2labman Newtons8
EXPERIMENT 8
To Determine the Wavelength of Sodium Light using Newton’s
Rings
Please read additional instructions on the bench for setting up the
PC and camera for this experiment
Introduction
Newton’s rings are interference fringes of equal thickness which are produced in the air film between a convex surface and an optical flat. It is interesting to note that these interference fringes, which demonstrate the wave nature of light, should be credited to Newton who was the chief proponent of the corpuscular theory.
The apparatus is set up as shown (Fig. 8.1). L is a convex lens placed on an optically flat plate of glass P, forming an air film of varying thickness. Light from the sodium lamp S, strikes a sheet of glass G, set at an angle such that the light is reflected downwards towards the lens and plate,
P. Some of the amplitude is reflected at the lower convex lens surface and some at the glass plate.
These two reflected rays travel upwards and enter the microscope and since they are coherent, they interfere in a way which depends on the phase difference introduced by the air film. Since the air film is symmetric about the point of contact, the fringes, which follow lines of equal thickness, will be concentric rings with their centre at this point. They are called fringes of equal thickness. This is an example of interference fringes produced by division of amplitude.
Background
Consider a ray of light incident on the airfilm at a point where its thickness is t. The optical path difference between the two reflected rays will be 2t. Taking into account the phase change of π for reflection at the rare to dense surface, the conditions for constructive and destructive intereference are 2t =
m+
1 λ 2
(constructive interference or bright rings)
8-1
Experiment8. To Determine the Wavelength of Sodium Light using Newton’s Rings
Travelling Microscope
Sodium Lamp
G
L
P
Figure 8.1: Apparatus for observing Newton’s rings
(8.1)
2t = mλ
(destructive interference or
To Determine the Wavelength of Sodium Light using Newton’s
Rings
Please read additional instructions on the bench for setting up the
PC and camera for this experiment
Introduction
Newton’s rings are interference fringes of equal thickness which are produced in the air film between a convex surface and an optical flat. It is interesting to note that these interference fringes, which demonstrate the wave nature of light, should be credited to Newton who was the chief proponent of the corpuscular theory.
The apparatus is set up as shown (Fig. 8.1). L is a convex lens placed on an optically flat plate of glass P, forming an air film of varying thickness. Light from the sodium lamp S, strikes a sheet of glass G, set at an angle such that the light is reflected downwards towards the lens and plate,
P. Some of the amplitude is reflected at the lower convex lens surface and some at the glass plate.
These two reflected rays travel upwards and enter the microscope and since they are coherent, they interfere in a way which depends on the phase difference introduced by the air film. Since the air film is symmetric about the point of contact, the fringes, which follow lines of equal thickness, will be concentric rings with their centre at this point. They are called fringes of equal thickness. This is an example of interference fringes produced by division of amplitude.
Background
Consider a ray of light incident on the airfilm at a point where its thickness is t. The optical path difference between the two reflected rays will be 2t. Taking into account the phase change of π for reflection at the rare to dense surface, the conditions for constructive and destructive intereference are 2t =
m+
1 λ 2
(constructive interference or bright rings)
8-1
Experiment8. To Determine the Wavelength of Sodium Light using Newton’s Rings
Travelling Microscope
Sodium Lamp
G
L
P
Figure 8.1: Apparatus for observing Newton’s rings
(8.1)
2t = mλ
(destructive interference or
References: 1. ‘Physics’, D. Giancoli 2. ‘Optics’, Hecht & Zajac 8-12