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Newton's Rings

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The rings of Newton’s in optics is a series of concentric lights and bands which are dark-coloured observed between two pieces of glass when one is convex and rests on its convex side on another piece having a surface which is flat. Thus we can say that a layer of air exists between them. The phenomenon is said to be caused by the interference of light waves that is the superimposing of trains of waves so that when their crests coincide then the light gets brightens but when trough and meets the crest. The light is said to be destroyed. The waves of the light reflected from both the top and bottom surfaces of the air film between the two pieces of glass interfere. Here, we are going to discover all the things related to Newton's ring. The rings are named after the English physicist of 17th century, Sir Isaac Newton who first investigated them quantitatively.

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What are Newton's Rings?

Newton's rings are said to be a phenomenon in which an interference pattern is created by the light's reflection between two surfaces. That is a spherical surface and an adjacent touching flat surface. It is named after great scientist Isaac Newton who investigated the effect in his treatise Opticks in 1704. When viewed with monochromatic light then these  Newton's rings appear as a series of concentric and alternating bright and dark rings centred at the point of contact which is between the two surfaces. When it is viewed with white light then it forms a concentric pattern of a ring of rainbow colours because the different wavelengths of light interfere at very different thicknesses of the layer of air between the surfaces.


The pattern is said to be created by placing a very slightly curved convex glass on an optical glass which is flat. The two pieces which are of the glass make contact only at the centre, that is we can say at other points, there is a slight air gap which is between the two surfaces. That is known as the increasing radial distance from the centre to the microscope. There is a small section of the two pieces with the gap which is increasing from right to left.  The light from a monochromatic single source of colour shines through the top piece and reflects from both the bottom surface of the top piece and the top surface of the flat optical and the two rays which are reflected combine and superpose. 


However, we can here say that the ray which is generally reflecting off the bottom surface travels a path which is quite long. The path which is the additional length is equal to twice the gap which is between the surfaces. In addition, we can say that the ray reflecting off the bottom piece of glass undergoes a 180° phase reversal. while on the other hand, the internal reflection of the other ray from the underside of the top glass causes no phase reversal.


Formation of Newton's Ring

The rings of Newton’s are formed as a result of interference which is between the light waves that are reflected from the top and bottom surfaces of the air film formed between the lens and glass sheet.

The phenomenon of the formation of rings of  Newton’s can be explained on the basis of the theory of wave of light that is:

  • An air film which is of varying thickness is formed between the lens and the sheet of glass.

  • When a ray is incident on the lens surface then it is reflected as well as refracted.

  • When the ray which is refracted strikes the glass sheet it generally undergoes a phase change of 180 on the reflection.

  • Interference occurs usually is said to occur between two waves which interfere constructively if the path difference between them is m+½* 1 and destructively if path difference which is between them is ml producing alternate bright and dark rings.

Newton Ring Effect

The brightness that is of the light which is reflected generally depends on the difference in the path length of the two rays:

  • That is the Constructive interference denoted by a: In areas where the path length difference which is between the two rays is equal to an odd multiple of half a wavelength that is denoted by λ/2 of the light waves. Then we say that the reflected waves will be in phase so the "troughs" and "peaks" which are of the waves coincide. Therefore, here we will notice that the waves will reinforce that add and the resulting light reflected intensity will be greater. As a result, an area which is bright will be observed there.

  • The destructive interference that is written as b: At other locations where the path length difference is said to be equal to an even multiple of a half-wavelength that is the wave which is reflected will be 180° out of phase. So a "trough" which is of one wave coincides with a "peak" that is of the other wave. Therefore we can say that the waves will cancel, that is they will subtract and the resulting light intensity will be weaker or say zero. As a result, an area which is the dark area will be observed there. Because of the phase that is 180° reversal due to reflection of the bottom ray the centre where the two pieces touch is dark.

  • This interference generally is said to result in a pattern of bright and dark lines or bands known as the "interference fringes" being observed on the surface. These are similar to contour lines which are on maps that are revealing differences in the thickness of the air gap. The gap which is between the surfaces is constant that too along a fringe. 

FAQ (Frequently Asked Questions)

Q1. What are Newton's Rings and How They are Formed?

Ans: The rings of Newton's are formed as a result of interference which is between the light waves that are reflected from the top and bottom surfaces of the air film which is formed between the lens and glass sheet. A film of air which is of varying thickness is formed between the lens and the glass sheet.

Q2. Why Newton's Rings Circular?

Ans: The path of difference which is between the ray that is reflected and the ray which is the incident depends upon the thickness of the air gap between the lens and the base. As the lens is said to be symmetric along its axis the thickness is said to be constant along the circumference which is of a ring of a given radius. Hence we see that Newton's rings are circular.

Q3. Explain What the Newton Ring Experiment is.

Ans: The apparatus for Newton's ring and the aim of the experiment are to study the formation of Newton's rings in the air-film in between a lens which is a plano-convex and a glass plate by using a nearly monochromatic light from a sodium-source and hence this is to determine the radius of curvature of the plano-convex lens.

Q4. What are the Applications of Newton's Ring?

Ans: FRED generally is said to be allowed for simulation of physical optics phenomena such as diffraction and interference. With this capability, the components such as the Gaussian beam of laser and interferometers can be accurately modelled and incorporated into a system that is optical.