3 ms·
Here is a lay explanation. Light travels at different speeds in different substances. But it always takes a path that is locally fastest - meaning that any ne
by btilly 6y ago
Here is a lay explanation.
Light travels at different speeds in different substances. But it always takes a path that is locally fastest - meaning that any nearby path would be slower. (This is called the Fermat principle.)
You can see this principle at work when you put a stick into water. Because light travels more slowly in water, the light first heads mostly straight up, then bends when it hits the air. The result is that light does not travel a straight path to your eye. Which means that the part of the stick in the water looks like it is where the light comes out of the water, rather than where the stick is. As a result you can see the stick visibly bend.
Now what is happening here is that you have a layer of warm air over cold air. Light travels faster in warm air. (That is because as air warms it expands, making it less dense. Less dense means that there is less getting in the way of the light and it can move faster.) Therefore that fastest path is for the light to go up into the warm air, go along the warm air, and then dive back down to your eyes.
In many places you can see the reverse of this on hot days where hot ground makes for a hot air layer next to the ground. When the conditions are right the light from the sky can reach your eyes by skimming along the ground, and you get blue patches in the ground. In a desert this can look like water in the distance.
- cevn 6y agoOK so, that's how the air ripples work for heat... because of the density of the air. It makes me wonder, if you had a 10 inch globe of vacuum suspended in air, how different looking thru it would appear vs the air. It seems like you might be able to detect it via sight alone.
- FreeFull 6y agoYeah, it'd definitely be visible. I imagine it'd be like looking at an air bubble while underwater, in terms of refraction, except that air doesn't have surface tension so the boundary wouldn't look quite the same.
- dredmorbius 6y agoThat's what a visible shockwave from an explosion is. Light bending through sharp pressure discontinuities. https://i.redd.it/jbx7x4a66em21.jpg https://i.redd.it/jbx7x4a66em21.jpg https://i.ytimg.com/vi/_31RYF3p42E/maxresdefault.jpg https://i.ytimg.com/vi/_31RYF3p42E/maxresdefault.jpg http://waitbutwhy.com/wp-content/uploads/2014/10/explosion-in-super-slow-motion-to-see-the-destructive-power-of-the-shockwave-in-action..gif http://waitbutwhy.com/wp-content/uploads/2014/10/explosion-i... The vapour cloud which often forms occurs in saturated air as a partial vacuum forms behind the shockwave: https://frontlinevideos.com/wp-content/uploads/2018/05/massive-car-bomb_525595f2-05d3-488f-89bc-9a34aa5c41e0.jpg https://frontlinevideos.com/wp-content/uploads/2018/05/massi... https://media.tenor.com/images/a6572b4115f67f2aeea90c529d598741/tenor.gif https://media.tenor.com/images/a6572b4115f67f2aeea90c529d598...