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Newton’s Rings
Definition: what are newton’s rings.
Newton’s rings are a series of concentric circular rings consisting of bright- and dark-colored fringes. When a plano-convex lens lies on top of a plane lens or glass sheet, a small layer of air is formed between the two lenses. Newton’s rings are formed by the interference phenomenon when monochromatic and coherent rays of light are reflected from the top and bottom surfaces of this air film. The film’s thickness varies from zero at the point of contact to a finite value in the wedge -shaped region.
By studying the pattern of concentric rings, it is possible to determine the wavelength of monochromatic light and the refractive index of a given transparent liquid medium present in a wedge-shaped film. If white light is used instead of monochromatic, the lens system produces colorful interference, since the condition for a maximum in the interference is dependent on the wavelength.
Newton’s rings have been named after English physicist and mathematician Sir Isaac Newton, who was the first to observe the effect in 1704.
Principle of Newton’s Rings Formation
The phenomenon of the formation of Newton’s rings can be explained based on the wave theory of light.
- An air film of varying thickness is formed between the lens and the glass sheet.
- When a ray falls on the surface of the lens, it is reflected as well as refracted.
- When the refracted ray strikes the glass sheet, it undergoes a phase change of 180° on reflection .
- Interference occurs between two waves that interfere constructively if the path difference between them is (m+1/2)λ and destructively if the path difference between them is mλ, thereby producing alternate bright and dark rings.
Newton’s Rings Experiment
The apparatus consists of the following components.
- Nearly monochromatic source of light (sodium light)
- Plano-convex lens
- Optically flat glass plates
- Convex lens
- Traveling microscope
The experimental setup for Newton’s ring is shown in the figure above. The convex surface of a plano-convex lens having a long focal length (large radius of curvature) is placed in contact with a plane glass plate and clamped together. Light from a monochromatic source (e.g., sodium lamp) is allowed to fall on a convex lens through a wide slit, which renders it into a nearly parallel beam. At first, light falls on a glass plate inclined at an angle of 45° to the vertical before reaching the lens-plate system at the bottom. Light is reflected from the upper surface of the glass plate and the lower surface of the lens. Due to the air film formed by the glass plate and lens, interference fringes are formed, which are observed directly through a traveling microscope. The rings are concentric circles. A dark central spot is obtained when viewed by reflection.
Theory and Equations
Consider a ray of light incident on the air film at a point where its thickness is t . The optical path difference between the two reflected rays will be 2t . Taking into account the phase change of 180° for reflection at the rare to the dense surface, the conditions for constructive and destructive interference are
2t = (m+1/2) λ (for constructive interference or bright rings)
2t = (m+1/2) λ (for destructive interference or dark rings)
where m is the order of the ring and can take the values m = 0, 1, 2, 3, …, n
If R is the radius of curvature of the lens and r is the distance of the point under consideration to the point of contact of the lens and glass plate, then
R 2 = (R-t) 2 + r 2
or, R 2 = R 2 – 2Rt + t 2 + r 2
or, 2t = r 2 /R = D 2 /4R
since t 2 << r 2 and D = 2r , the diameter of a ring.
Combine this result with the condition for the m th and n th dark rings. Then, the diameters of the two rings are given by
D m 2 = 4Rmλ
D n 2 = 4Rnλ
Subtracting the two equations and rearranging
D n 2 – D m 2 = 4R(n-m)λ
λ = (D n 2 – D m 2 )/4R(n-m)
Thus, the wavelength λ can be determined from this equation.
If a liquid of refractive index μ is introduced between the lens and the plate, then the path difference is given by 2μt . The diameter of any ring will be less with the liquid in place than without it. The equation for wavelength is given by
λ = (D n 2 – D m 2 )μ/4R(n-m)
In a laboratory experiment, students are required to calculate the radius of curvature R of the lens. It is calculated by spherometer using the following relation
R = l 2 /6h + h/2
Here, l is the mean length of the three sides of the equilateral triangle formed by joining the tips of three outer legs, and h represents the height of the central screw above or below the plane of the outer legs.
Image courtesy: Softsolder.com
Newton’s Rings Video
Newton’s rings in transmitted light.
Newton’s rings are studied in transmitted light as well. The interference conditions for reflection and transmitted light are complementary. The diameters of the bright rings calculated for transmitted light using the equations above correspond precisely to the diameters of the dark rings in reflection. In particular, the center of Newton’s rings is bright in transmitted light and dark in reflection.
Applications of Newton’s Rings
Newton’s rings are a phenomenon that can be viewed daily. Some of its applications are as follows.
- For testing the uniformity of a polished surface by studying the interference pattern the surface makes when placed in contact with a perfectly flat glass surface
- To control the thickness of paint that is used on posters. Too much paint would exceed total weight requirements, and too little would result in faint imprints.
Ans. The plano-convex lens is circular. All the bright and dark fringes are the loci of the points of the film of equal thickness. As the equally thick films are formed along the diameter of the circular shape, the fringe pattern is also circular.
Ans. At the point of contact of the lens with the glass plate, the thickness of the air film is minimal compared to the wavelength of light. Therefore, the path difference introduced between the interfering waves is zero, the condition of minimum intensity. Consequently, the interfering waves at the center are opposite in phase and interfere destructively.
Ans. Sodium light is used in Newton’s rings experiment because it is monochromatic, and the two spectral lines of sodium can be resolved without difficulty.
- Interference: Newton’s Rings – Animations.physics.unsw.edu.au
- Laboratory Manual for Newton’s Rings Method – Nitj.ac.in
- Newton’s Rings – Iiserpune.ac.in
- Newton’s Rings: Wavelength of Light – Vlab.amrita.edu
- Newton’s Rings – Iitr.ac.in
- Newton’s Rings – Sciencedemonstrations.fas.harvard.edu
- Demonstrations in Optics : Newton’s Rings – Physicsed.buffalostate.edu
- Newton’s Rings Experiment – Brainkart.com
- Newton’s Rings – Niser.ac.in
- Investigation of Interference Patterns and Newton’s Rings- Slideshare.net
Article was last reviewed on Friday, February 3, 2023
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3 responses to “Newton’s Rings”
The diagram may be mislabeled. Perhaps R and RT are the wrong way around.
Thank you for your comment. We have edited the image.
this content is very nice,very useful for me and thank u so much
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Experiment name
To determine wavelength (λ) of light by newton’s rings experiment..
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| Newton's rings setup An extended interference pattern is most easily seen using light with only a very narrow band of wavelengths. Here we use a sodium vapour lamp, in the box at left in this photo. The lens geometry is sketched below. The geometry and the interference pattern
The sketch at left shows the global geometry. The convex side of the lens has a radius of curvature R and we consider one of the rigns with radius r where the air film between lens and flat has thickness t . We analyse this geometry below. At right we see a photograph of the interference pattern. The large dark patch at the centre is around the point where the convex lens surface touches the glass flat below it. In this area, the thin film of air between the lens and the flat has a thickness much less than the wavelength of light. This is surrounded by bright and dark circles, due to constructive and destructive interference respectively. Reflections and the conditions for constructive and destructive interferenceThis sketch shows the geometry close to the point* of contact between the convext lens (top) and glass flat (bottom). The other surfaces (the top surface of the lens and the bottom surface of the flat) are assumed to be so far away that reflections from them don't produce interference fringes: see Coherence length . Consider first the arrows closest to the point of contact, where the separation t between interfaces is rather less than a quarter wavelength, λ /4. Now consider the reflection from the top surface and that from the bottom: the two parallel rays travelling upwards. The component of phase difference between these two that is due to their different pathlength, which is much less than π. Now consider the phase difference due to reflections. (See Reflections and phases for an introduction.) At the upper surface, the ray reflects going from glass towards air – high n to low n – so the phase change is zero, as indicated on the sketch. At the lower surface, the ray reflects going from air towards glass – low n to high – so the phase change is π. So, with small phase change due to path length but a π phase change due to the two reflections, these two rays are about π radians or half of one cycle out of phase, which gives destructive interference. Consequence: in this region close to the point of contact, there is destructive interference in reflection . Next consider the middle set of rays in the sketch. Here the thickness is t = λ /4, so the path difference is 2 t = λ /2. This gives a phase difference of π. Add this to the π from the reflections and the phase difference is 2π – the two rays are out of phase by one complete cycle. So, provided the coherence length is sufficiently long, these rays give constructive interference in reflection . Finally, the rays in the sketch at left are for an air film thickness of t = λ /2, so the path difference is 2 t = λ . This gives a phase difference of 2π. Add this to the π from the reflections and the phase difference is 3π – the two rays are out of phase by one and a half complete cycles. This time we have destructive interference in reflection . And so on: our thin film has a varying thickness, so we have different interference conditions at different thicknesses. This leads to the alternating rings of constructive interference (bright rings) and destructive interference (dark), as shown in the photograph. We can use these rings to determine the radius of curvature of the lens surface, as shown below. But first, let's stop to think about: Newton's rings in transmissionThe radius of the rings gives the radius of curvature of the lens. Let's consider a dark ring with radius r at a point where the separation is t . The right angled triangle shown in red has a height R–t so Pythagoras' theorem gives us
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Newton's Ring | Full Experiment | Practical File All Lab Experiments 56.8K subscribers 8.2K 358K views 3 years ago Wave n Osc. + Optics Experiment & Viva
What will happen if a transparent liquid in Newton's rings experiment replaces air in the interspace? Newton's rings are broader near the center and sharper towards the edge.
The Newtons Rings Experiment is clearly explained in this video. Aim: To determine the wavelength of sodium light using Newton rings set up.
Performing the simulation: The simulation virtualizes the Newton’s rings experiment. The user can view the effect of Newton’s rings formed when the medium changes. Select any one type of medium. Different ring pattern can be seen by changing the radius of curvature of the lens and wavelength of light source. .
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EXPERIMENT 16 NEWTON'S RINGS I. THEORY Sir Isaac Newton discovered bright and dark concentric rings when he held two lenses in contact. This phenomenon can only be explained as constructive and destructive interference, which requires a wave theory of light.
In this video we are going to conduct Newton s ring experiment. The inference pattern dark or bright fringes appeared here and it was firstly observed by Sir Isaac Newton in 1917.
To determine wavelength (λ) of light by Newton's rings Experiment.
Introduction Newton's rings are interference fringes of equal thickness which are produced in the air film be-tween a convex surface and an optical flat. It is interesting to note that these interference fringes, which demonstrate the wave nature of light, should be credited to Newton who was the chief pro-ponent of the corpuscular theory.
Newton's rings is analysed as an interference pattern and we derive the equation relating the len's radius of curvature to the radii of the dark rings. Light, interference, thin films. Physclips provides multimedia education in introductory physics (mechanics) at different levels.
What it shows: Waves reflecting from two surfaces can interfere constructively and destructively. In this case it is light waves that are being reflected at glass/air and air/glass interfaces. The interference produces a concentric ring pattern of rainbow colors in white light, or dark and light rings in monochromatic light.
Final Reading = Main Scale Reading +( Vernier Scale Reading * Least Count) >Place the crosswire on 20th dark ring on the left side. >Note the MS and vernier readings and move towards right to note the readings for 18th , 16th , 14th ....2nd rings. >Keep moving towards other side of the central ring and note down the reading for 2nd , 4th ...
Newton's rings is a phenomenon in which an interference pattern is created by the reflection of light between two surfaces, typically a spherical surface and an adjacent touching flat surface. It is named after Isaac Newton, who investigated the effect in 1666. When viewed with monochromatic light, Newton's rings appear as a series of ...
All Lab Experiments 56K subscribers 4.6K 191K views 4 years ago Wave n Osc. + Optics Experiment & Viva Most Important Viva Questions of Newton's Ring Experiment. ...more
Newton's rings is an interference pattern caused by the reflection of light between two surfaces - a spherical surface and an adjacent flat surface.
Apparatus Required : A plano-convex lens of large radius of curvature, optical arrangement for Newton's rings, plane glass plate, sodium lamp and travelling microscope.
To study the interference of light by observing the formation of Newton's rings. To determine the radius of curvature of a plano-convex lens for a particular monochromatic light.
These rings are known as Newton's ring. For a normal incidence of monochromatic light, the path difference between the reflected rays (see Fig.1) is very nearly equal to 2 t where and t are the refractive index and thickness of the air-film respectively. The fact that the wave is reflected from air to glass surface introduces a phase shift of .
Newton's rings is a phenomenon in which an interference pattern is created by the reflection of light between two surfaces; a spherical surface and an adjacent touching flat surface.
Aim of the experiment. To study the formation of Newton's rings in the air-film in between a plano-convex lens and a glass plate using nearly monochromatic light from a sodium-source and hence to determine the radius of curvature of the plano-convex lens. Apparatus required. A nearly monochromatic source of light (source of sodium light) A ...
Newton's rings, in optics, a series of concentric light- and dark-coloured bands observed between two pieces of glass when one is convex and rests on its convex side on another piece having a flat surface. Thus, a layer of air exists between them. The phenomenon is caused by the interference of.
In this video, we will understand the setup to obtain Netwon's ring in the laboratory. Also, we will explain how to represent it in the exam. It will help y...