Michelson Interferometer-Physics-Lab Report, Exercises of Physics

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MEASURING WAVELENGTH OF LIGHT THROUGH

MICHELSON INTERFEROMETER

Submitted to:

Dr. Asloob Ahmad Mudassar

Submitted by:

Yasir Ali

M.Phi. Physics DPAM

PIEAS

Introduction:-. Laser light are assumed to be monochromatic light. They are not so

monochromatic but its wavelength is spread over a small range and centered on a wavelength which is called wavelength of source. Wavelength is an important parameter of laser. Laser applications requires laser of specific wavelength. It is a well-established fact that different dental procedures require different laser wavelengths. Wavelength is important because specific body tissues interact in unique ways depending on the particular laser source. So laser wavelength is an important parameter. It is found mostly by Michelson Interferometer.

Michelson interferometer:- Michelson Interferometer was introduced by Albert

Michelson in 1881,is an instrument that brought the era of modern physics; most notably, it validated Einstein's theory of special relativity and dismissed the presence of ether through which light was thought to have propagated. The Michelson interferometer is a precision optical instrument that splits a beam of light and allows each beam to follow different optical paths of lengths, L1 and L2, and then to recombine (the light beams) by superimposing them so that they interfere. If the difference in the path lengths traveled by the two rays, L2- L1, is an integral number wavelength of the (monochromatic) light, then constructive interference occurs. If L2-L is equal to half a wavelength, then destructive interference occurs and no light is observed. A precision measurement of the path lengths L2 and L1 will allow a precision measurement of the wavelength of the monochromatic light used.

Figure 1. Michelson interferometer

Figure 1 shows the Michelson Interferometer. Light from the source passes through the beam splitter, divides the light along two paths. One part is transmitted to mirror M 1 the other is reflected to mirror M2. These two rays reflect back to the beam splitter where they recombine and proceed toward the eyepiece where interference is observed.

two beams are derived from the same source, both beams undergo these random phase changes but do so simultaneously so that the phase relation between them is unaffected. In this case, the interference fringes will remain stationary and can, therefore, be observed. On the other hand, it should be noted that if the difference between the optical path lengths of the two beams is larger than the coherence length, fringes will not be observed, even though the beams are derived from the same source.

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Fig 2: Wave train from a laser.

We see that the centre of the fringe pattern has a maximum intensity if

d = nλ/

and a minimum intensity if

d = (n + 1/2) λ/2.

Procedure:-. This experiment was performed in following steps.

1. Alignment:-. Place laser in front of Michelson interferometer such that light from laser falls on beam splitter and after beam splitter light falls on two mirrors. Reflected light from mirrors should pass again through beam splitter, this time beam splitter does not split again light beams but they are recombined and directed to screen. Mirrors should be adjusted such that light reflecting from mirrors should recombine and fall on screen. Movable screws are attached, are used for mirrors and beam alignment.
2. Fringes:-. After alignment fringes can be observed on screen. Adjust mirrors position such that fringes on screen become clear as shown in figure. This is done by making minimum angle, near zero degree, between beams after reflecting from mirrors. More over a converging lens should be fixed between the screen and beam splitter so that fringes are produced at some finite distance.

Fig. Fringes produced on screen

3. Counting fringes:-. Now, starting from any point, move one of the mirrors slowly and count the fringes changing with mirror motion on screen. Count about 100 fringes and note distance travelled from micrometer fixed with mirrors.
4. From fringes counted, wavelength of source can be calculated as

Where N is number of fringes counted, L is length through which mirror moves and λ is wavelength on light.

Observations:- During experiment, we observed some important phenomena’s.

1. Placing and rotating polarizer in front on laser light, changed intensity of light of light on screen which showed that laser source is polarized laser light source.
2. Fringes formed on screen were perpendicular when beams after reflection made angle in horizontal plane and vice versa.
3. Light on screen were not so uniform, even for single bright fringe on screen, light were not looking uniform. The reason for this is, as we know there is no one screen which is perfectly smooth, so is screen of experiment too. So light reflected from rough screen has some phase differences and they interfere constructively or destructively so light on screen were not smooth but had some variations.

Wavelength measured by Michelson interferometer

Length (μm) Fringes λ measured λ - λ 0 (λ 0 0.6328μm)

30 97 0.61^855 - 0.022%

35 110 0.636364^ 5.63E-3%

128 0.625 - 0.0123%