Just as echoes from within a canyon give a clue to the relative distance of the canyon walls the same applies to light even though it travels at ~300,000Km/s
When a wall-light is switched on the immediate wall is lit first then the ceiling and floor and finally the opposite wall - and when the light is switched off this wall 'disappears' last but its all too quick for the eye to see!
The same applies on a cosmic scale but because of the huge distances between space objects a burst of light [even travelling at 300,000km/s] can take months and even years to illuminate its surroundings - a classic example are the light echos surrounding the exploding star called V838 Mon below as recorded by the Hubble ST over some months.
A burst of light can therefore be used to probe space to build up over time a 3D image surrounding the light source and is an invaluable tool for the astronomer
Further light echos references found in
The Astronomer AGM video on this forum
