What Is Newton’s rings.
NEWTON’S RINGS
To study the interference in a non-uniform thin film, Newton’s rings experiment is used. The formation of rings can be understood by arranging a plano-convex lens and a glass plate as shown in the figure.
Formation of Newtons rings When a plano convex lens with its convex surface is placed on a glass plate then an air film is formed between the gap. If a monochromatic light is incident normally on the convex lens and the air film is observed using a microscope then alternate dark and bright rings are observed. It is due to the interference of reflected rays from top and bottom surface of the non uniform air film. These rings are first observed by the Newton and hence these are known as Newton’s rings.
Experimental arrangement .
The experimental arrangement of Newton’s rings is shown in the figure. When a monochromatic light is incident on a glass plate kept at 45o then some part of light is reflected normally towards the plano convex lens and refracted into the lens. Now some part of this light is reflected by the upper surface of air film and other part is reflected by the lower surface of air film. The two reflected rays interfere with each other and form circular rings.
Path difference = δ = 2 µt cosr + λ /2
For air , µ=1
then path difference = δ = 2t cos r + λ /2
For normal incident rays, r = 0
then, δ = 2t cos (0) + λ /2
Path difference = δ = 2t + λ/2
At point of contact, t = 0, so δ = λ /2
i.e, at point of contact t = 0,
path difference = λ/2, i.e., the reflected light rays at the point of contact has a phase change of π. Hence, the incident and reflected light rays are out of phase and causes destructive interference. So, it will form dark region at the center.
The condition for bright ring is 2𝑡 + 𝜆 2 = 𝑛𝜆
2𝑡 = (2𝑛 − 1) 𝜆/2 𝑤ℎ𝑒𝑟𝑒 𝑛 = 1,2,3, ….
The condition for dark ring is
2𝑡 + 𝜆 2 = (2𝑛 + 1) 𝜆/2
2t = nλ, n = 0, 1, 2, 3, …..