## The Michelson-Morley Experiment of 1887

written by: Asheeshโขedited by: Lamar Stonecypherโขupdated: 5/18/2011

The Michelson-Morley experiment is a great experiment in the history of science, and the instrument of this experiment is used for studying the wave nature of light.

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The Michelson-Morley experiment was performed in 1887 by American physicists Albert Abraham Michelson and Edward Morley.

At that time, there were many theories regarding the movement and velocity of light. According to those theories, the movement of light was considered as a wave. The medium of light was assumed to be invisible and the effect of the medium was difficult or impossible to evaluate with precision. That medium was called โluminiferous aether" or "the ether." According to some theories, the ether was considered to be at rest and the earth moves through it. Due to this fact, Michelson believed that the speed of light, as observed from the Earth, is highly dependent upon the direction of travel of light with respect to the Earthโs orbiting motion.

When he was studying in the physics laboratory of Hermann von Helmholtz in Berlin, Michelson designed an instrument named an interferometer with the help of Edward Morley and started to study the properties of light. This instrument was based upon the principle of interference and became a famous instrument for the study of light and atomic spectrum A few points regarding this experiment are as follows.

Principle of Michelson's Interferometer:

The amplitude of a light beam coming from a source is divided into two parts of the same intensities; the first is due to partial reflection and the second is due to transmission. After this, these beams are sent in two directions and at a right angle and brought together after they suffer reflection from plane mirrors to produce interference fringes.

Construction of Michelson's Interferometer:

This instrument is constructed by two highly polished plane mirrors and two glass plates or slabs. These glass plates are of same thickness and are kept in the same angle, i.e. they are always in a parallel direction. The glass plate, placed at first in the path of the light beam coming from the source, is kept half silvered to make a reflected and a transmitted beam of the same intensity.

Both the plates are placed at 450 to both the mirrors and these mirrors are mutually perpendicular to each other. There are provided with screws on the back side of these mirrors so that they can be adjusted to be exactly perpendicular. You can understand the construction according to the image. (Click the image to enlarge it). The mirror, placed on the top, is mounted on a carriage and can be moved exactly parallel to itself with a micrometer screw. This screw is fitted with a graduated drum which can read a displacement of approximately 10-5cm. The interference bands can be observed with the help of a telescope placed at the bottom in the image.

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Working of Michelson's Interferometer:

First of all, the light coming from the source must be made parallel and this is done by a collimating lens placed at the middle of the light source and first glass plate. These parallel light beams are then made to fall on the first glass plate. These beams are partly reflected in a vertical direction and in a horizontal direction by this glass plate. The reflected beam travels normally towards the plane mirror which is at the top and so it is reflected along the same path and comes out towards the telescope. The transmitted light beam is received by the second mirror normally, reflected along the same path, and then moves towards the telescope after reflection from the back surface of the first glass plate. So, the two beams, moving along the common path, are collected by the telescope after the division of amplitude of the light coming from the source. We may get interference fringes depending upon the distance traveled by them.

This instrument is used as a powerful tool for the study of the atomic spectrum. Because of its importance, Albert Michelson was awarded the Nobel Prize in 1907.

This instrument is use as base of modern physics. Because, modern physics is directly dependent upon the properties of an atom and this experiment is used to study of the properties of atomic spectrum.

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### Applications of Michelsonโs Interferometer

There are many applications of this instrument. A few of them are as follows:

• This instrument is used to determine the wavelength of light.
• It is used for determining refractive index or the thickness of a transparent medium.
• This instrument is very helpful to study the properties of spectral lines.
• Michelson also used this instrument for calibration of the standard meter.

The main drawback of the Michelson-Morley experiment was that it was unable to explain the existence of the ether. Michelson thought that light travels like a wave and a wave needs a medium to travel. After some time, Einstein explained it according to his Theory of Relativity and that is the velocity of light is constant, therefore, there is no existence of any medium, i.e. ether does not exist.

After some time, when The Quantum Theory arose, it developed a new concept about light. According to Quantum Theory, light is composed of particles and also behaves like a wave. After this concept, they started to use this experiment to study of wave nature of light.

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### References

Optics- P.K. Chakrabarti

Modern Engineering Physics - A.S. Vasudeva