PADI IDC Revision – Physics

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PADI IDC Revision – Physics – Light

Typical questions that you’re likely to find in PADI knowledge reviews or exams:

Let’s take a look at this video, and then answer the questions that follow.

So! now let’s try a few questions!

What causes things to look larger underwater?


How much bigger do things appear to be underwater?


What is the name of the phenomenon that makes things look smaller and further away?

Visual Reversal

What is the main cause of the phenomenon that causes things to look further away?


PADI IDC Revision – Physics – Sound

Let’s see if you can answer these questions?

How much faster does Sound travel in water than in air? Becuase the sound travels faster, where do most sounds seem to come from?

Take a look at this video:

So here are a couple of questions.

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How many times faster does sound travel in water than in air?

4 times faster

Because sound travels faster underwater, where does it seem to come from?


PADI IDC Revision – Physics – Heat

Heat travels much faster in water than in air.  In fact it travels 20 times faster, and that’s because the molecules in water form weak bonds between themselves.
Watch this short video to find out more:

now let’s look at a couple of questions

How much faster does heat travel in water than in air?

20 times

Why does heat travel much faster in water than in air?

Because water molecules form weak bonds between themselves.

PADI IDC Revision – Physics – Pressures Underwater

When you’re answering any question about Pressure in any PADI exam or knowledge review, it’s essential that you first of all look for a couple of things.  The first is to see if the question refers to Salt or Fresh water,  and the other is to look to see it it’s Gauge or Ambient pressure. So many times when people get the question wrong it’s because they didn’t look for these words.

Take a look at this video. I think it might help.

PADI IDC Revision – Physics – Pressures underwater Now let’s look at some questons

What is the Gauge Pressure in 19 metres of salt water?

1.9 atm/bar

What is the Ambient ( sometimes called Absolute )  Pressure in 19 metres of salt water?

2.9 atm/bar

What is the Gauge Pressure in 18 metres of fresh water?

1.7 atm/bar (1.8 divide by 1.03)

Is the Gauge Pressure for a given depth  in salt water be the same/greater/less than the same depth of fresh water?

Greater than

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PADI IDC Revision – Physics – Effects of Pressure

Learn how to use the “Box Method” to make these calculations really simple!

PADI IDC Revision – Physics – Effects of Pressure, Now for some more questions

If a balloon is filled to 2 litres at 45 metres, and then released, assuming that it doesn’t explode, what will it’s volume be upon reaching the surface?

11 litres
(Using box method.. 2 x 5.5 = 11)

Approximately how much air mist be pumped down from the surface to fill a 60 litre container if the container was in 50 metres of water?

360 litres
(Using box method.. 60 x 6 = 360)

What is the partial pressure of  Nitrogen in Air if breathed up a mountain, where the surrounding pressure is 0.6 atm?

0.48 %
Even this can be worked out by the Box method!
the absolute pressure is 0.6 ( it says so in the question!)  the approximate percentage of Nitrogen in the air is .80%   so the calculation would be: 0.8 x 0.6 = 0.48)

A diver breathes from a cylinder that contains 0.03 % carbon monoxide. At a depth of 25 metres, this would have the same effect as breathing  … % at the surface?

0.105 %
(Using box method: 0.03 X 3.5 = 0.105)

At a depth of 20 metres, approximately how much denser is the air a diver breaths as compared to the surface?

3 times
(even this can by solved using box method: 1 x 3 = 3)

If a diver breathes a complete cylinder of air in 70 minutes.  at the surface, how long would it take to breathe the whole cylinder at 25 metres?

20 minutes
Using the box method: (70 divide by 3.5 = 20 minutes)

PADI IDC Revision – Physics – Displacement

This is a subject that is far easier than some people think!  Take a look at this video to see why, and then have a go at the sample questions.

PADI IDC Revision – Physics, Displacement. Now let’s see how you get on with more questions!

If an object weighs 720 Kg and displaced 120 litres, how much more fresh water would need to be displaced to bring the object to the surface?

600 litres

( as it’s in fresh water,   720 – 120 = 600 )

If an object weighs 377Kg and displaced 89 litres, how much more sea water would need to be displaced to bring the object to the surface?

277 litres

( as it’s in salt water,  377 divide by 1.03 = 366.
Then .. 366 – 89 = 277)

If a full scuba cylinder was placed outside in full sunshine on a hot day. What would happen to the pressure inside the cylinder?

The pressure inside the cylinder will increase

If an object that weighs 970 Kg and displaced 420 litres, is in fresh water how much air would need to be added to a lifting bag to bring the object to the surface?

550 litres
( as it’s in fresh water,  970 – 420 = 550)

If an object that weighs 68 Kg and displaced 15 litres, is in salt water how much air would need to be added to a lifting bag to bring the object to the surface?

51 litres

(as it’s in sea  water,  68 divide by 1.03 = 66
Then .. 66 – 15 = 51 litres)

If a fully inflated balloon was placed in a cold room, what would happen to the volume?

The volume of the Balloon would decrease

Your PADI IDC Revision – Physics

So you see! Physics is much easier that you might think! If you Followed the tips on this page, watched the videos, and then tried the sample questions, you should be very close to being prepared for your PADI DM or PADI IDC

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PADI IDC Revision - Physics

PADI IDC Revision