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

Search

Loading



Sponsors

USC University of Southern California Dornsife College Physics and Astronomy Department McMaster University Physics and Astronomy Department

Science Quote

'A hundred times every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving…'

Albert Einstein
(1879-1955)

Deal of the Day

Image of Hawk Talon v3 - RC Helicopter w/Gyro. Click on this image for more info. Hawk Talon v3 - RC Helicopter w/Gyro
Regular Price: $49.99
Today's Price: $19.00
Check out this easy to control remote-control helicopter! Buy it now! Deal ends at midnight!

Top Selling

Here are our physics & astronomy bestsellers:
Mini Plasma Ball
Slide Making Kit
The Ultimate Geek Pen
Top Secret - Spinning Top
Thermodynamic Drinking Bird
Solar Radiometer
Wood Grain Newtons Cradle
Deluxe 3D Mirascope
Periscope
Solar Science 4M Kit