Q.What are Newton’s Rings?
A.Alternate dark and bright rings formed due to presence of air film when plano convex lens is placed on glass plate is called newtons rings.
Q.How are Newton’s rings formed?
A.They are formed as a result of interference between light waves reflected from the upper and lower surfaces of the air film developed between the convex surface of plano convex lens and plane glass plate.
Q.Why are they circular?
A.This is so because the air film formed is wedge shaped and loci of points of equal thickness are circles concentric with point of contact.
Q.what is the function of the 45° inclined glass plate?
A.It turns the light rays coming from an extended source to ninety degrees and so the rays fall normally on the plano convex lens.
Q.Why do rings get closer as their order increases?
A.The diameter of dark rings is proportional to the square root of natural numbers while bright rings are proportional to the square root of odd natural numbers hence the don’t increase at the same rate.
Q.Why is the center of the ring dark?
A.At the point of contact the path difference is zero but one of the rays is reflected so the effective path difference becomes λ/2 thus the condition of minimum intensity is created hence it is a dark spot.
Q.What if the glass plate is replaced with plane mirror?
A.Then we will not get interference fringe because the intensity of light reflected from mirror will be so great that it won’t be visible and we will get uniform illumination.
Q.What if sodium light is replaced with white light?
A.Few colored fringes will be observed near the center.
Q.What will happen if we replace the lens with plane glass?
A.Then interference will take place but the shape of the rings will be irregular.
Q.What will happen if few drops of liquid is introduced between the lens and glass?
A.The diameter of the rings will decrease because the diameter of the rings is inversely proportional to the refractive index.
Q.What will happen if we use a lens of small radius of curvature?
A.Then the rings will be of smaller diameter and there is chance of error while taking the readings.
Q.What if a plane glass making some angle with the glass stripe is used in place of lens?
A.Then we will have dark and bright fringes in shape of a line. Newtons ring experiment
NEWTON'S RINGS are the circular interference pattern first discovered by Newton.
Formation of fringes If monochromatic light is allowed to fall normally on the lens from a source 'S', then two reflected rays R1 (reflected from upper surface of the film) and R2 (reflected from lower surface of the air film) interfere to produce circular interference pattern. This interference pattern has concentric alternate bright and dark rings around the point of contact.
Theory of Fringes: The effective path difference between the two reflected rays R1 and R2 for a wedge shaped film from equation 2.18
If the light is incident normally on the lens, r = 0 and near to point of contact Therefore ….2.21 At point of contact t = 0 therefore the effective path difference ∆ = λ/2 which is odd multiple of λ/2 Therefore the Central fringe is dark. Condition of Maxima (Bright Fringe): The effective path difference
Condition for Minima (Dark Fringe): The effective path difference; substituting this in equation 2.21 From 2.22 and 2.23 it is clear that for particular dark or bright fringe t should be constant. Every fringe is the locus of points having equal thickness. Hence the fringes are circular in shape. Diameter of Newton’s Rings: To calculate the diameter of fringes, assume a plano-convex lens is placed on a plane glass plate as shown in figure say R be the radius of curvature of lens. In ΔO’ML As t << R; neglecting t2 (small value) Diameter of Dark Rings
Substitute t from eq. 2.24 to eq. 2.23 As Dn = 2rn
The medium enclosed between the lens and glass plate is if air therefore,
The diameter of dark ring is proportional to square root of natural numbers Diameter of Bright Rings Substitute t from eq. 2.24 to eq. 2.22 As Dn = 2rn
The medium enclosed between the lens and glass plate is if air therefore, = 1. The diameter of nth order bright fringe will be
The diameter of bright ring is proportional to square root of odd natural numbers Spacing between Fringes The Newton’s rings are not equally spaced because the diameter of ring does not increase in the same proportion as the order of ring and rings get closer and closer as ‘n’ increases. For example the diameter of dark
ring is given by Where n = 0, 1, 2, 3, 4…. From above, we conclude that the fringe width reduces with increase in ‘n’. Newton’s Ring with White Light If the monochromatic source is replaced by the white light few coloured rings are seen around dark centre later illumination is seen in the field of view. |