Its unit is second. Frequency is defined as number of waves produced in one second. Waves are generally produced by disturbance in a medium. Examples are water waves and light waves. Longitudinal waves are those which travel in the same direction as its displacement.
Example is sound wave.
If the light source is extended in a plane, light pulses are like traveling plane surfaces. These imaginary surfaces are called Wave fronts. Any point on the wave front has the same physical property as another point on the same wave front. Wave fronts are nearly plane surfaces when the light source is at a long distance from where it is observed. Let us consider two identical waves.