In vacuum, the speed of light

In a vacuum, the speed of light (c) is a fundamental physical constant, approximately 3 × 10^8 m/s [1]. Unlike in material media where light experiences dispersion, the speed of light in a vacuum is independent of its wavelength and frequency. While the wave equation v = λf holds true, any change in frequency is perfectly compensated by a change in wavelength to maintain the constant speed c. Furthermore, the speed of light in a vacuum is determined by the electromagnetic properties of free space—specifically the vacuum permittivity (ε₀) and permeability (μ₀)—and does not depend on the intensity or amplitude of the wave. In contrast, material media cause light to slow down based on the refractive index, which often varies with wavelength, leading to phenomena like dispersion [1]. Therefore, in a vacuum, light speed remains invariant regardless of its properties.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > QUESTIONS > p. 150