Part 3: Detecting layers of oil
Oil does not easily mix with water and, because it is also
usually less dense than water, a drop of oil will want to rise above the
water's surface.
As it does so, gravity pushes the edges downward and spreads
the drop into a thin and growing layer. You can easily duplicate this by
squirting a drop of salad oil into a pan of water. Notice, as you do so,
how rapidly the drop spreads and how thin the layer becomes. Ben Franklin
was one of the first to write about this phenomenon. He speculated that
the layer would spread into a single layer of molecules--called a mono-molecular
layer.
In practice--and in the ocean--the viscosity of oil usually
prevents it from forming a mono-molecular layer, but at the point where
oil is surfacing from a seep, you can watch a continuous rain of drops bursting
into rainbow circles, then spreading further until they merge into an even
thinner, glassy layer. The layer then drifts away from the source with the
wind and current.
Experiments suggest that this layer is about 0.1 µm
thick--too thin to see, but thick enough to suppress the tiny ripples on
the surface of the water. These tiny ripples are called 'capillary waves'
and they are what gives water its sparkling appearance.
So even when an oil layer is too thin to see, you can detect
it by the way sunlight glares off the smoothed water. In the picture of
the oil drop you can see the rainbow sheen from a newly surfaced drop, but
you can also see the surrounding area of glassy slick. It turns out that
the glare, or 'specular reflection' from an oil seep is visible from miles
away when the sun is at the right angle. | 
