Page 2 - Snorkelling Preview

Page 2 Copyright Wet Paper Publications 2014
Having two eyes allows us to see depth. In water, this
depth is affected by the bending of light as it passes from
water to the glass of the face mask and then into the eye.
The wearing of a face mask allows us to see more clearly
underwater as it provides an air space so that the rays of
light can focus on the retina.
This is shown in Figure 2.1.
Light and size
Light behaves differently in water than it does in air
because as it moves through a different medium it bends.
When a ray of light passes from water (more dense) to
air (less dense) it bends away from the surface of the
face mask.
The process by which light is bent as it passes from one
medium to another (eg from water to air) is called
refraction.
In water, rays of light are not bent as far and images focus
behind the retina. This is why fish appear blurred when
seen without a mask.
How a face mask works
When a mask is used, the rays of light enter the eye
normally and the image is clear. However, the mask has
a magnifying effect and objects appear closer and one
third bigger.
When there is a layer of air between our eyes and water,
objects appear to be one third larger and closer than they
actually are as shown in Figure 2.2.
Colour and depth
As depth increases, the range of visibility and the intensity
of colour, especially at the red end of the spectrum,
decreases due to the diffusion and absorption of light,
hence brightly coloured marine life appears greeny-blue.
Also with depth, rays of light become more diffused, ie,
spread out, and more of the light is absorbed.
Different colours can also penetrate different depths as
shown in Figure 2.3. Red is absorbed fastest, whereas
blue penetrates the furthest.
Look again at Figure 2.3.
What colour will a red fish appear to be at a depth of 6
metres and could this science fact affect the colour types
of algae found in the sea?
Implications for research
When conductingunderwater observations and recording
data, you need to be aware that size and colour may be
different and you are faced with the moral dilemma of
stressing or killing animals to bring them to the surface to
validate their size and colour.
You could devise a method to account for this by taking
some typeofmeasuringdeviceor colour chart underwater.
The problem is greatly magnified by underwater flash
photography and if you go on to obtain your SCUBA (self
contained underwater breathing apparatus) certification. Figure 2.3 Different colour frequencies can also penetrate
different depths.
Figure 2.1 A mask helps us focus
Bob Moffatt
Figure 2.2 A mask makes objects appear bigger
Bob Moffatt
Bob Moffatt