Final answer:
The cone of light is a region in space formed by the world lines of light beams passing through an event. It has sides at a 45° angle and remains the same in all inertial frames.
Step-by-step explanation:
The cone of light refers to the region in space where light beams pass through the origin event A. According to the postulates of relativity, the light cone has sides at an angle of 45° if the time axis is measured in units of ct. The cone of light remains the same in all inertial frames. Every point in the space-time diagram has a light cone associated with it.