Newton’s Rings Phenomenon
Ring-like fringes appear where lens touches plate.
There is a 180o change in phase when a ray reflects at a boundary where refractive index increases.
Central spot (t=0) dark (two rays out of phase), so
Destructive interference: D = 2 t = m l
Constructive interference: D = 2 t = (m + 1/2)l.
Notes:
Even before Thomas Young’s discovery, Isaac Newton had noticed an interference phenomenon, but he hadn’t realized the significance. This phenomenon is called “Newton’s Rings,” and is illustrated at upper left.
The phenomenon appears when a convex lens with small curvature rests against a plate glass. The rings appear when the region between the lens and glass plate is viewed as shown in the illustration.
Newton’s Rings are explained as the interference between light reflected from the two surfaces. It turns out that the light that is reflected while travelling from air to glass suffers a 180-degree phase change, so it interferes destructively with the light reflected inside the lens. Therefore, the bright fringes happen when the difference in path length (twice the thickness of the air layer between the lens and glass) is an odd number of half wavelengths.