Momentum is defined as

Where V is the velocity, P is the momentum, and m is the mass.

For the continuum medium, the momentum density is defined as

Where

is the momentum density,

is the mass density, V is the field velocity.

As we know, the electromagnetic field momentum density is:

The electromagnetic field energy density is given as:

Based on the relationship between mass and energy

Then we can get the mass density of electromagnetic field as:

Thus the velocity of the electromagnetic field is given as:

Thus

Then

From the above velocity equation of electromagnetic field, we find that the value of velocity is always less than the speed of light c, if and only if when the following two conditions are satisfied:

And

The value of electromagnetic field velocity will become the speed of light c.

The equation can be rewritten as following: