+Oscillation of Electric Charges within the Atom.+--We have seen how these waves might be produced by the oscillation of two electrified rods, and it is supposed that the light coming from luminous bodies is produced in a similar way. There are many reasons for believing that there exist in the atoms of all substances, minute negatively electrified particles which may rotate in small orbits or oscillate to and fro within the atom. There also exists an equal positive charge within the atom. As the negative particles rotate or oscillate in the atom, it is evident that the field between them and the positively electrified part of the atom alternates, and so electro-magnetic waves are sent out. {22} CHAPTER II GRAPHIC REPRESENTATION OF WAVES A system of ripples on the surface of water appears in vertical section at any instant somewhat as in Fig. 10. The dotted line AB represents the undisturbed surface of the wafer, and the solid line the actual surface. If the disturbance which is causing the ripples is an oscillation of perfectly regular period the individual ripples will be all alike, except they will get shallower as they become more remote from the disturbance. [Illustration: FIG. 10.] +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 {23} 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. [Illustration: 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.