Wave-length.—The distance between two successive crests will be the same everywhere, and this distance or the distance between any two corresponding points on two successive ripples is called the wave-length. Evidently, the wave-length is the distance in which the whole wave repeats itself.
Phase.—The position of a point in the wave is called the phase of the point. Thus the difference of phase between the two points A and C is a quarter of a wave-length. As the waves move on along the surface it is evident that each drop of water executes an up and down oscillation, and at the points C, C the drop has reached its highest position and at the points T, T its lowest.
Amplitude.—The largest displacement of the drop, i.e. the distance from the dotted line to C or to T, is called the amplitude of the wave. The time taken for a drop to complete one whole oscillation, i.e. the time taken for a wave to travel one whole wave-length forward, is called the period of the wave. The number of oscillations in one second, i.e. the number of wave-lengths travelled in one second, is called the frequency.
FIG. 11.
Although there is no visible displacement in the waves of light and heat, yet we may represent them in much the same way. Thus if AB, Fig. 10, represents the line along which a ray of light is travelling, the length NP is drawn to scale to represent the value of the electric field at the point N, and is drawn upwards from the line AB when the field is in one direction and downwards when it is in the opposite direction.
Thus the direction of the field at different points in the wave XY, Fig. 11, is shown by the dotted arrows as if due to electrified rods of quartz and ebonite placed above and below XY.
In the case of the electromagnetic wave, the amplitude will be the maximum value to which the electric field attains in either direction, and the other terms—wave-length, phase, period and frequency—will have the same meaning as for water ripples.
Wave Form.—Waves not only differ in amplitude, wave-length, and frequency, but also in wave form. Waves may have any form, e.g. Fig 12. Or we may have a solitary irregular disturbance such as is caused by the splash of a stone in water.