When a particle moves through space, it carries kinetic energy itself wherever the particle goes, the energy goes with it. The energy is associated with the particle and is transported from one region of the space to the other together with the particle just like we ride a car and are taken from Amity Noida to Amity Bijwasan with the car.
There is another way to transport energy from one part of space to the other without any bulk motion of material together with it. Sound is transmitted in air in this manner. When you say “Hello” to your friend, no material particle is ejected from your lips and falls on your friend’s ear. You create some disturbance in the part of the air particles either by pushing them ahead or putting them back. The density of the air in this part temporarily increases or decreases. These disturbed particles exert force on the next layer of air, transferring the disturbance to that layer of air. In this way, the disturbance proceeds in air and finally the air near the ear of the listener get disturbed. The disturbance produced in the air near the speaker travels in air, the air itself does not move. The air that is near the speaker at the time of uttering a word remain all the time near the speaker even when the message reaches the listener. This type of motion of energy is called a wave motion.
A wave that travels from one point of the medium to another is called a progressive wave. A progressive wave may be transverse or longitudinal.
Plane Progressive Harmonic Wave
If during the propagation of a wave through a medium, the particles of the medium vibrate simple harmonically about their mean positions, then the wave is said to be plane progressive
harmonic wave. In a harmonic progressive wave of given frequency, all particles have same
amplitude but the phase of oscillation changes from one particle to the next.