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:

• Color, Thin Films, and Interference
  by the Department of Physics and Astronomy, Arizona State University.

Answered by: Frank DiBonaventuro, B.S., Physics, The Citadel. Air Force Officer