Quiz - Physics - Visual Reversal

[Pedro Burrito]

I apologize for the delay. Some mf'er with a backhoe has cut a fibre cable and internet, TV and telephony is out throughout my area from early this morning until tomorrow evening. Everyone is still working from home so there are many angry Suisse right now.

I've been in Suisse for almost three years now. I've noticed that one of their favorite sports is digging. It doesn't matter what, they dig. All utilities are underground as well as the required bomb shelters that everyone must have. So they dig. Often without notice, some guys will show up, block the road and dig a trench, causing traffic delays for weeks. The elite diggers, however, are the ones that cut something. Then they will light their cigarettes and ponder the vagaries of a world where they cannot take part in their passion without cutting something. This takes hours before they will actually report the problem, if they ever do. Swisscom did not even know what the problem was for about 7 hours!

Without further ado, our next question.

From the Physics Section of the PADI Dive Theory Exam:

A phenomenon called "visual reversal" makes objects viewed underwater appear ___ than they actually are.

a. closer

b. more distant

c. brighter in color

d. darker in color


I will post a daily question from my exams to help newer divers and to encourage more experienced divers to interact gracefully and helpfully with the newer divers.

Reminder - this is a post in the Basic Forum and it is a green zone. Please be nice and on topic.

Thank you for your patience while we try to give people something to discuss other than Covid-19 and/or Politics. I will post the answer covered by the spoiler tag later today.

 

[Pedro Burrito]

Spoiler: Answer

b. more distant

 

[agilis]

I voted closer because that's the conventional wisdom. Actually both A and B are correct. Objects at a distance are usually perceived as being further away than they actually are because of a lack of clarity and peripheral detail. There's a psychological name (Ganzfeld?) for this, but I can't quite recall. Objects only a few feet away will seem closer, but only these nearby objects are affected by the phenomenon.

 

[HKGuns]

There were a lot of fiber cuts in the US a couple of weeks ago. We had three corporate fiber paths cut in a single day. They were triple redundant, the odds of that happening under normal circumstances is astronomical, even higher when no-one is supposed to be working.

 

[TMHeimer]

I voted closer because that's the conventional wisdom. Actually both A and B are correct. Objects at a distance are usually perceived as being further away than they actually are because of a lack of clarity and peripheral detail. There's a psychological name (Ganzfeld?) for this, but I can't quite recall. Objects only a few feet away will seem closer, but only these closer objects are affected by the phenomenon.

I was gunna say there is no possibility THIS one could be argued. You are right. I was wrong. We do know what they're getting at with the question -- most will get it right because of what they've read. But your "only these closer objects" I'm sure is correct, but not something I read in the PADI materials.
It's a clear question based on what an average diver has read in the manuals.

 

[scubadada]

I believe refraction is responsible for closer and larger, turbidity causes visual reversal, and the answer to this question.

 

[michael-fisch]

And I thought that the mandatory bomb shelters were only necessary in permanent dwellings, IOW all the weekend residences were exemt.
Michael

 

[tursiops]

I believe refraction is responsible for closer and larger, turbidity causes visual reversal, and the answer to this question.

Agreed. Both effects are in play. Up close, you don't notice the turbidity as much, so refraction (and the objects being apparently closer/larger) dominates. Further away, turbidity dominates, and you get the visual reversal.

Funny story. I worked with an optical oceanographer for a while who had a project that involved visual estimates of visibility distance...he was looking to correlate instrumented measurements of optical clarity with the visual estimates. He got a bunch of Navy SEALs to gather a lot of his data for him as part of some of their in-water training time. As time went on, in unchanging conditions, the viz nevertheless got larger and larger....turns out it was just each group of SEALs trying to outdo the group before them..."I can see farther than you can!"

 

[TMHeimer]

Agreed. Both effects are in play. Up close, you don't notice the turbidity as much, so refraction (and the objects being apparently closer/larger) dominates. Further away, turbidity dominates, and you get the visual reversal.

Funny story. I worked with an optical oceanographer for a while who had a project that involved visual estimates of visibility distance...he was looking to correlate instrumented measurements of optical clarity with the visual estimates. He got a bunch of Navy SEALs to gather a lot of his data for him as part of some of their in-water training time. As time went on, in unchanging conditions, the viz nevertheless got larger and larger....turns out it was just each group of SEALs trying to outdo the group before them..."I can see farther than you can!"

That's what I thought. My own thought is that when stuff is further away in turbid water it also makes them harder to see--ie to figure out exactly what they are (well, here I know that 99% of them are rocks....). But that's just a thought.

 

[dorsal]

There is the Padi answer and the technically correct answer.