What is Refraction and Visual Reversal ?

Refraction is the tendency of light to bend as it passes from one medium to another -such as from water to air. It results as light travels at different speeds through various substances due to the different densities.

For example, light travels through air at a faster rate than it does through water, so when light enters the water it enters at a different angle. Unless of course it enters at a pure 90 degree angle, like the sun when it is straight over head.

So for divers to see clearly under water, we wear masks. Now the light must travel from water, glass, and air to reach our eyes. During each pass the light must bend, shape though each medium before we see the object. AS a result objects will appear closer to us at a 4:3 ratio than their actual size. Meaning that an object that is 4 feet away will appear to be only 3 feet away.

There is a tendency to underestimate the distance due to refraction. At greater distances and under certain conditions the objects may appear farther away than this ratio suggests. These phenomenon is called visual reversal, where objecgts appear father away than they actually are.

This reversal depends upon depth and seems to be a result of decreased brightness, and reduced contrast. Also can result from absence of visual / distance cues like what we have on land.
In highly turbid ( low viz ) water, even relatively close objects tend to look farther away.

As a rule and in closing, in estimating distances, the closer the object the more likely it will appear closer than it actually is due to refraction. The more turbid the water, the more likely that close object will appear farther away than it is due to visual reversal.

Evaluation of Dive Conditions

If there is a dive planned for a certain location, review and look at the weather forecast for that area of diving and determine before you head out if diving is going to happen or not.


If you live on the coast lines, you may be able to go online and check out surf reports, and tide changes. You would like to dive in an area when you have a slack tide, that is the point between high and low tide. This is where the water has less movement.

Ok now you have determined that the weather is right and the tides are ok. ANd now you are at the site.

PADI trains divers to dive within their training and experience limitations. It is a good idea to check the water conditions once there before you unpack your gear.

When you get there at the site take into account weather at the site, water temperature, bottom composition, waves, depth, local area hazards, and anything else that will have a direct bearing on your dive.

Look for possible entry and exit points and and procedures as part of the pre dive.

At this point decide whether you can make a safe dive.

REMEMBER THIS IS YOUR DECISION - YOU ARE ULTIMATLY RESPONSIBLE FOR YOUR OWN SAFTEY. Only you can make the final decision to dive..

If you do not feel like you can make a safe dive at this site, maybe scrub this site and check put the alternate site.

Diving in poor or potentially dangerous conditions is not fun. You are diving to have fun, adventure, and challenge - not to expose yourself to unreasonable risks.

Nitrogen Narcosis - What Is It ?

Deep diving can bring to a diver with another difficulty that relates to nitrogen diving - nitrogen narcosis a euphoric anesthetic effect nicknamed Rapture of the Deep. As you might remember the air we breathe is 21 % Oxygen and 78 % Nitrogen and the other 1% is various gases. On the surface the air gives us no real effects, but at depth and under pressure, nitrogen can give us nitrogen narcosis.

When breathing air nitrogen develops an increase partial pressure at about 100 feet. The exact mechanism surrounding this narcosis is not fully understood, but almost any gas can cause this narcosis under high pressures. Theory suggests that nitrogen becomes dissolved in the lipids(fats) in nerve cells, which interfere with signal transmission from neuron to neuron.

Underwater and at depths reaching 100 feet, nitrogen has a noticeable intoxicating affect that intensifies as you do deeper.

Narcosis may make a diver feel dizzy, sleepy, and may affect memory underwater. They also may feel falsely secure, exercise poor judgement and become uncoordinated.
Nitrogen Narcosis affects divers differently and can affect the same diver differently on different days. It can make a diver feel anxious or uncomfortable, which can lead to panic or other poor decisions.


When you ascend the effects of the narcosis recede quickly , with no after effects. It is not dangerous or harmful in itself, but it impairs judgement and coordination.

This is why divers who do dive deep consistently will dive and be trained to dive with enriched air. This meaning diving with air that is greater than 21% Oxygen. Although diving with increased oxygen can have some for the same effects and carry a different set of risks. This is why a diver must be properly trained in enriched air diving. A topic for future blogs and lessons.

If while diving you start to feel intoxicated, uncoordinated, or confused, immediately ascend to shallower depths to relieve the feeling. If your buddy acts impaired help him to a shallower depth.

To prevent Nitrogen Narcosis, simply avoid deep dives. Take and be trained in the Padi Deep Dive Certification Course. In this the instructors can watch and observe you under water and at depth.