Different wavelengths of light travel at the same speed in a vacuum because the vacuum does not interact with the electromagnetic waves in a way that depends on their wavelength or frequency.
- Nature of electromagnetic waves: Light is an electromagnetic wave composed of oscillating electric and magnetic fields. In a vacuum, there are no particles or material structures to influence the wave’s propagation differently based on its wavelength.
- Maxwell’s equations: These fundamental equations of electromagnetism predict that electromagnetic waves travel at a fixed speed cc in a vacuum. This speed depends only on the electric permittivity (ε0\varepsilon_0) and magnetic permeability (μ0\mu_0) of free space—not on the wavelength or frequency of the wave.
- No medium-induced dispersion: In materials like glass or water, light slows down and different wavelengths travel at different speeds due to interactions with the medium—a phenomenon called dispersion. But in a vacuum, there is no medium to cause such effects, so all wavelengths propagate uniformly.
- Constant energy transfer: Since energy in electromagnetic waves is carried by the oscillating fields and not through particle collisions or vibrations, all wavelengths—radio, visible, ultraviolet, etc.—carry their energy at the same fixed speed through the vacuum.