Newton's Rings Lab Report

Abstract

The purpose of carrying out this experiment was to investigate the phenomenon of Newtons’s Rings, to gain a better understanding of the theory Newton developed as well as to calculate the radius of curvature of a plano-convex lens and the thickness of a section of optical fibre.
The effect is named after Isaac Newton who first studied it in 1717.

The pattern observed appears as a series of concentric bright and dark fringes, which has its centre at the point of contact between two surfaces. The experiment was conducted by allowing monochromatic light from a sodium lamp, which is a monochromatic source, to fall normally onto the plano-convex lens. The light underwent reflection and refraction and was observed by a travelling microscope.

It was shown that the theory of Newton’s rings has practical

The plano-convex lens is replaced with another glass plate and a section of optical fibre was placed between them at one end. Light falling normally on the plates will be reflected back out with a phase difference.

Figure 3: Apparatus set up for finding the thickness of an optical fibre, showing the optical fibre between the two glass plates

This is due to the fact that some of the coherent light waves were reflected from the top plate and others from the bottom, this path difference resulted in the interference of these waves with one another. This caused an interference pattern similar to the below image:

Figure 4: Image similar to the observed interference pattern. [3]

This pattern is also linked to Equations 1&2 as stated previously. The position of every fourth fringe was recorded giving the value which is required in Equation 3 below. (See Table1 for the recorded data)

Knowing the wavelength of sodium to be 589nm the angle, ,at the apex between the two glass plates can be calculated using Equation 3 as