In vacuum, because ε= 1， μ= 1, the electromagnetic wave velocity is с。 In 1856, W.E. Weber and r.h.a. collausch measured с The value of is approximately equal to 3.1 × 10 ^ 10 cm / s. In 1862, j.b.l. Foucault measured the speed of light equal to 2.98 × 10 ^ 10 cm / s. The two values are very close. Since electromagnetic waves can also propagate in vacuum, if we think that light is electromagnetic waves, we don't need to introduce the assumption of ether at all. This solves the problem that Fresnel and others failed to solve. It is known from optics that the speed of light in a medium is the speed of light in vacuum divided by the refractive index n.

Because electromagnetic waves have mutually perpendicular electric field strength and magnetic field strength, the problem of what vector the so-called optical vibration vector corresponds to in electromagnetic theory arises. Strictly speaking, according to electromagnetic theory, the complete description of light wave requires the strength of electric field and magnetic field. But there is a certain relationship between them. Given the electric field strength, the magnetic field strength is determined at the same time. On the other hand, when studying the interaction between light wave and matter, it involves the interaction between electromagnetic field and charged particles (electrons and nuclei).

In general, the effect of magnetic field intensity is one factor smaller than that of electric field intensity υ/с。 here υ Is the velocity of charged particles, which is often much less than the speed of light с。 Therefore, it is considered that in general, the electric field intensity vector should correspond to the optical vibration vector.