Wave Properties
(A) Propagation of Waves
Water waves in a ripple tank are usually used in the laboratory to study the propagation of waves and properties of waves.
In a ripple tank, ripples produced in the tank are projected on a screen, with bright lines representing wave crests and dark line representing wave troughs.
A wavefront of a water wave is a line of neighbouring points which are vibrating in phase. Therefore, the boundary of the shape of a wave crest is a wavefront. The wavefront is at right angles to the direction of travel of the wave.
A ray is a straight line with an arrow that shows the direction of travel of the waves.
(B) Wave properties
Water waves show reflection, refraction, diffraction and interference. All types of waves should display all these four properties.
(i) Reflection
When water waves are reflected, they obey the laws of reflection: the angle of incidence is equal to the angle of reflection.
(ii) Refraction
Refraction takes place when water waves pass from deep water to shallow water.
As the waves enter the shallow water,
•the wave speed decreases,
•the wavelength decreases,
•the refracted waves bend towards the normal.
When water waves pass from shallow to deep water, the opposite occurs. In all cases, the frequency stays the same.
(iii) Diffraction
Diffraction is the bending of waves around corners without a change in wave speed.
Diffraction is more prominent if the slit or obstacle is comparable in size to the wavelength. The narrower the slit, the more the wavesspread out from it. The smaller the obstacle, the more the waves bend round it.
(iv) Interference
Interference is the overlapping of two identical waves. It is observable when two sources produce waves that are in phase and of the same frequency and waves overlap each other.These sources are called coherent sources. (Two sources are said to be coherent if they have a constant phase different.)
Constructive interference occurs when two waves that are in phase reinforce each other to form a wave of greater amplitude.
Destructive interference occurs when two waves that are in antiphase cancel each other to give an area of calm water.
The lines joining the places of destructive interference are called nodal lines. The lines joining the places of constructive interference are called antinodal lines.
Conditions of interference (when the two sources are vibrating in phase):
Constructive interference takes place at a position where the path difference from the two sources is equal to whole number ofwavelengths, i.e., 2, 3 and so on.
Destructive interference takes place at a position where the path difference from the two sources is equal to, , and so on.
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Wave Properties