Why can you not see through fog when it’s ahead of you but you can once you’re in it?
In: Physics
It’s a bit of a perception illusion.
As the top commenter pointed out, sometimes you can see through it (if it’s a small patch of fog) because once you are in the fog, there is less ‘density’ of fog between you and the objects on the other side of the fog.
But I think your real question is, why does it seem that you can’t see anything that’s IN the fog, but once you’re in the fog, you can see some things that are out of the fog.
This is where the illusion comes in.
When you are outside of the fog, looking into it, it seems like you can’t see anything in there because perceptually your wider vision is clear and this one area is obscured. You actually CAN see pretty far into the fog (equally as far as you can see out of it) but you eyes and brain are adjusted to the light level and clear landscape around you. Once you are inside the fog, you adjust to the low-contrast environment and it becomes easier to pick out faint objects that are outside of the fog (as the top commenter said, this is also aided by the fact that there is less ‘density’ of fog between you and an outside object in this case).
Another thing contributes to this perceptual illusion. Often when we encounter fog we are driving. If there is a patch of fog ahead that you are driving into, it can appear totally opaque party because there is only open road in front of you, IE there are no tall, distinct objects in the road that you would otherwise be able to faintly make out. Once you are IN that patch of fog, the trees / houses / ETC on the side of the road are visible because they are distinct objects, giving the slight illusion that you just drove into an opaque area but can somehow now see things in that area. In other words, it wasn’t as opaque as you though, it’s just that there wasn’t very much directly ahead of you TO SEE (except open road or faded pavement, which don’t stand out in contrast through the fog.
A (freaky) example of this is if you’ve ever had the misfortune to encounter a deer in the road in a patch of fog (I’m looking at you, Vermont State highways). It looks like a patch of dense fog with a deer just floating in the middle of it. In reality, the deer is standing on the road, but the faded pavement and striping can’t be made out in the fog and neither can the clear road / sky beyond. What you realize in this case is that you actually CAN see pretty far into the fog, it’s just that there isn’t always much TO SEE.
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