NEWTONS RING EXPERIMENT
1.1 BACKGROUND OF THE STUDY
Recently, a novel method has been proposed and setup in our undergraduate laboratory to determine the wavelength of laser light using modified Newton’s rings. These rings are similar, yet distinctly different from conventional Newton’s rings. For instance, these fringes are essentially Fizeau, (2010) fringes like Newton’s rings, but the order is maximum for central disc unlike that in Newton’s rings. Detailed comparison of these two is provided in the reference (Longhurst, 2006), along with its principle, experimental details, coherence requirements etc. In fact, this experiment can easily be adopted in the undergraduate laboratory due to its simplicity. In the literature, there is an elegant article on classroom demonstration of Newton’s rings where the authors derive the formula for wavelength for the case when the convex surface of the lens is illuminated from a point source.
1.1.1 The principle of Newton’s rings
The schematic diagram of the experimental setup is shown in the figure 1. Laser beam, expanded using a microscope objective, is collimated using a lens. The collimated beam is incident on the plano-convex lens after passing through a beam splitter BS, inclined to the beam at an angle of 450. There are two reflected beams. One is from the plane surface: it retraces the path upto the BS, gets reflected again at the BS which remains parallel and the second is from the curved surface of plano-convex lens which is a converging beam. These two beams, on superposition, give rise to interference fringes similar in appearance to Newton’s rings. These fringes are directed by the beam splitter to the observation plane placed in a perpendicular direction, convenient for measurements. Fringe diameters at the unit magnification plane (UMP) can be measured using a travelling microscope. A unit magnification plane is one in which the ring diameters are identical to those at the plano-convex lens.
Fig.(1) : Schematic diagram for Modified Newton’s rings: a + b = fo, MO: Microscope objective, L: Collimating lens, BS: beam splitter, PL: plano-convex lens, UMP: Unit Magnification Plane, TM: travelling microscope.
Placing the observation screen at the UMP is critical to the experiment since the fringes are not localized and thus fringes are formed wherever the two beams overlap. The UMP can be located as follows. As shown in figure 1, the reflected beams from the plane and curved surfaces of the lens, can be easily distinguished. When an observation plane is placed at an arbitrary distance, one can see a spot of light on circular beam of constant diameter. The rays from the plane surface, remain collimated unlike the ones from the curved surface which focus at a distance fo, from the lens. The UMP lies exactly at a distance fo, from the focal point of the beam reflected from the curved surface, that is, at a distance 2fo, from the plano-convex lens, PL along the path of the beam. In the figure 1, a+b is approximated to be equal to fo, neglecting the thickness of the lens compared to its focal length.
1.2 STATEMENT OF THE PROBLEM
Reflection-interference occurs along the air wedge, and is seen as a series of concentric rings from above. You may assume the radius of convergence is much larger than the thickness of the wedge. When a liquid is introduced into the air between the glass and table, the radius of the tenth bright fring changes from 1.501.50 cm to 1.311.31 cm from the center of the pattern. Calculate the index of refraction of the liquid
1.3 AIM AND OBJECTIVES OF THE STUDY
The main aim of the research work is to examine Newton’s ring experiment. The objectives of the study are:
1. To determine the processes involved in Newton’s ring experiments
2. To determine the application of Newton’s ring experiment to real life situation
3. To investigate on the factors affecting the Newton’s ring experiment
1.4 RESEARCH QUESTION
The study came up with research questions so as to ascertain the above stated objectives of the study. The research questions for the study are:
1. What are the processes involved in Newton’s ring experiments?
2. What are the applications of Newton’s ring experiment to real life situation?
3. What are the factors affecting the Newton’s ring experiment?
1.5 SIGNIFICANCE OF THE STUDY
The study on Newton’s ring experiment will be of immense benefit to the entire physics and mathematics departments in tertiary institutions in Nigeria. The study will contain practices of Newton’s ring experiment. The findings of the study will help tackle the existing problems as regard to Newton’s ring experiments. The study will also serve as a repository of information to other researchers that desire to carry out similar research on the above topic. Finally the study will contribute to the body of the existing literature on Newton’s ring experiment
1.6 SCOPE OF THE STUDY
The study on Newton’s ring experiment will cover on the processes involved in Newton’s ring experiments and the applications of Newton’s ring experiment to real life situations in Nigeria
1.7 ORGANISATION OF THE STUDY
This section deals with the organization of the research work in chapters; the chapter one of the research work will cover the background of the study, the statement of problem, the aims and objectives of study, significance and the scope of study, the chapter two will deal with the review of related literature Newton’s ring experiment. The chapter three of the research work will cover the areas of materials and method. The chapter four will cover the area of experiment and discussion of results while the chapter five will cover the summary, conclusion and possible recommendation for the research work
1.8 DEFINITION OF TERMS
Newton’s ring: 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)