The propagation of light without a medium is a fundamental concept in physics known as the wave nature of light or electromagnetic waves. Light, as well as other forms of electromagnetic radiation, can travel through a vacuum or empty space without requiring a material medium to propagate.
The understanding of light as an electromagnetic wave originated from the work of James Clerk Maxwell in the 19th century. Maxwell's equations mathematically described the behavior of electric and magnetic fields and their interactions, leading to the prediction of electromagnetic waves. These waves were found to consist of oscillating electric and magnetic fields that are perpendicular to each other and also perpendicular to the direction of propagation.
When an oscillating electric field is generated, it induces a perpendicular oscillating magnetic field, and vice versa. These fields mutually support each other and propagate together as electromagnetic waves. The changing electric field produces a magnetic field, and the changing magnetic field generates an electric field. This self-sustaining process allows the wave to propagate through space.
In a medium such as air, water, or a solid material, light can also propagate by interacting with the atoms or molecules of the medium. This interaction leads to phenomena like absorption, scattering, and refraction. However, in a vacuum, where there are no particles to interact with, light still propagates as an electromagnetic wave.
The key point is that electromagnetic waves do not require a material medium for their propagation. Instead, they rely on the interaction between electric and magnetic fields, which can exist independently in space. This wave-like behavior of light allows it to travel through the vacuum of space, making it possible for us to observe distant stars, galaxies, and other celestial objects.