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 “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 Earth’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.