ANALELE ŞTIINŢIFICE ALE UNIVERSITĂŢII “AL. I. CUZA” IAŞI
Tomul III, s. Biofizică, Fizică medicală şi Fizica mediului 2007
INTERFEROMETRIC METHOD TO EVIDENCE THE
WATER POLLUTION WITH HYDROPHOBIC
SUBSTANCES
Nicoleta Puica Melniciuc1, Dana Ortansa Dorohoi2, Servilia Oancea1
KEYWORDS: interference on thin layers.
A method permitting to appreciate the thickness of the hydrophobic pollutant layers on the
Seas and Oceans waters is described here. The position of the maxima obtained by the Sun
light interference on the transparent hydrocarbon layers depends on their thickness and a
spectrometer can be used to establish the spectral composition in the interference field.
1.
INTRODUCTION
Pollution of Oceans’ or Seas’ waters by hydrocarbons is essentially caused of
the frequent loses of carburant. The pollution consists from a great surface covered by
a thin film of hydrocarbons. In the Sun white light these films become colored,
functioning as layers of constant thickness that divide the radiation beams. Light
propagation in the sea is a function of both the quantity and optical quality of
suspended particles in the water. Water pollution can be studied using different
techniques [1-4].
In this paper we intend to analyze the influence of the thickness pollutant layer
on the aspect of interference fringes obtained for small angles of reflection.
Interference fringes of equal inclination, obtained in white light, under small
incidence angles, are colored. Their color is dependent on the layer thickness. The
spectral composition of the radiations reflected by the varnish is governed by the
interference on thin layers [5]. The interaction between two or more beams is known
as interference. The spatial area in which the beams simultaneously exist and interfere
is called interference field. The flux density distributions within the interference field
can be characterized by means of surfaces having constant flux densities.
r
ϕ(R ) = const.
1
2
University of Agricultural Sciences and Veterinary Medicine, Iasi, Romania
Faculty of Physics, “Al.I.Cuza” University, Iasi, Romania
(1)
74
Nicoleta Puica Melniciuc, Dana Ortansa Dorohoi, Servilia Oancea
r
r
where ϕ( R ) is the energetic flux density in a point P( R ) within the interference field.
r
r r
The flux density ϕ(R ) can be expressed as a function of e R , the resultant electric
()
r
field intensity of radiations interfering in the P(R ) point, by the following
relationship:
()
()
r
r r
ϕ R = χ〈 e 2 R 〉
(2)
where:
1
(3)
cμ 0
Experiments prove that the interference field often contains level surfaces of extreme
values:
r
(4)
ϕ M (R ) = C M
r
(5)
ϕ m (R ) = C m
r
r
where ϕ M (R ) and ϕ m ( R ) are the maximum and the minimum of flux density,
respectively. The distributions of maxims and minims are known as interference
r
fringes. In the close vicinity of a point P(R ) within the interference field (Fig.1), the
r
interference fringes are described by the interfringe and the visibility, V( R ) .
χ=
Fig. 1: Interference of radiations coming from punctual source,
reflected by thin layers.
r
According to the definition given by Michelson (1890), the visibility V(R )
within the interference field, can be expressed as:
ϕ ( Rr ) − ϕ ( Rr )
r
m
(6)
V(R ) = M
ϕ ( Rr ) + ϕ ( Rr )
M
m
75
INTERFEROMETRIC METHOD TO EVIDENCE…
where the maximum and minimum flux densities are measured in the vicinity of
r
P( R ) point. The visibility is a measure of the interference fringes observability.
r
The value: V(R ) = 0 resulting from the condition: ϕ ( Rr ) = ϕ ( Rr ) shows that
m
M
the fringes cannot be seen.
r
r
The value: V(R ) = 1 resulting from the condition: ϕ m R = 0 expresses the
optimum situation for fringes observation.
By subtracting the path differences between the two rays interfering in
r
point P(R ) , the overall path difference between rays SI1 and SIx is obtained. It
depends on the thickness of the covering layer, on its refractive index and on the light
incidence angle on the covering surface.
(7)
Δ12 = 2 ⋅ n 2 ⋅ h ⋅ cos r
()
In the present case, the reflections on the two separating surfaces, Σ1 and Σ 2 are
equivalent, which results in a null supplemental path difference.
Maximum value of flux density of the reflected beam is given by the condition:
1⎞
⎛
(8)
2 ⋅ n 2 ⋅ h = ⎜ 2p + ⎟ ⋅ λ max ; p = 0, 1, 2, ...
2⎠
⎝
while the minimum value is obtained when:
λ
(9)
2 ⋅ n 2 ⋅ h = 2p ⋅ min ; p = 0, 1, 2, ...
2
2.
MATERIALS AND METHODS
In order to argue that the pollution with hydrocarbons can be evidenced by
interferometric method, we analyzed the interference field obtained with these
substances on a thick water layer. An optical system with optical fibber assured the
light entrance in a spectrometer able to analyze the interferometric field. When an
initial etalonation is made, the apparatus directly displays the thickness of the pollutant
layer [6].
The same method is applied when a sea or a portion of ocean are polluted by
hydrocarbons.
3.
RESULTS AND DISCUSSIONS
The number of the maxims of interference obtained when a thin layer of
hydrocarbon pollutant (n=1.52) laying on the water surface (n=1.35) is observed at a
small angle of incidence, depends on the thickness of the pollutant layer, as relation
(8) predicts for the maxima of interference.
This assertion is sustained by the Figs. 2-4 in which the reflected flux by a thin
layers observed at normal incidence is shown for different thickness of the
hydrocarbonic layers on water. In relation (8) the difference between the two
reflections of air/hydrocarbon and hydrocarbon/water surfaces was taken into
76
Nicoleta Puica Melniciuc, Dana Ortansa Dorohoi, Servilia Oancea
consideration by the supplementary pathway introduced at reflections from a more
dense to a few dense transparent materials.
Φ (R)
120
100
80
60
40
20
0
403 421 441 463 487 514 545 579 617 661 712 772
λ (nm)
Fig. 2: Reflected radiations in maxims of flux density, at normal incidence, on a
hydrocarbon layer (n= 1,543) with 3μm thickness.
Φ (R)
120
100
80
60
40
20
0
385
411
441
475
514
561
615
686
771
λ (nm)
Fig. 3: Reflected radiations in maxims of flux density at normal incidence on a
hydrocarbon layer (n= 1,543) with 1μm thickness.
Only three monochromatic radiations are reflected by when the pollutant layer
is of 1μm thickness, at normal incidence. Only one monochromatic radiation from the
visible range is reflected by a hydrocarbon layer of 0,5μm thickness at the normal
incidence.
Φ (R)
120
100
80
60
40
20
0
386
441
543
617
772
λ (nm)
Fig. 4: The reflected radiations in maxims of flux density at normal incidence on a
colophony layer (n= 1,543) with 0,5μm thickness.
INTERFEROMETRIC METHOD TO EVIDENCE…
77
The spectral composition of the reflected at normal incidence light is poor when
the pollution layer is smaller than 1μm. The monochromatic radiations giving minims
of interference on the colophony layer do not contribute at visual observation.
From data presented in figures 2-4 it results that at normal incidence, the
number of interference maxims grows with thickness the pollution layer increasing.
The graphs were realized for normal incidence. For angles of incidence others
than the normal one, relation (8’) show us that a smaller number of maxima can
appear by interference.
⎛ 1
⎞
λ
2 ⋅ n 2 ⋅ h ⋅ cos⎜⎜ sin i ⎟⎟r = (2p + 1) ⋅ ; p = 0, 1, 2, ...
(8’)
n
2
2
⎝
⎠
For each incidence angle, the researcher can predict the thickness of the
pollution layer and then, appreciating its area, can estimate the volume of the lost
hydrocarbons and can decide the protective actions.
4.
CONCLUSIONS
The thickness of transparent insoluble in water layers from the water surface
determine the aspect of the interference fringes obtained in white Sun light for small
angles of reflection. When the covering layer is very thin, it can function such as
interferential filter. From the color of reflected radiation one can estimate the thickness of
the layer.
REFERENCES
1.
2.
3.
4.
5.
6.
Szermer M., Daniel M., Napieralski A., 2003. Design and Modelling of Smart
Sensor Dedicated for Water Pollution Monitoring, Nanotech, 1:110-114.
Devolder S., Leroy O., Wevers M., De Meester P., 1994. Relation between
phase differences and small layer thicknesses, Ultrasonic Symposium, 1994,
Proceedings 1994 IEEE, 1085-1090.
Balin I., 1999. Differential optical absorption spectroscopy (DOAS) Air
pollution measurements, Ecole Polytechnique Federale de Lausane, PhD
Thesis.
Kim K-H., 2004.Comparison of BTX Measurements Using a Differential
Optical Absorption Spectroscopy and an On-Line Gas Chromatography
System, Environmental Engineering Science. 21(2): 181-194.
Pop V., 1988. Bazele opticii, Ed. Univ. „Al. I. Cuza”, Iaşi, 1988.
Evans R.M., 1965. An Introduction to Color, John Wiley & Sons, Inc. New
York, London, Sidney.