When visible light travels through a vacuum or empty space without encountering any objects, it continues to propagate in a straight line. This phenomenon is known as "free space propagation" or "rectilinear propagation." The behavior of light in such conditions is governed by the laws of physics, specifically the principles of wave optics.
Visible light consists of electromagnetic waves with various wavelengths within the range of approximately 400 to 700 nanometers. These waves do not require a medium to travel; they can propagate through a vacuum. As light travels, it maintains its frequency, wavelength, and speed.
In the absence of obstacles or interactions, light will continue to travel indefinitely in a straight line at a constant velocity of approximately 299,792 kilometers per second in a vacuum (usually denoted as "c"). This speed is the fastest possible speed in the universe and is often referred to as the speed of light.
So, when visible light travels through empty space without encountering any objects, it essentially continues its journey indefinitely, propagating as a straight beam of light until it eventually encounters an object, such as a planet, star, or other matter, or until it dissipates over extremely long distances due to various factors such as the expansion of the universe.