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Why gasoline puddles forms a bright spot?

Gasoline is not miscible with water. So, getting for example, in a puddle on the road, it spreads over its surface and forms a very thin film. This film has excellent physical property is to create such a rosy picture.

Interference of light on the film of gasoline

Why is this happening?

Light rays falling on gasoline film, divided: part of the beam is reflected from the surface of the gasoline film (boundary of air and gasoline), and part passes through the gasoline layer, reaches the boundaries of the gasoline-water and is reflected from the border (another part goes deep into the water, but our question is this component that does not matter).

Interference of light in thin film

The result is two reflected beams, and the second of them on our way to the eye behind the first one, because he twice had to overcome the thickness of the film. These two beams are superimposed on each other, resulting in space is a redistribution of energy. The resulting fluctuations in either amplified or attenuated. Amplification occurs when the refracted wave 2 (see picture) behind the reflected wave 1 to an integer number of wavelengths. If the second wave lags the first by half a wavelength or an odd number of half-waves, there will be a light attenuation.

This phenomenon is called in physics the interference of light.

Interference of light in thin film
The beam of red light emitted from the point Y, is the sum of two rays:
part of the beam 1 passing through the film, and part of the beam 2,
reflected from the outer surface.
The path length XOY is a multiple of the wavelength of the incident on tape light
therefore, both of the beam formed in the phase and amplified.

Interference of light in thin film
In this case the blue rays when the film thickness
add up out of phase, because
distance XOY is not proportional to the wavelength.
The result - rays are formed in opposite
and extinguished: the blue color is not reflected from the film.

In order to occur the phenomenon of interference, two reflected beams must be synchronous, consistent, i.e. their wavelengths should be the same, and the phase shift is constant (physicists call such waves are coherent). Conventional light sources are not coherent, because they consist of a large number of atomic emitters that operate independently from each other and inconsistent. Waves, reflected from the outer and inner surfaces of a thin film, are coherent because they are parts of the same light beam.

If the light rays have the same wavelength, then there would be one color (a light source is called monochromatic), the interference picture looked like the alternation of light and black stripes (respectively, the interference maxima and minima). But the sunlight is white, there are waves of the entire visible spectrum. So the picture that is obtained at the petrol film solar light - colored, rainbow.

The fact that the path difference of the beams reflected from the film depends on its thickness. At a certain thickness, the condition of maximum run for some wavelength, and film in the reflected light will change its color corresponding to this wavelength. If the foil has a variable thickness, as is the case with petrol film on the water, the interference fringes become rainbow colouring as in different parts of the film, the condition of maximum runs for different wavelengths.

However, this does not mean that on film with a uniform thickness interference to observe impossible: the effect of interference is determined not only by the film thickness, but also by other factors, for example, the angle of incidence of the light beam, the refractive index of the film.

The phenomenon of interference of light can be observed only in thin films, the thickness of which is comparable with the wavelength of the light falling on them (but not necessarily more of it). Because light is the sum of the radiation with different wavelength. When passing through a thick film of differences in the passage of the rays will be the most different and reflected rays will not be coherent. That is, of course, some waves will be in phase, and some are in antiphase, but incoherent waves will be much more, and the interference pattern just "smeared". However, in thick films interference can watch the light source must be monochromatic.

The interference of light can be observed not only on gasoline films on the water.

When oil spills into the sea water surface is covered with iridescent stains - but only in those cases where the oil film is thin, no more than a micron in thickness, i.e. the scale of the disaster is relatively small.

The interference causes a rainbow iridescence on the surface of the CD-ROM drive.

Raguenet bubbles is also a result of interference. The wall thickness of the bubble a little more wavelengths of the visible spectrum. Decreasing the wall thickness of the bubble gradually changes color. When the thickness of 230 nm is coloured in orange, at 200 nm - green, with 170 nm blue. The film thickness varies unevenly, so it has a speckled appearance. When the water evaporates, the wall thickness of the bubble is smaller than the wavelength of visible light, the bubble ceases to Shine with the colors of the rainbow, becomes almost invisible before burst - this occurs when the wall thickness of about 20-30 nm.

The phenomenon of interference:

on the wall of a soap bubble

on the CD-ROM

on the oil film

on the oxide of the metal film


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2014-01-04 14:25:05
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