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Demonstration of Newton’s Ring Experiment Using a USB Camera

P. Kopatta1* and P. Limsuwan2

1Division of Physics, Mahidol Wittayanusorn School, Puttamonthon, Nakhon Pathom 73170, Thailand

2King Mongkut’s University of Technology Thonburi (KMUTT)Bangkok 10140, Thailand

*Corresponding author. E-mail: pornchai_kopatta@

Abstract

Newton’s ring is one of the most well-known classic experiment of thin film interference. In the experimental setup, a convex lens is placed on an optical flat. The sodium (Na) lamp is generally used as a chromatic light source. The yellow light from Na lamp is reflected by 45( glass slide onto the top surface of convex lens. The light is then transmitted into the convex lens and finally the circular fringes are formed. In conventional experiment, the fringes are observed through the eyepiece of a travelling microscope. However, this method is very difficult for students to setup the experiment for observing circular fringes. Furthermore, the students in the classroom cannot observe the fringes at the same time. In this work, a USB camera was attached to the eyepiece of the travelling microscope for observing the circular fringes. The USB camera was connected to a computer notebook. The picture of circular fringes was then displayed on a computer screen and also on a large projector screen. Therefore, this technique can be used for the demonstration in the classroom and all the students can observe the Newton’s ring at the same time.

Keywords: Newton’s rings, USB Camera

Introduction

Newton’s rings are formed due to interference between the light waves reflected from the top and bottom surfaces of the air film formed between the convex lens and glass plate. The monochromatic light such as Na lamp is generally used as a light source. The yellow light from Na lamp is reflected by 45( glass slide onto the convex lens and glass plate to produce the interference as mentioned above and results in the formation of circular fringes. The central fringe is usually a dark fringe. The circular fringes can be observed by a microscope. The real diameter of the whole circular fringes that can be observed is only 1-2 mm. Therefore, it is difficult for student to setup the Newton’s ring experiment to observe the circular fringes through the eyepiece of microscope. Moreover, this experiment is usually carried out in the dark room and the student can observe the fringes only one by one. Therefore, it is not possible to demonstrate Newton’s ring experiment in the lecture room. In this work, the conventional Newton’s ring experiment was modified by attaching a WebCam USB camera to the eyepiece of travelling microscope. Then, the circular fringes was recorded and displayed on a computer screen and also a large projector screen. Therefore, this technique can be used for the demonstration of Newton’s ring experiment in the classroom and students in the whole class can observe the circular fringes at the same time.

Materials and Methods

Theory [1-3]  

When a plano-convex lens with its convex surface is placed on a plane glass plate, an air film of gradually increasing thickness outward is formed between the lens and the glass plate as shown in Fig.1. The thickness of film at the point of contact is zero. If monochromatic light is allowed to fall normally on the lens, and the film is viewed in reflected light, alternate bright and dark concentric rings are seen around the point of contact. These rings were first discovered by Newton, that's why they are called Newton’s rings

[pic]

Figure 1: Experinental arrangement for Newton’s

rings

Newton's rings are formed due to interference between the light waves reflected from the top and bottom surfaces of the air film formed between the lens and glass plate as shown in Fig. 1.

  The phenomenon of the formation of the Newton's rings can be explained on the basis of wave theory of  light. An air film of varying thickness is formed between the lens and the glass plate. When a light ray is incident on the upper surface of the lens, it is reflected (ray 1) as well as refracted. When the refracted ray strikes the glass plate, it is reflected (ray 2) and undergo a phase change of 180( . Interference occurs between ray 1 and ray 2 which interfere constructively if path difference between them is (m+1/2)( and destructively if path difference between them is m( producing alternate bright and dark rings as shown in Fig. 2.

[pic]

Figure 2: Newton's rings observed through a

microscope.

  Let the radius of curvature of the convex lens is R and the radius of ring is 'r' as shown in Fig. 3. Consider light of wave length '(' falls on the lens. After refraction and reflection two rays 1 and 2 are obtained. These rays interfere each other producing alternate bright and dark rings. At the point of contact the thickness of air film is zero and the path difference is also zero and as a 180O path difference occurs, so they cancel each other and a dark ring is obtained at the center.

[pic]

Figure 3: Showing the radius of curvature of lens.

As we move away from the central point , path difference is also changed and alternate dark and bright rings are obtained. Let us suppose that the thickness of air film is 't'. By using the theorem of geometry,

[pic]

where AB = t

BC = 2R-t

BD = r

Since ‘t’ is very small as compared to ‘r’, therefore, neglecting ‘[pic]’. Then,

r2 = 2Rt (1)

In thin films, path difference for constructive and destructive interferences are

Bright 2nt = (m+1/2) ( (2)

Dark 2nt = m( (3)

where n= refractive index For air n = 1 Therefore,

Bright 2t = (m+1/2) ( (4)

Dark 2t = m( (5)

where m = 0,1, 2, 3, …

For central dark ring m = 0

For 1st dark ring m = 1

For 2nd dark ring m = 2

From Eq.(5) t = [pic] (6)

Putting the value of t in Eq.(1)

[pic]

= mR(

[pic]

D2 = 4mR( (7)

where D is the diameter of the ring of order m.

Materials

1. A travelling microscope attached on the rail equipped with a veriner scale

2. A sodium light source, (=5,893 nm

3. A convex lens and a glass plate

4. A Webcam USB camera

Methods

Figure 4 shows the diagram of Newton’s ring experimental setup. It consists of travelling microscope a attached on a rail of stainless steel frame. A veriner scale is attached in parallel with the rail. Therefore, during the measurements of the ring diameter by travelling microscope, the distance or ring diameter is measured by veriner scale. At the eyepiece of travelling microscope, it is attached with a WebCam USB camera to record the Newton’s rings and transmit to computer for the display on the screen.

[pic]

Figure 4: Diagram of Newton’s ring experimental

setup.

Results and Discussion

Fig. 5 shows the photograph of Newton’s ring experimental setup.

The front view of Fig.5(a) shows the light box which contains the Na lamp. The 45( glass slide, convex lens and glass plate cannot be observed. The yellow light can pass through a hole on the light box to reflect at 45(glass slide.

From the side view of Fig.5(b), 45( glass slide can be observed. However, due to the yellow light of Na lamp which reflect from 45( glass slide onto the lens surface is to bright the convex lens and glass plate cannot be observed.

Fig.5(c) shows the WebCam USB camera attached on the eyepiece of travelling microscope and USB cable is connected to a notebook computer.

Fig.5(d) shows the appearance of Newton’s rings on the notebook computer screen

Fig.5(e) shows the large circular rings on the notebook computer screen.

[pic]

a) Front view

[pic]

b) Side view

[pic]

c) WebCam USB camera

[pic]

(d) Newton’s rings were displayed on

computer screen

[pic]

(e) Newton’s rings on computer screen

Figure 5: Photograph of Newton’s ring experimental

setup.

Conclusions

In this work, a conventional Newton’s ring experimental setup was modified by the attachment of a WebCam USB camera to the eyepiece of travelling microscope. The circular fringes were recorded and transmitted to the computer to display on the computer screen. The circular fringes could be further displayed on a large projector screen. Therefore, by this method the teacher can demonstrate the interference from Newton’s experiment in the classroom and all students can observe the circular rings at the same time.

References

1. Serway, Jewett, 2004, Physics for scientists and

engineers with modern physics, 6th ed., Singapore,

Thomson Learning, pp. 1189-1193.

2. Young & freedman, 2000, University Physics with modern physics, Addison Wesley Longman, Inc. pp. 1149-1153

3. Halliday, David, Resnick, Jearl Walker, 2005,

Fundamental of Physics,7th ed., New York,

Johnwiley & Sons, pp. 972-977.

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