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Question
Why is petrol/oil in water colorful?
Asked by:
H.M.
Answer
The reason that petrol appears colorful when it
contacts water has to do with the relative density
of the materials, both optical density and mass
density. Petrol, as a liquid chemical, has a
significantly lower mass per volume, or density,
than the water it sits on top of. This causes a
very thin film of petrol to spread out onto the
waters surface. This film is so thin, in fact,
that the distance between it's top and bottom is
something approaching the wavelength of the visible
light spectrum. The petrol is also partially
reflective, and as the light enters the top layer
of the first film, some is reflected, and the rest
is transmitted (let through). Of that percentage,
the light reaches the boundary of the water, which
is, again, partially reflective. This causes there
to be actually two separate rays of light reflected
back upward to the eye, both of them with different
phases, due to the points in time at which they
reflected. These phases will either interfere with
each other, either destructively, constructively,
or some where in between. Total destructive
interference means you will see a dark spot, and
constructive interference means that you will see
a bright spot that is strongly reradiating a color
whose wavelength corresponds to the thickness of
the film of petrol. Thus a film of petrol on a
body of water will appear colorful, because each
particular area of the film is not a uniform
thickness.
Further reading:
Answered by:
Frank DiBonaventuro, B.S., Physics, The Citadel. Air Force Officer
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