, or the bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves.
The most significant property of water that would affect the speed of waves travelling on its surface is the depth of the water
Water waves travel fastest when the medium is the deepest. Thus, if water waves are passing from deep water into shallow water, they will slow down.
This decrease in speed will also be accompanied by a decrease in wavelength.

So as water waves are transmitted from deep water into shallow water, the speed decreases, the wavelength decreases, and the direction changes.

Click on the diagram below to play!

There are two laws of reflection.
Law 1
The normal and the incident and reflected rays are coplanar. They lie in the same plane.

Law 2

The angle of incidence, i = the angle of reflection, r

Reflection of Circular Wave

Resonance occurs when the forced frequency of an external agent equals the natural frequency of the oscillating system.
When resonance occured, it produces maximum amplitude of vibration.

Resonance can be proved by Barton Pendulum experiment

Resonance causes Tacoma Bridge to collapse