Why does speed become constant?

[jonny456]

Now I have a basic idea of why it does, I’m just wondering what would be the best way to word it.

Also, does anyone have a idea on how to work out a value for initial acceleration off a graph? Is it the gradient of the line?

Any help would be greatly appreciated.

[Zephir]

...you're using the light both for space distance, both time interval measurements at the same time.