Interference - Newton's Rings:
The Newton's rings are formed when a plano convex lens (lens for simplicity sake) is placed on a plane glass plate, at the point of contact. The light from Sodium vapour lamp or Mercury vapour lamp is made to fall on the pattern by a transreflecting glass plate kept at 45 degrees angle and the reflected light is observed with a 10x Microscope. With the Sodium vapour lamp concentric bright and dark fringes that taper off from the center of the pattern are observed.
These fringes, according to the Wave theory of light, are formed due to interference of waves from the convex surface of the lens and those emanating from the upper surface of the plate that is coming in contact with the lens. The radii of the fringes is used to compute the wave length of Sodium light. Using this wave length of Sodium light, the wave length of various lines in the spectrum of white light are computed depending on the relative position of these lines vis-a-vis yellow line(s) of the Sodium light.
The author's view is that when the lens and the glass plate are brought together the fluid layers on the surfaces mingle to form a plano convex shaped "complex fluid". Depending on the pressure exerted on this complex fluid and the temperature, ripple like density changes take place in the complex fluid and these density ripples taper off from the center (fig-1a) with Sodium light.
These density changes in the complex fluid can be construed as compressions and rarefactions taking place alternatively. And these density changes taper off from the centre towards the edge of the complex fluid. The width of the fringes depend on the density of the fluid/complex fluid. Between two compressions, the complex fluid has differential density gradient. When white light falls on this pattern, refraction of the white light takes place owing to the density gradient giving rise to color bands (Fig 1b).
When green color light (obtained from white light using filters or laser) is used, only that portion in the fringe pattern where green color appears will look bright green and the remaining portions appear dark including the compressed fluid portion. Same is the case with blue light derived from white light using filters. That is why different color lights give rise to bright and dark fringes of varied diameters. The same is the case with the bright and dark fringes with yellow light from Sodium vapour lamp in Fig-1a. In normal circumstances the lens is seen to be floating on the complex fluid.