Dermine the wavelength of the incident light by Newton’s ring method

Objective: –

To determine the wavelength of the incident light by Newton’s ring method. 


When a parallel beam of monochromatic light is incident normally on a combination of a plano-convex lens L and a glass plate P, as shown in the fig below, a part of each incident ray is reflected from the lower surface of the lens n and a part after refraction through the air film between the lens and the plate, is reflected back from plate surface. These two reflected rays are coherent. Hence the reflected rays will interfere and produce a system of alternate dark and bright rings with the point of contact between the lens and the plate as the center. These rings are known as Newton’s rings.

If Dm is the diameter of the m-th bright ring, counted from the center, we have

where R is the radius of curvature of the lower surface of the lens L, and λ is the wavelength of light. For the (m + n) th bright ring

Where Dm+n is the diameter of the (m+n)th ring. From (i) and (ii) we get

Equation (iii) is used as the working formula for calculating R


  • Clean the plate G and lens L thoroughly and put the lens over the plate with the curved surface below B making angle with G.
  • Switch in the monochromatic light source. This sends a parallel beam of light. This beam of light gets reflected by plat B falls on lens L.
  • Look down vertically from above the lens and see whether the center is well illuminated. On looking through the microscope, a spot with rings around it can be seen on properly focusing the microscope.
  • Once good rings are in focus, rotate the eyepiece such that out of the two perpendicular cross wires, one has its length parallel to the direction of travel of the microscope. Let this cross wire tangential to one ring as shown. Note down the microscope reading.
  • Move the microscope to make the crosswire tangential to the next ring neared to the center and note that reading, continue with this purpose till you pass through the center. Take readings for an equal number of rings in the both sides of the center.

Experimental Results:

 Determination of the vernier constant of the horizontal scale of the microscope

Table for the measurement of diameter of the rings


Table for the measurement of the wavelength:

                 R =  


Percentage error:

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