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Date of experiment: 16/03/2012
Date of report: 30/03/2012
Investigation of Interference Patterns and
Experiment conducted by Sultan LeMarc.
This experiment looks to investigate the physical optics of the interference
pattern called Newton’s rings and calculate the wavelengths of various filters as
well as the radius of curvature of a plano-convex lens. Through the
consideration of the wavelength dependence of the diameter of the rings,
mathematical relationships are derived and applied on the measured data to
verify the known wavelength values. The results produced by the data
successfully show the relationship between diameters of the rings, their order,
the refractive index of the medium causing the interference pattern and the
wavelength of the incident light. Graphical analysis confirms that the diameters
of the rings are larger at higher orders, and larger still for longer incident
The aim of this experiment is to gain an understanding of the wave behaviour of
light through the examination of interference patterns of light, more specifically
through the production of Newton’s rings. The investigation will look to
demonstrate the dependence of interference patterns on the wavelength of
incident light through the manipulation of lenses and filters on an optical bench.
The objective is to gain additional quantitative insight through the measurement
of the Newton’s rings in order to calculate the radius of curvature of the lens
and the incoming wavelength of the light.
In order to understand the interference of light, the model of light’s wave nature
must be considered instead of the model of rays. Optical effects that depend on
behaviour of this nature are considered to be part of the physical optics field.
Interference of light, as an electromagnetic wave, arises by the combination of
the electromagnetic fields that constitute the individual light waves. This
phenomenon is governed by the principle of superposition, which states that
when two or more waves travelling in the same medium at the same time
overlap, the resultant displacement at any point in space or time is the vector
sum of the individual instantaneous wave displacements.
Interference in light waves from two sources was first demonstrated by Thomas
Young with an experiment now called Young’s double-slit experiment. This
showed that light can display characteristics of both waves and particles by
passing light through two apertures. The light waves passing through the
apertures interfered and thus superimposed, producing interference patterns of
bright and dark parallel bands called fringes.
Figure 1: Interference pattern produced by Young’s double-slit experiment.
This established the principle of wave-particle duality and the distribution of the
brightness is explained by the interference of the wavefronts. This behaviour of
light can be modelled using classical wave theory. The Huygens’s principle
states that each point on a wavefront generates a secondary spherical wavelet
which then superimpose individually during the interference.
The phase of a wave is the fraction of its period that a given point completes.
When two or more components of a wave are of the same frequency and phase,
they superimpose such that their amplitudes are reinforced and constructive
interference occurs. This is a result of the crest of one wave coinciding with the
crest of the other, producing the bright fringes.
When the waves are out of phase by half a period, where one is a minimum
when the other is a maximum, they superimpose such that the resultant
amplitude is the difference between the individual amplitudes. If the waves are
of equal amplitude they cancel each other producing complete annulment,
producing the dark fringes.
Figure 2: Sinusoidal representation of constructive and destructive interference.
Sinusoidal waves are characteristic of monochromatic light, which is light of a
single wavelength. Monochromatic light is effective in observing interference
patterns as it has waves of the same wavelength and same amplitude. In
addition, all waves from a monochromatic source are permanently in phase, and
this constant phase relationship for the same frequency makes it coherent.
Lasers are an example of monochromatic light however ordinary light sources
can be made monochromatic with the use of optical filters. These are devices
which selectively modify the component wavelengths of mixed light in order to
transmit light of a specific wavelength corresponding to its colour. Filters may
be made of coloured glass, plastic or gelatin. An alternative is to use a
monochromatic filter, also known as a monochromator, which mechanically
selects a narrow band of wavelengths of light.
Interference effects are commonly observed in thin films as a result of wave
reflection from the two surfaces of the film. Light waves are reflected from the
two boundaries of thin films such that interference, both constructive and
destructive, takes place between the reflected waves.
Incident light wave shining on the firstboundary of a thin film with thickness t is
partly reflected before it enters the film, whilst the remaining wave which are
transmitted through to the second boundary are again partly reflected back out.
The reflected waves interfere with each other and the type of interference is
determined by the phase relationship and the various wavelengths of different
colours. This is why colours fringes or rings are produced by oil on water or
soap films. This difference in phase of the waves, and thus the degree of
constructive and destructive interference, is determined by the angle of
incidence of the incident wave, the thickness of the film and the refractive index
of the film.
Figure 3: Interference pattern of rings produced when white light isincident on a thin oil film.
The refractive index, also known as the index of fraction, commonly denoted by
n, is a measure of the refraction of light when passing from one medium to
another. It is related to the angles of incidence θ i and refraction θr by Snell’s
law; n = sinθi/sinθr.
It also equates to the ratio of the speed of a given wavelength in a vacuum to its
speed in a medium. They are characteristic values unique to each medium. The
refractive index value of a vacuum is 1, whilst that of air is 1.002 ± 2.5 × 10 -9
The phase relationships which determine the degree of constructive and
destructive interference are dependent on how the refractive indices of the film
and the medium adjacent to it compare. As for the case of an oil film and air in
Figure 3, the reflected waves from the upper boundary undergo a polarity
inversion; a phase change of 180o (π) with respect to their respective incident
The second set of reflected waves from the lower boundary of the film undergo
no phase change because they are reflected from a medium with a lower
refractive index. These waves travel an extra distance of approximately double
the thickness of the film, 2t, if close to the normal before recombining with the
reflected waves of the first boundary. For the waves that are not close to the
normal, the path difference is larger than 2t.
Figure 4: Reflected and refracted light paths through a thin film.
As the two sets of waves are out of phase by 180 o, the path difference is
equivalent to λn/2, where λn is the wavelength of the light in the film.
When 2t = λn/2, the waves combine in phase and constructive interference takes
place. The general condition for constructive interference in thin films for
waves that are 180o out of phase is given by:
If the extra distance travelled by the secondary reflected waves corresponds to a
multiple of λn, the waves combine out of phase and destructive interference
takes place. The general condition for destructive interference in thin films is
If neither or both waves have a 180o phase shift, the conditions are reversed.
The production of Newton’s rings is fundamentally based on suchthin-film
interferences. When a plano-convex lens is placed on a plane glass surface, with
its convex surface facing the glass, an air film of increasing thickness is formed
between the lower surface of the lens and the upper surface of the glass surface.
A film is considered to be thin when its thickness is of the order of the
wavelength of light.
Figure 5: Plano-convex lens on plane glass sheet. Size of air film exaggerated.
The thickness of the film at the point of contact is zero and increases from the
centre towards the edge of the lens. The point at which the thickness of the air
film is constant is at the central point of contact.
As a result of this air film, when parallel light is normally incident on the flat
side of the convex lens, it is partially reflected from the lower surface of the lens
and the upper surface of the glass. The reflections of interest are those involving
the surfaces in contact, reflections from top of the lens and bottom of the glass
do not contribute to the pattern.
The reflected waves do not experience a change of phase at the lens-air
boundary as the light is propagating through a higher refractive index medium
to a lower refractive index medium. In contrast, the light that is transmitted past
to the air-glass boundary is reflected with a change of phase of 180o as the air
has a lower refractive index than the glass.
As a result, both of the reflected waves from the contact boundary travel in the
same direction and are coherent because they are derived from the same
incident wave by division of amplitude.
The interference between the reflections produces a concentric ring pattern of
spectrum colours for white light, or alternate dark and light rings in
monochromatic light. The interference rings can be observed both in reflection
and in transmitted light. These concentric rings are called Newton’s rings. The
distances between the interference rings are not constant as the convex
boundary is curved.
Figure 6: Diagram for formation of Newton’s rings through the transmission of light.
The radius of curvature of the convex lens, R, is related to the radius of the flat
surface r and the thickness of the air film t by:
Figure 7: Image of Newton’s rings interference pattern.
The interference pattern shows a central dark spot because at the point of
contact of the glass and the lens the thickness of the air film is effectively zero
relative to the wavelength of light. Therefore there is a zero path difference
between the interfering waves at this point and out of phase by 180 o, resulting to
destructive interference and producing a dark spot. In transmitted light the
interference pattern is complementary to the reflected interference pattern but
the central spot is bright due to constructive interference. The type of
interference is determined by the conditions of equation (1) and (2). The
conditions for interference in transmitted light are the opposite to the conditions
for reflected light.
The fringes are circular with equal thickness throughout the circumference due
to each fringe being a locus of constant film thickness at a particular point. The
rings in the pattern get closer as the order m increases because the diameter does
not increase in the same proportions.
The radius of the mth bright fringe is given by:
The radius of the mth dark fringe is given by:
The radius of a dark ring is proportional to the square root of the radius of the
curvature of the lens.
The diameter,Dm, of the mth fringe from the centre is related to the radius of
curvature, the refractive index, nx, of the mediumbetween the lens and glass,
and the wavelength λ of the lightby:
This equation governing the interference pattern shows that the diameter is
inversely proportional to the refractive index. This means that the greater the
refractive index of the medium between the lens and the glass, the smaller the
diameter of the rings. The radius of any given ring will be less with for example
a liquid in place than with air. This effect can be used to the measure the
refractive indices of various mediums.
Furthermore, equation (6) also shows that the greater the wavelength of a given
light, the greater the diameter of the rings. This means that red light would
produce the largest rings whilst blue light would produce the smallest.
Subsequently, this can be used to measure the wavelength and frequency of a
The optical setup used to produce a Newton’s rings interference pattern, through
transmission of light, to find the radius of curvature and the wavelength of the
incoming monochromatic light is shown by the optical bench below:
Figure 8: Diagram of set up on optical bench.
The light source is connected to the power supply and fixed on the optical
bench, aligned with the three lenses. The plano-convex lens with the plane glass
surface was placed in between two collimator lenses. This lens produced the
interference pattern, whilst the collimator lenses either side of it aligned the
incident light in order to replicate the source at an infinite distance. This made
the incident light parallel, first into the plano-convex lens and then again for the
projection screen. The plano-convex lens has a ruled scale of 20mm printed on
it which is also projected.
The projection screen allowed the interference pattern to be observed by
viewers. A larger projection screen, such as a plain wall, was not required as
rings up to the 10th order fitted within the range of the screen. The experiment
was conducted in full covers from external light in order to prevent light
attenuation, allowing the projection of light to be as bright as possible.
The experiment used three different coloured filters, blue, green and red, with
stated wavelengths of 437nm, 546nm and 578nm respectively. The radius of
curvature of the plano-convex lens was stated to be 12.13 ± 0.005m.
First, the experiment looked to confirm these claimed filter wavelengths with
the known radius of curvature value of the lens and the refractive index value of
air. Equation (6) was used with λ as the subject:
Each of the three filters was individually tested. They were attached to the first
collimating lens by inserting them into the lens bracket. The optical bench
allowed the lenses to be moved at varying distances until the projection of the
interference pattern was in focus.
The distances had to be adjusted for different filters to achieve a focused
projection. However, once the vertical position settings of the lenses were fixed
to align them with the optical axis, they were not adjusted in any trials. The
screws that tightened the lenses in place were fixed to a limit such that they did
not provide excessive contact pressure and risk deformation of the lenses. The
filters were allowed to cool from the heat of the light before being handled.
Measurements of the diameters of the dark rings up to the 10th order were
recorded for each filter using the ruled scale. The maximum order of rings was
measured to 10th as the rings of higher orders were not well defined and thus
could not be distinguished adequately. The diameters were measured as
opposed to the radii because they are the larger lengths; this reduced the relative
instrumental error of the measuring scale. The measurement was taken from the
middle of the ring’s thickness.
Due to the limited resolution of the scale, a plain sheet was attached to the
projection screen and the edges of each ring were marked on to it along with the
ruled scale. This allowed the marked diameters to be measured more accurately
by a ruler with consideration of the scale conversion. This was repeated for all
Lastly, the data recorded was then used to calculate the radius of curvature of
the lens by assuming that the stated wavelengths of the filters are correct using
equation (6) with R as the subject:
Blue Filter (λ: 436nm)
± 0.25 (mm)
Table 1: Measurements and calculations for blue filter of wavelength 436nm.
The theoretical diameter was calculated by equation (5) to show what the
diameter for the given wavelength and order should be and to provide a
reference for deducing the deviation of the experimental values.
The wavelength λ was calculated using equation (7) using the literature values
of nx = 1.002 ± 2.5×10-9 and R = 12.13 ± 0.005 m. The error in wavelength ∆λ
was calculated using:
The average wavelength of the blue filter was calculated as λB1= 538.6 ± 12.0
nm. The error on this average value was calculated at 65% confidence level by:
The radius of curvature R was calculated by equation (8) using the stated
wavelength value λ = 436nm.The error in the radius of curvature ∆R was
The average radius of curvature was calculated as RB1 = 14.97 ± 0.35 m. The
error on this average was calculated at 65% confidence level with σ = 1.10.
Green Filter (λ: 546nm)
± 0.25 (mm)
Table 2: Measurements and calculations for green filter of wavelength 546nm.
The average wavelength of the green filter was calculated as λG1= 656.2 ± 13.6
nm. The error on this average value was calculated at 65% confidence level
with σ = 43.2. The average radius of curvature was calculated as RG1 = 14.58 ±
0.30 m. The error on this average was calculated at 65% confidence level with
σ = 0.96.
Red Filter (λ: 578nm)
± 0.25 (mm)
Table 3: Measurements and calculations for blue filter of wavelength 578nm.
The average wavelength of the redfilter was calculated as λR1= 672.3± 8.7 nm.
The error on this average value was calculated at 65% confidence level with
σ = 27.6.
The average radius of curvature was calculated as RR1 = 14.11± 0.18 m. The
error on this average was calculated at 65% confidence level with σ = 0.58.
Figure 9: Graph of the results from Table (1), (2) and (3), plotting diameter as a function of
order, D2 vs m for all three wavelengths.
The line of best fit for the data points was determined using the method of least
squares fit. The equations of least squares fit require the values of S x, Sy, Sx2 and
sxy of the data to be calculated for each of the three tables using:
The plots for all three wavelengths show that the diameter of the rings increases
as their order increases, which confirms what is seen in the illustration in Figure
The plots also showed that the diameters were largest for the largest
wavelength, the red light, and were smallest for the smallest wavelength, the
The gradients, m, were calculated using the least squares fit formulae and
The errors on the gradients, ∆m, were calculated using:
The gradient of the plot for the blue filter is mB = 23.6 mm2, giving values of
λB2= 487.4 ± 5.8 nm and RB2 = 13.6 ± 0.16 nm.
The gradient of the plot for the green filter is mG = 29.5 mm2, giving values of
λG2= 609.2 ± 6.8 nm and RG2 = 13.5 ± 0.09 nm.
The gradient of the plot for the red filter is mR = 32.2 mm2, giving values of
λR2= 665.0 ± 7.8 nm and RR2 = 13.9 ± 0.08 nm.
The average radius of curvature values established from Table 1, 2 and 3 is
Rav1 = 14.55 ± 0.25 m. The average established from the values of Figure 9 is
Rav2 = 13.67 ± 0.11 m.
The errors on these averages are calculated by:
The biggest source of error in the investigation was the measurement of the ring
diameters. Although measuring the diameter as opposed to the radius of each
ring meant that the relative instrumental error of the scale was reduced, the
nature of the measurement itself resulted to the rise of uncertainty in the
Firstly, the point at which the interference pattern is in focus was difficult to
determine as there was a range of arrangements on the optical bench which
produced a projection that could be deemed to be in focus. As a result there was
an appreciable range on focus in which the ring sizes differed considerably.
In addition, deducing the centre of the each ring’s thickness as the start and end
points of the diameter gave rise to further error in the measurements as it
introduced the systematic parallax error. This meant that the judgement of the
centre was not constant and thus varied for each measurement. Also the limited
resolution of the projected scale compromised the measurements as the rings
and the scale had to be marked out on a plain sheet and measured with a ruler.
Lastly, there was no precaution to ensure that the lengths being measured are of
the diameter, exactly perpendicular to the edge of a ring, and not of a chord.
Alongside taking repeat measurements, this source of error can be improved by
various ways. To address the issue of determining the focus, an iris diaphragm
can be added to the optical set up, placed in front of the projection screen. This
will optimize the bright-dark contrast and make the point of focus more
distinguishable. This can also be addressed by taking measurements over the
range of different focus points and averaging the data. A level with a scale rule
can be used to ensure that the length being measured is exactly straight.
The results of the investigation show trends that agree with the background
physics. The results of the first task, in which the diameters are measured and
the known radius of curvature value is used to find the wavelength, show that
the diameters of Newton’s rings increase the larger the order gets.
The results also demonstrate the wavelength dependence of the diameters, as the
longer wavelength of the red filter had the larger diameter at given order than
the shorter wavelength of the blue filter. This relationship is reflected by
equation (6) where wavelength is directly proportional to the diameter. This is
because the larger wavelength has a larger path difference in the air film.
A similar relationship can be seen between the diameter and the radius of
curvature of the lens which is producing the interference pattern. Equation (6)
dictates that the radius of curvature is also directly proportional to the diameter.
The two sets of results for each value, calculated by two different approaches,
are summarised in Table 4. The percentage differences from the literature
values are included below each value.
curvature R (m)
(Equation 7, 8)
538.6 ± 12.0
656.2 ± 13.6
672.3 ± 8.7
14.55 ± 0.25
487.4 ± 5.8
609.2 ± 6.8
665.0 ± 7.8
13.67 ± 0.11
Table 4: Analysis of calculated values and comparison with literature values.
The analysis shows that graphical approach to establishing the values of the
wavelength and radius of curvature was more accurate as all four of the values
established have a lower percentage difference from the literature value
compared to those from the numerical calculation.
Moreover, the relative errors of the values from this method are also smaller,
making it the more effective method for deducing these quantities.
Although the closest a value has got to its corresponding literature value is by
an 11.58% difference, the results for both sets show the wavelengths within the
correct range. Also they have the correct relative magnitudes for red, green and
blue filters, with blue as the shortest wavelength and red as the largest.
To summarise, the experiment achieved its objective to consider the wave
behaviour of light through the investigation of interference patterns and
Newton’s rings. The results successfully demonstrated the dependence of
interference patterns on the wavelength of the light and how such patterns can
be produced through the use of lenses. More specifically, Newton’s rings can be
produced by using a plano-convex lens adjacent to a optically flat glass surface
in order to exploit the thin-film phenomena.
The observations made and data recorded successfully allowed mathematical
relationships to be applied in order to calculate the radius of curvature of the
lens and the incident wavelength of three different filters. The results confirmed
that the radius of curvature of the lens is within close range of the known value
of 12.13 m, as well as verifying the stated wavelengths of the filters. A thorough
quantitative analysis showed that the graphical approach of plotting the ring
diameter as a function of the order provided the most accurate values.
Extensions of this experiment include the use of different mediums as the film
between the convex lens and the glass surface. Example of an alternative
medium is a liquid such as water, or particular gases. Furthermore, the thin
film’s thickness can be varied in order to examine the relationship between the
radii of the rings and the thickness of the film. This can be achieved by using
two convex surfaces in contact with each other. Finally, the apparatus can be
used to measure the expansion coefficients of crystals.
In conclusion, this experiment was a success as a whole as the entire set of
results correctly followed the principles of physics and demonstrated the
mathematical relationships by producing hypothesised results. Overall the
investigation has been effective in exploring Newton’s rings and considering the
wave nature of light in its interaction with other waves.
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