Quiz - Physics - Colors

[Pedro Burrito]

From the PADI Dive Theory Exam:

As depth increases underwater colors disappear due to water's ability to _____ light. The first color to disappear is ____.

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.

 

[Storker]

The physics is slightly different in cold salt water, where we often have quite a bit of microscopic particles - usually microplankton - in the water. This makes a secondary phenomenon significant. The same phenomenon is seen in rivers fed from glaciers, although the particles in those rivers are clay rather than microplankton

Bonus question for the geeks: What is this secondary phenomenon, how does that affect the color of the water, and why?

 

[tursiops]

Bonus question for the geeks: What is this secondary phenomenon, how does that affect the color of the water, and why?

Spoiler

Geek here: scattering, type depends on the size of the particles. If really small particles, Rayleigh scattering, tends to give a blue color (Why is the sky blue?), so even in shallow water things look blue. That's from memory, how'd I do?

 

[Storker]

Spoiler

Geek here: scattering, type depends on the size of the particles. If really small particles, Rayleigh scattering, tends to give a blue color (Why is the sky blue?), so even in shallow water things look blue. That's from memory, how'd I do?

Very close, but not quite a cigar.

Scattering is correct. And particles between the light source and the observer scatter the light. However, the shorter the wavelength the stronger the scattering. Which is why sunsets are reddish (the blue light is scattered and doesn't reach your eyes directly) while the sky is blue (the scattered light comes from everywhere in the sky).

Bottom line, when the water absorbs the reds and yellows, and the particles scatter the blues, all you're left with is the greens. And cold water is green water.

https://flic.kr/p/DhErBa

 

[tursiops]

Spoiler

Very close, but not quite a cigar.

Scattering is correct. And particles between the light source and the observer scatter the light. However, the shorter the wavelength the stronger the scattering. Which is why sunsets are reddish (the blue light is scattered and doesn't reach your eyes directly) while the sky is blue (the scattered light comes from everywhere in the sky).

Bottom line, when the water absorbs the reds and yellows, and the particles scatter the blues, all you're left with is the greens. And cold water is green water.

https://flic.kr/p/DhErBa

Nice. Thanks. My only serious cold-water diving has been the St Lawrence seaway (ice diving class) and Antarctica. I don't remember green.......

 

[Storker]

My only serious cold-water diving has been the St Lawrence seaway (ice diving class) and Antarctica. I don't remember green....

Well, when the water is that cold, plankton life is somewhat inhibited. Besides, when it's that cold, there's also (usually) less light to fuel the plankton growth. So, less particles in the water body. During winter, when there's little to no life going on, our waters are more blue-ish.

I perhaps should have said "moderately cold water"

 

[Pedro Burrito]

Spoiler: Answer

b. absorb/red

I do not see colors well, particularly red. My instructor for deep dives wore a red shirt to demonstrate the loss of red at depth. I spent the entire dive shining a light on his shirt to see if I could tell the difference. He was annoyed with me.

 

[BlueTrin]

Spoiler: Answer

b. absorb/red

I do not see colors well, particularly red. My instructor for deep dives wore a red shirt to demonstrate the loss of red at depth. I spent the entire dive shining a light on his shirt to see if I could tell the difference. He was annoyed with me.

This is a hilarious story.

Fortunately he didn’t choose to wear a red mask to demonstrate

 

[edwants2dive]

Nice. Thanks. My only serious cold-water diving has been the St Lawrence seaway (ice diving class) and Antarctica. I don't remember green.......

Well, when the water is that cold, plankton life is somewhat inhibited. Besides, when it's that cold, there's also (usually) less light to fuel the plankton growth. So, less particles in the water body. During winter, when there's little to no life going on, our waters are more blue-ish.

I perhaps should have said "moderately cold water"

When the water is that cold the light over my head goes off and tells me I shouldn't be diving here... lol

Good discussion very informative

 

[rhwestfall]

Nice. Thanks. My only serious cold-water diving has been the St Lawrence seaway (ice diving class) and Antarctica. I don't remember green.......

Lake Ontario...