Newton's rings by with light (coloured rings)

Theory of Newton's rings:

5)Newton's rings by with light (coloured rings):

          Diameter of the Newton's ring depends on the wavelength of light used .  White  light  consists  of
 ( seven colours) continuous range of colours.  Each colour possesses a particular wavelength. So each colour produces its own system of rings. Only first , it cannot be seen.

           At a point, near the  point of  contact if  for a particular  wavelength  ( say violet colour ),  the condition of brightness will be satisfied,  then  it gives maximum intensity for that colour. At the same point, for other colours, the condition of darkness may be satisfied. Consequently that point will have average intensity due to red colour. The same is true for other colours.

6) Determination of wavelength of monochromatic light using Newton's rings:

              Let S be a monochromatic source of light which is at the focus of a lens L₁. So horizontal parallel rays fall on the glass plate B which is placed at 45° to the horizontal line. The glass plate B, partially reflects these parallel rays normally on the lens L placed on the glass plate G. Interference occurs between the rays reflected from the top and the bottom surfaces of the air film. Newton's rings formed can be viewed with the help of traveling microscope focussed on the air film. The position of traveling microscope is adjusted to get the centre of Newton's rings at the position of the intersection of the cross-wire of the eye piece. The microscope is moved until one of the cross wire becomes tangential to the 20th dark ring . This microscope reading is noted. Then the microscope is moved such that the cross wire becomes tangential to 15th, 10th, 5th dark rings respectively. Reading of microscope for each case should be noted . Microscope reading corresponding to the same rings on the other side of the centre should be noted. The reading are noted in the observation table:

Dₙ = Diameter of nth bright ring

Dₙ² =2(2n-1)λR             .....(1)

Dₙ₊ₚ= Diameter of (n+p)the bright ring

Dₙ₊ₚ²= 2[2(n+p)-1]λR   .....(2)

Dₙ₊ₚ² - Dₙ² = 4.pλ.R

λ = Dₙ₊ₚ² - Dₙ² 

              4pR                   .....(3)

              The radius of curvature of lens L can be found out by any simple method. Substituting the value of R and the mean value of P in equation (3), λ can be calculated.