Page 38 - (DK) Ocean - The Definitive Visual Guide

36 OCEAN WATER

Light and Sound

LIGHT AND SOUND BEHAVE VERY DIFFERENTLY in water than in air. Most
light wavelengths are quickly absorbed by water, a fact that both explains
why a calm sea appears blue and why ocean life is concentrated near its
surface – almost the entire marine food chain relies on light
energy driving plant growth. Sound, in contrast, travels
better in water, a fact exploited by animals such as dolphins.
Yellow
Orange
Red
Violet
Blue
Green
Light in the Ocean
White light, such as sunlight, contains a mixture of light wavelengths,
650nm
30m 590nm
ranging from long (red) to short (violet). Ocean water strongly absorbs
(100ft)
DEPTH 570nm red, orange, and yellow light, so only some blue and a little green and
violet light reach beyond a depth of about 40m (130ft). At 90m (300ft),
most of even the blue light (the most penetrating) has been absorbed,
while below 200m (650ft), the only light comes from bioluminescent
60m 400nm
organisms, which produce their own light (see p.224). Because they rely
510nm
(200ft)
on light to photosynthesize, phytoplankton are restricted to the upper
layers of the ocean, and this in
turn affects the distribution of
90m other marine organisms.
(300ft) 475nm Intriguingly, many bright red
animals live at depths that are
LIGHT PENETRATION
The red and orange devoid of red light: their colour
components of sunlight are provides effective camouflage,
absorbed in the top 15m since they appear black.
(50ft) of the ocean. Most
other colours are absorbed
in the next 40m (130ft).
Wavelength is measured in
nanometres (nm).

COLOUR RESTORATION
At a depth of 20m (65ft), most animals and plants
look blue-green under ambient light conditions (top).
Lighting up the scene with a photographic flash or torch
reveals the true colours of the marine life (bottom).

FISH VISION
Fish have excellent vision, which helps
them f ind food and avoid predators.
Many can see in colour. The lens
of a fish’s eye is almost spherical
INTRODUCTION This squid produces moved backwards and forwards
and made of a material with a
high refractive index. It can be
FIREFLY SQUID
to focus light on the retina.
a pattern of glowing
spots (photophores).
When viewed by a
FISH EYE
predator swimming
The lens of a fish’s eye bulges through the iris
below, the spots help
(the dark central part) almost touching the cornea
camouflage its outline
(outer part). This helps to gather the maximum
amount of light and gives a wide field of view.
against the moonlit
waters above.