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NV6104
Newton’s Ring Apparatus
NV6104
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Learning Material
Ver 1.1
Ph.: 91-731- 6546638
E-mail: info@nvistech.com
Toll free : 1800-103-5050
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Nvis Technologies Pvt. Ltd.
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Pardeshipura, Indore- 452 010 India
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141-B, Electronic Complex,
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NV6104
Newton’s Ring Apparatus
NV6104
Table of Contents
1. Introduction
3
2. Features
4
3. Technical Specifications
5
4. Instruction for assembling Newton’s Ring Apparatus
6
5. Theory
8
Experiment 1
To determine the Wavelength of sodium light by measuring the diameters of
Newton’s Rings
6. Theory
15
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Experiment 2
To determine the Refractive Index of a liquid transparent medium such as
water using Newton’s Ring Apparatus
7. Warranty
11
16
20
8. List of Accessories
21
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NV6104
Introduction
NV6104 Newton’s Ring Apparatus is one of the basic experiments at graduation level. With the help of this
apparatus, the wave nature of light is confirmed. It is based on the phenomenon of interference of light
waves obtained from single coherent light sources (of same frequency and constant or zero phase
difference).
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Figure 1
Figure 1 shows the apparatus for obtaining Newton’s Rings (light source not shown). It has three parts: (1)
Microscope with horizontal measurement (2) Newton’s Ring assembly and (3) Sodium vapour lamp as the
Monochromatic light source.
Main purpose of the apparatus is to understand the concept of interference with a simple experiment. It has
got its name after Isaac Newton, who first analyzed it though Robert Hooke first observed it.
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The phenomenon of Newton's Ring is the result of interference between the partially reflected and partially
transmitted rays from both the lower curved surface of the lens as well as upper surfaces of the plate.
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When a plano-convex lens of large focal length is kept on a plane glass plate such that the convex surface is
in contact with the plate, a wedge-shaped air film is formed between the lens and the plate. Due to the
formation of wedge - shaped film, the path difference between the reflected rays varies away from the point
of contact. This gives rise to a ring pattern. This ring pattern can be seen along the reflected part, top of the
glass plate inclined at 45º through microscope by following the procedure of experiment given in this
manual.
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When viewed with a monochromatic light, it appears as a series of concentric, alternating bright and dark
rings centered at the point of contact between the two surfaces. The thickness of the film is radially
symmetrical and increases outwards from the point of contact. The outer rings are spaced more closely than
the inner ones since the slope of the lens surface increases outwards resulting in smaller separation of the
outer rings.
Similarly, when viewed with white light, it forms a concentric ring pattern of seven colours because the
different wavelengths of light interfere at different thicknesses of the air layer between the surfaces. On
replacing the sodium vapour lamp with mercury lamp, coloured rings can be obtained.
By studying the ring pattern, we can determine the wavelength of the monochromatic light and also the
refractive index of a given transparent liquid medium present in the wedge - shaped film. These two
experiments have been described in this manual in detail.
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Features
Traveling microscope with x-y-z axes movement.
Horizontal measurement scale with fine and coarse movement knobs.
Cross wire in the field of view for ring’s diameter measurement.
Newton’s ring assembly consisting of plano-convex lens mounted on an optically plane
glass plate.
Adjustable plain glass plate is provided to be inclined at 45º with respect to the vertical
plain.
Sodium vapour lamp as the monochromatic (an average of D1 & D2 lines = 5893 Ǻ)
and broad light source.
Light emitting diode for taking measurement in the dark room.
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Spherometer for radius of curvature measurement.
Extensive learning material
2 year warranty.
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Technical specifications
Sodium Vapour Lamp:
Wavelength
:
5893Ǻ
Power Supply
:
Input voltage 230V ± 10%, 50Hz
Operating wattage
:
35W
Type
:
Plano-convex
Focal length
:
100cm
Diameter
:
4cm
:
10X
:
5.7 Kg
Horizontal movement limit
:
9cm
Least count of circular scale
:
0.001cm
:
5”x3”x3”
Lens:
Newton’s Ring Microscope:
Weight
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Dimensions
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Magnification
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NV6104
Instruction for assembling Newton’s Ring Apparatus
Parts of Newton’s Ring Apparatus
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1. Before assembling the Newton’s Ring Apparatus make sure that all the lenses are clean if not
clean all the lenses with the help of cotton cloth.
2. Now take the mounting unit, place Plane Glass Lens in the circular gap provided for holding
lenses. After that, carefully place Plano Convex Lens such that it convex side touches the plane
glass lens.
3. Dark circular fringe is seen as soon as you place Plano Convex Lens.
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4. Now fix lenses on the mounting unit with the help of lens cover. (Carefully rotate the lens cover
till its base just touches the lenses)
5. Now adjust position of the dark fringe such that it comes at the center, for that place all three
leveling screw in the holes on the lens cover and very carefully rotate them for changing the
position of dark fringe.
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Precaution: Avoid over tightening of leveling screw as it can break the lens.
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6. Now insert the glass plate in the hole provided on vertical rod of mounting unit, then place the
spring and fix it with the help of tightening screw.
7. Now mount this mounting unit above the base unit and fit it with the help of mounting screws.
8. Adjust its position such that it makes the angle of 450 with respect to vertical plane.
The Newton’s Ring Apparatus will look as shown in the figure below
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NV6104
Theory
The formation of maximum intensities at some points and minimum intensities at the other due to the
superposition of two coherent light waves (of same frequencies and constant phase difference) is called
interference of light. The interference fringes are observed as an alternate pattern of bright and dark fringes.
The interference at a point where the intensity of light is maximum, is called constructive interference
(corresponds to bright fringe). For constructive interference, the two waves should have either same phase or
a constant phase difference of
Φ = 2nπ where n = 0, 1, 2,
or, a constant path difference of
Δ = nλ
While the interference at another point where the intensity of light is minimum, is called destructive
interference (corresponds to dark fringe). For destructive interference, the two waves should have either
same phase or a constant phase difference of
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Φ = (2n+1) π where n = 0, 1, 2,
or, a constant path difference of
Δ = (2n+1)λ/2
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Interference fringes are obtained by dividing the single coherent source into two sources. This can be
achieved either (1) by division of wave front, that is by taking (or considering) two secondary wavelets on
the same wave front and superposing them or (2) by division of amplitude, that is by separating the
amplitude of single wave and reuniting them.
When light is incident on a thin film (thickness of the order of wave length of the incident light), it suffers
partial reflection and partial transmission at both upper as well as lower surfaces of the thin film. The
transmitted light ray again suffers reflection at the lower surface. Interference occurs between the rays in the
reflected and transmitted parts.
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Similarly, in a wedge-shaped film, partial reflection as well as partial transmission also takes place.
Moreover, the path difference changes from point to point which results into an interference fringes.
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Figure 2
Figure 2 shows an air wedge-shaped film formed between the convex and plane glass plate inclined at an
angle say, θ. The refractive index of the film is μ. Let ray AB is incident from a broad monochromatic
source almost normally on the film. It suffers partial reflection (ray BE) and partial transmission (ray BC) on
the convex surface. Again ray BC suffers partial reflection (ray CD) and partial transmission (not shown) on
the plane surface at C.
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Newton’s Ring is a interference pattern obtained by the division of amplitude from a single light source.
The bright rings are caused by constructive interference between the reflected light rays BE and CF for a net
path difference = nλ where, n = 0, 1, 2,…….. and λ is the wavelength of the incident light.
While the dark rings are caused by destructive interference between the same light rays BE and CF for a net
path difference = (2n + 1) λ/2 where, n = 0, 1, 2,……...
For wedge-shaped thin film, path difference between the rays BE and CF is given by
Δ = 2 μt cos r
where, t is the thickness of the film at B (or at D) and r is the angle of refraction at B.
Since the angle of incidence is almost normal, so we can assume
cos r =1
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Note that here we ignore the reflections from top of the plano-convex lens and bottom of the plane circular
glass plate since these reflections just contribute to the overall glare. The reflections of interest are only
those involving the surfaces in contact.
Now by Stoke’s law, there is no phase change at the glass-air interface of the convex lens (because the wave
is going from a higher to a lower refractive index medium) whereas the reflection at the air-glass interface of
the plane glass plate undergoes an additional path difference of λ/2.Therefore, net path difference =
Δ+
λ
λ
= 2 μt +
2
2
For bright fringe, net path difference = nλ where, n = 0, 1, 2,….…..
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2 μt = (2n-1)
or,
λ
2
and for dark fringe, net path difference = (2n + 1) λ/2 where, n = 0, 1, 2,……..
2 μt = nλ
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or,
At the centre, the two glass surfaces are in intimate contact and there is no reflection because it is as if there
were no surface that is, t = 0 or, 2μt = 0 which is the condition for dark fringe. Hence the center of the
pattern is always dark.
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Figure 3
Again, in the right angled triangle OAB of Figure 3,
2
2
2
OB = OA + AB
or, R2 = (R-t)2 + r n2 where, rn is radius of the nth ring
2
2
or, rn = 2Rt (for t << 2Rt)
2
or, t = rn /2R where, R is the radius of curvature of the Plano-convex lens
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or, t = Dn2/8R where, Dn is the diameter of the nth ring.
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NV6104
In practice, it is not possible to find the exact centre of the bull's eye in order to obtain rn. Rather, the
traveling microscope can measure an approximate diameter, Dn for the interference ring.
th
th
Therefore, substituting the value of t in 2μt = nλ, we obtain the diameters of the n and (n+p) dark fringes
respectively as
4nRλ
4 (n+p) Rλ
2
2
Dn =
and D n+p =
μ
μ
for p ≥ 1.
By subtracting these equations, we can obtain the wavelength of incident light by
λ=
2
2
(Dn+p - D n ) μ
4pR
Since the human eye is more sensitive to small changes in low intensity, we will measure positions of dark
fringes throughout the experiment. This relation also holds true for bright rings.
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NV6104
Experiment 1
Objective :
To determine the Wavelength of sodium light by measuring the diameters of Newton’s rings
Apparatus required :
1
Newton’s Ring Microscope
2
Sodium Vapour Lamp with Encloser
3
Circular slit plate
4
Light emitting diode source
Procedure :
1
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The Newton’s Ring microscope has two parts, Microscope with horizontal measurement and a
Newton’s Ring assembly. First detach the Newton’s Ring assembly and clean the adjustable glass
plate, plano-convex lens and its adjoining glass plate with a clean cloth.
Note : Do not Detach The Plano-Convex Lens From Glass Plate Frequently. It Will Disturb The
Measurements.
2
Replace the plano-convex lens over the glass plate and tight them carefully with the help of three
leveling screws (if present). A interference ring pattern can be observed with the naked eye as shown
in Figure 4.
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Precaution : Avoid Over-Tightening Of The Screws. Tight the Screws to Bring the Central Dark Fringe At
The Centre Adjust Its Diameter to Be Equal to Nearly 3mm.
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3.
Figure 4
Arrange the Newton’s Ring assembly as shown in Figure 5. Note that the glass plate P’ should be
inclined at 45º with respect to the vertical plain.
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Figure 5
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First connect the sodium vapour lamp Power Supply with mains. Then connect it with the light source
box with the help of mains cord. Then switch ON the Power Supply.
Precaution : Never Connect The Sodium Vapour Lamp Directly To The Main Power Supply.
Wait for 30 minutes till the lamp glow bright yellowish.
6
Insert the circular slit plate into the slit-holder. Adjust to fully illuminate the glass plate P’ inclined at
45º.
7
Calculate the least count of the traveling microscope
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5
Pitch
of
the Number
of Least count of
micrometer screw, divisions on the microscope,
circular scale, N
P (in cm)
P/N (in cm)
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8
Take a view through the eye piece (E) of the tube as shown in Figure 6.
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Figure 6
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9
There are two type of movements provided in the microscope, coarse and fine. Align the microscope
tube, with the help of coarse movement knob no. 2 and 4 as shown in Figure 6, to bring it over the
glass plate P’.
10
Now move the microscope tube with the help of fine movement knob no. 3 close to the plate P’ to
obtain clear image of surface. Slowly move towards upward direction. Newton’s rings pattern is
observed. Adjust by moving the microscope to and fro, if necessary, to view the full pattern. Adjust
further for better contrast between bright and dark fringes as shown in Figure 7.
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Figure 7
Bring the cross-wire, using knob no.1 shown in figure 6, in the central dark fringe such that their centre
should coincide with each other.
12
Slide the cross-wire to the left till the vertical cross-wire line lies tangentially at the 20th dark ring.
Note the reading on the main and circular scale using the light emitting diode.
13
Now slowly slide the microscope to the right and note the reading when the vertical cross-wire lies
th
th th
th
tangentially at the 16 , 12 , 8 and 4 dark rings respectively.
14
Keep sliding the microscope to the right and again note the readings when the vertical cross-wire lies
tangentially at the 4th, 8th, 12th, 16th and 20th dark rings respectively.
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Least Count of circular scale = ……….……..cm
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Where, Dn and Dn+p are the diameters of nth and n+pth ring respectively, N is the nth ring and
P is the interval between the rings selected.
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15
Take the mean Dn +p2 – Dn2.
16 Substitute the values of mean (Dn+p2- Dn2), R and μ = 1 (for air) in the following formula to obtain the
Wavelength of light from sodium vapour lamp:
Dn+p2 – Dn2)
4pR
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NV6104
Theory
The refractive index of a medium is a measure for how much the speed of light is reduced inside the
medium. It is the ratio of velocity of light in air or vacuum to that in the given medium. For example, typical
glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the speed in air or
vacuum. Light rays change direction when they cross the interface from air to a medium or vice versa due to
change in the refractive index. Moreover, light rays are reflected partially from surfaces that have a
refractive index different from that of their surroundings.
Now, wavelength of light from sodium vapour lamp can be calculated by
λ=
2
2
(D n+p - D n ) μ
4pR
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Where, Dn and Dn+p are the diameters of nth and (n+p)th Newton’s ring,
μ is the refractive index of the medium of wedge-shaped film,
R is the radius of curvature of plano-convex lens and
p ≥ 1.
From the above equation, we can obtain the following relations for air (μ = 1) and water films formed inbetween the plano-convex and glass plate combination
2
2
(D n+p - D n )
(in air)
= 4pRλ and
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2
2
(D n+p - D n )
(in water)
=
4pRλ
μ
Dividing the above two equations, refractive index of the transparent liquid medium (water in our case) can
be calculated as
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2
2
(D n+p - D n ) (in air)
μ=
2
2
(D n+p - D n )(in water)
Note that this relation also holds true for bright rings.
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From the above relation, we can infer that the Newton’s rings formed in water or any other transparent
liquid medium, will contract as compared to that in air since refractive index of water (μ=1.33) or any other
liquid is always greater than that of air (μ=1).
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NV6104
Experiment 2
Objective :
To determine the Refractive Index of a transparent liquid medium such as water using Newton’s Ring
Apparatus
Apparatus required :
1
Newton’s Ring Microscope
2
Sodium Vapour Lamp with Encloser
3
Circular Slit Plate
4
Light Emitting Diode Source
Procedure :
1
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The Newton’s Ring microscope has two parts, Microscope with horizontal measurement and a
Newton’s Ring assembly. First detach the Newton’s Ring assembly and clean the adjustable glass
plate, plano-convex lens and its adjoining glass plate.
Note: Do not Detach The Plano-Convex Lens From Glass Plate Frequently. It Will Disturb The
Measurements.
2
Replace the plano-convex lens over the glass plate and tight them carefully with the help of three
leveling screws (if present). An interference ring pattern can be observed with the naked eye as
shown in Figure 8.
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Precaution: Avoid Over-Tightening of The Screws. Tight the screws to Bring the Central Dark Fringe At
the Centre. Adjust Its Diameter to Be Equal to Nearly 3mm.
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3
Figure 8
Arrange the Newton’s Ring assembly as shown in Figure 9. Note that the glass plate P’ should be
inclined at 45º with respect to the vertical plain.
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Connect the sodium vapour lamp Power Supply with mains. Then connect it with the light source box
with the help of mains cord. Then switch ON the Power Supply.
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4
Figure 9
Precaution : Never Connect The Sodium Vapour Lamp Directly To The Main Power Supply.
Wait for 30 minutes till the lamp glows bright yellowish.
6
Insert the circular slit plate in the slit-holder provided in the lamp source. Adjust to fully illuminate the
glass plate inclined at 45º.
7
Calculate the least count of the traveling microscope.
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5
Pitch
of
the Number
of Least count
micrometer screw, divisions on the microscope,
P (in cm)
circular scale, N
P/N (in cm)
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Take a view through the eye piece (E) of the tube shown in Figure 10.
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Figure 10
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9
There are two type of movements provided in the microscope, coarse and fine. Align the microscope
tube, with the help of coarse movement knob number 2 and 4 as shown in Figure 10 to bring it over the
glass plate P’.
10
Now move the microscope tube with the help of fine movement knob no. 3 close to the plate P’ to
obtain clear image of surface. Slowly move towards upward direction. Newton’s Rings pattern is
observed. Adjust by moving the microscope to and fro, if necessary, to view the full pattern. Adjust
further for better contrast between bright and dark fringes as shown in Figure 11.
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Figure 11
Bring the cross-wire in the central dark fringe, using knob no.1 shown in figure 10, such that their
centre should coincide with each other.
12
Slide the cross-wire to the left till the vertical cross-wire line lies tangentially at the 20 dark ring.
Note the reading on the main and circular scale using the light emitting diode.
13
Now slowly slide the microscope to the right and note the reading when the vertical cross-wire lies
tangentially at the 16th, 12th, 8th and 4th dark rings respectively.
14
Keep sliding the microscope to the right and again note the readings when the vertical cross-wire lies
th th
th
th
th
tangentially at the 4 , 8 , 12 , 16 and 20 dark rings respectively.
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11
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Least Count of circular scale = ……….……..cm
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Where, Dn and Dn+p are the diameters of nth and n+pth rings respectively,
n is the nth ring and
p is the interval between the rings selected.
15
2
Take the mean Dn +p2 – Dn for air film.
16
Now, open and take out the plano-convex lens and put few drops of water on the plane glass plate and
again carefully mount back the lens over the plate.
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17
Again repeat the steps from 2 nd to 15th of procedure of this experiment for the water film formed
between the convex and plane surfaces.
18
Substitute the corresponding values of (D n+p2 - Dn2) for air and water films in the following formula to
determine the value of
refractive index of water:
Dn+p2 – Dn2) (In air)
(Dn+p2 – Dn2)(In water)
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Warranty
1)
We guarantee the product against all manufacturing defects for 24 months from the date of sale by us
or through our dealers.
2)
The guarantee does not cover perishable item like cathode ray tubes, crystals, batteries, photocells etc.
3)
The guarantee will become void, if
a) The product is not operated as per instruction given in the learning material.
b) The agreed payment terms and other conditions of sale are not followed.
c) The customer resells the instrument to another party.
d) Any attempt is made to service and modify the instrument.
The non-working of the product is to be communicated to us immediately giving full details of the
complaints and defects noticed specifically mentioning the type, serial number of the product and date
of purchase etc.
5)
The repair work will be carried out, provided the product is dispatched securely packed and insured.
The transportation charges shall be borne by the customer.
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4)
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List of Accessories
1. Newton’s Ring Microscope………………………………………………………1 No.
2. Sodium Vapour Lamp……………………………………………………………1 No.
3. Sodium Vapour Lamp Encloser…………………………………………………1 No.
4. Circular Slit Plate…………………………………………………………………1 No.
5. Power Supply………………………………………………………………………1 No.
6. Mains Cord………………………………………………………………………...1 No.
7. Learning Material CD……………………………………………………………..1 No.
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