Why can you not see through fog when it’s ahead of you but you can once you’re in it?
In: Physics
Most of these answers are assuming you’re asking about a situation that isn’t concerned with light source or direction, and that you are _only_ concerned with whether you can see through the _density_ of fog.
Let’s say the fog covers 5km^2 with whatever density is needed to make your hand nearly hidden at 2ft. That’s a lot of fog. Now, does it matter if that fog is 10km^2 instead? How about 2km^2? If you are 1km in and thus 1km closer to the edge, is that shortened distance to the end of the fog enough to make a difference in whether you can see your hand? No, not really.
Now let’s get closer to the answer to the question I think you’re asking. Let’s say _you_ are not in the fog; your eyes are 1ft outside the boundary, but your hand is 1ft inside the boundary. Let’s also assume this is during the day with the sun overhead. What will happen? More than likely, you won’t see your hand at all. But why? You could sort of see it when you were _in_ the fog, but now there is the distance of 1ft of fog between your hand and eyes and you can’t see your hand at all.
You’re experiencing, in part, [refraction](https://en.m.wikipedia.org/wiki/Refraction) and [reflection](https://en.m.wikipedia.org/wiki/Reflection_(physics)). This means a few things are occurring:
* The light coming from the sun is reflecting off the fog boundary into your eyes
* The light is not penetrating deeply enough into the fog because of refraction, which means less light is bounced off your hand
* The light that does make it to your hand is also refracted on its way back out toward your eyes
The first point is key. Essentially, when you are outside the fog, the light coming into your eyes from the fog boundary is brighter than anything else in the fog, so that’s what you end up seeing. When you’re in the fog, you don’t have that reflection off the boundary, so the scattered and refracted light is more visible to your eyes. This is also why it’s a bad idea to turn on your car’s brights when you’re in fog, as it so happens.
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