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What might cause the speed of light to vary?
The speed of light varies depending on what kind of material it's traveling through. It's fastest when in vacuum, and it's different, and slower, in air, water, alcohol, oil, glass, jello, etc.
What is the ratio of speed of light in a vacuum to the speed of sound in air?
In a rough, round figure? 930,000 to 1. Sound travels roughly 1/5 of a mile per second in air. Light travels 186,000 miles per second in a vacuum. In five seconds sound travels one mile. In five seconds, light travels 930,000 miles in a vacuum.
Why is light speed always constant?
A great question. First--It isn't always constant. Light can travel slower than "c" ("the speed of light"). In a perfect vacuum light travels at "c". It's slower in glass, air, water, etc., light can even stop! But "c" is the upper speed limit. Second--Not only light but ANY disturbance in the universe has a speed limit of "c". Even the influence of gravity travels at the speed of light. Third--Einstein didn't just wake up one day with a good idea. The constancy of the speed of light (and relativity in general) explained many curious observations that had been made, and solved many physics problems. Fourth--There have been competing theories that allow a variable "c" but all the experiments indicate that it is indeed a constant value.
What is meant by optical density equals 1?
It means that the speed of light in that medium is the same as in vacuum. This is ALMOST the case for stuff like "air". If you measure closely enough, it is only true for vacuum, all other materials have an optical density > 1.
The speed of light and the speed of sound?
The speed of air at 20 degrees Celsius is 343 meters per second. The speed of light is 299 792 458 meters per second.
