The length of time that it takes for the light from the outburst to pass the observer indicates the size of the AGN
light is a form of electromagnetic radiation emitted by objects like stars, light bulbs etc. It is a kind of kinetic energy which enables us to see the world around us.
The amount of time taken for light to travel between two points is usually taken into consideration when the distance is very large. The most commonly used unit of time with respect to the distance travelled by light is a light year which is used to determine the distances between stars and galaxies in outer space.
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