Quiz - 3 - Diving Knowledge Workbook - Diving Physics

[Pedro Burrito]

From the Diving Physics Section of the PADI Diving Knowledge Workbook Version 2.02 © PADI 2009:

1.2 Question 1

Refraction is caused by the process of:

a. light traveling at different speeds as it passes through different substances.

b. water absorbing various wave lengths of light beginning at the red end of the spectrum.

c. the changing speed of light due to sunspot activity.

d. light traveling at the same speed as sound once it enters water and encounters resistance. ​

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

a. light traveling at different speeds as it passes through different substances.

As light passes through transparent substances of differing densities, the speed at which it travels changes. This change of speed causes the light wave to bend or "refract."

 

[Couv]

@Pedro Burrito I vote for a new question just after the previous question's spoiler is posted.

 

[TMHeimer]

I vote for a new question just after the previous question's spoiler is posted.

Doesn't matter to me. I get up around 11 each day.

 

[Couv]

Doesn't matter to me. I get up around 11 each day.

:-D I'm going to count that as two votes to the affirmative.

 

[Angelo Farina]

Ah, the official answer, again, points to different density as the cause of the phenomenon!
It seems that for PADI density is the explanation of everything!
Instead, also in this case, refraction is a complex phenomenon, where density perhaps plays some role, but which is governed by complex sub-atomic phoenomena, related to photon absorption and re-emission.
In fact, a completely new concept of density, called optical density, has been defined.
See here: Physics Tutorial: Optical Density and Light Speed
The optical density of a medium is not the same as its physical density. The physical density of a material refers to the mass/volume ratio. The optical density of a material relates to the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. The more optically dense that a material is, the slower that a wave will move through the material.
Here some numbers for understanding that the relationship between "normal" density and optical density (a.k.a. refraction index) is not univocal. Let see two gases and two liquids.

Gas Density (kg/m3) at 0 °C, 1013 mbar Optical Density (refraction. Index n)
Oxygen 1.429 1.000271
Nitrogen 1.2506 1.000298
Hence oxygen is more dense than Nitrogen, but has a slightly lower refraction index.

Liquid Density (kg/m3) at 20 °C, 1013 mbar Optical Density (refraction. Index n)
Water 998.2 1.333
Ethyl Alcohol 789 1.36
Again, water is more dense than ethyl alcohol, but has a lower refraction index.

Also in this case, we cannot say that the refraction index of water is higher than air BECAUSE the water is more dense, this is another factoid...

 

[Rickk]

I am an engineer and have some knowledge of physics.

I understand that dive physics needs to be explained in simple terms to most divers, not all are from a STEM background and some are still in Jr High.

However simple explanations do not need to be wrong and they should follow the generally accepted presentations.

I am taking the SSI diving science on line. They have a question where they give an answer to 3 decimal points, That implies an accuracy of +- 0.5 mm. Anyone ever look at your depth that closely? To the nearest 1 is good enough for diving.

They also give .13 bar instead of 0.13 bar as the difference in pressure that can cause lung injury. Anyone miss the decimal point in the first number?

They also get the reason why speed of sound is different in water than air and give the wrong answer. (They say due to higher density, however density slows sound while incomprehensibility increases speed of sound. )


It is possible to teach physics in simple terms to the accuracy sufficient for diving, but it should never be taught wrong or in ways that make it easy to make serious errors.

 

[boulderjohn]

not all are from a STEM background and some are still in Jr High.

OW certification in most agencies is available to 10-year olds (5th graders in the US).

I have seen many a ScubaBoard post mock OW training materials as being written at such a basic level. They have to be basic if they are to teach successfully to the full range of student ability. I have taught classes that included 6th graders and college professors. Fortunately, the college professors I have taught have all been smart enough to understand the situation and not make asses of themselves.

 

[Angelo Farina]

OW certification in most agencies is available to 10-year olds (5th graders in the US).

True.
But this is not excuse for suggesting wrong cause-effect relationship.
Why PADI feels the need for providing an explanation of the reason for which the sound speed is higher underwater, the light speed is slower, etc.?
The student should only know that water is different than air, without any explanation of the reason why...
And then he must understand the effect that the difference in physical properties causes to his vision, hearing, thermal flow, etc...

 

[Storker]

the official answer, again, points to different density as the cause of the phenomenon!

What's weird is that that isn't in the answer itself, only in the additional explanation.

Spoiler: Answer

a. light traveling at different speeds as it passes through different substances.

As light passes through transparent substances of differing densities, the speed at which it travels changes. This change of speed causes the light wave to bend or "refract."