-- The speed of light in air is very close to 3 x 10⁸ m/s. Whatever the actual number is, it's equivalent to roughly 7 times around the Earth in 1 second. So for this kind of problem, you can assume that we see things at the same time that they happen; don't bother worrying about how long it takes for the light to reach you.
-- For sound, it's a different story. Sound in air only travels at about 340 m/s. It takes sound almost 5 seconds to go 1 mile.
-- Now, the lightning and thunder happen at the same time. The light travels to you at the speed of light, so you see the lightning pretty much when it happens. But the sound of the thunder comes poking along at 340 m/s, and arrives AFTER the sight of the lightning.
The length of time between the sight and the sound is about 99.9999% the result of the time it takes the sound to reach you.
If the thunder arrived at you 3 seconds after the light did, then the sound traveled
(340 m/s) x (3 s) = 1,020 meters . (about 0.63 of a mile)
(If you're worried about ignoring the time it takes for the light to reach you ...
It takes light 0.0000034 second to cover the same 1,020 meters,
so including it in the calculation would not change the answer.)
Explanation: HIV is a viruse that only shows symptoms after a few weeks. in Fact HIV does not show any physical symptoms. So, no you can not tell if someone has or is been infected by HIV by directly looking at them.
