waves

The most familiar type of waves such as waves
on a string, water waves, sound waves, seismic
waves, etc. is the so-called mechanical waves.
These waves require a medium for propagation,
they cannot propagate through vacuum. They
involve oscillations of constituent particles and
depend on the elastic properties of the medium.
The electromagnetic waves that you will learn
in Class XII are a different type of wave.
Electromagnetic waves do not necessarily require
a medium - they can travel through vacuum.
Light, radiowaves, X-rays, are all electromagnetic
waves. In vacuum, all electromagnetic waves
have the same speed c, whose value is :
c = 299, 792, 458 ms–1.