Question

how practically speed of light is calculated ? How ?

Answer 1

Although the speed of light can be measured through some ideal means, I do not thing it's actually measured.
Light has a constant value if one is talking about standards.

It's the length of the path that light takes to travel in vacuum during a time interval of 1/299,792,458 of a second, that is, 299,792.458 km/s.

Because the speed of light in the vaccum is a constant, it makes perfect sense to say it's not really needed to be measured. But one can find different values when light travels in different media, the thicker the media ( say, honey for instance ) the slower it's supposed to travel.

Answer 2

The Apollo astronauts left mirrors on the moon which have been used to measure the speed of light. A terrestrial laser pulse is sent to the moon and the time it takes to come back to the Earth is measured. From this time and the known distance to the moon, the speed of light is calculated. Of course the speed of light was known long before the Apollo program but this was a pretty neat experiment.

Answer 3

The mirror on the moon is a distance measurement experiment. The orbit of the moon is getting a little bit larger with time.

Answer 4

So from studying MCAT material, I came across a passage that explained a way to measure the speed of light. On the bottom of the drawing is a light source that emits light to mirror A. Mirror A is partially-silvered (which I guess means some light is reflected to mirror B and some light goes right through). Concerning ourselves with the light that is reflected to mirror B and reflected back through mirror A, Shrek's cousin can then view this light through the Eye Piece (EP).

Now here is the interesting part. "W" is the toothed wheel, which I drew as a side view of what you can imagine as a gear (attached). Adjusting the wheel's rotation to at a particular known speed so that light from mirror A passes through one of the openings of the wheel toward mirror B, then passes through the adjacent opening on the way back to and THROUGH the partially silvered mirror A so that the observer can see...

After understanding this, if you want help with the mathematical calculation, lemme know..