Michelson-Morley
Experiment, Michelson Interferometer, Morley Interferometer
ETHER HYPOTHESIS
A material medium is a necessity for the propagation of
waves. It was considered that light propagate through ether as the sound wave
propagate through air. Ether affects all space. An interesting question is
whether relative motion between the earth and ether can be detected. If such a
motion can be detected, we can choose a fixed frame of reference in stationary
ether. In 1887, Michelson and Morley set out to measure the relative velocity
of earth with respect to the ether. The principle of the experiment lies is
nothing, the shift in fringes in Michelson Interferometer due to the difference
in time taken by light to travel along and opposite the direction of motion of
the earth. The time taken by a beam of light to travel along the direction of
motion of earth is greater than that of traveled distance opposite to the direction of earth.
Surprisingly, despite best efforts, the presence of ether could not be
detected.
MICHELSON-MORLEY EXPERIMENT
The main aim of Michelson-Morley Experiment was to find
evidence confirming the presence of an absolute Frame of Reference called
Ether. Ether was thought of as a medium which fill all spaces and penetrates
all matter. It was considered to be rigid, Invisible, Mass less and perfectly
transparent elastic, non-resistive, continuous and with negligible density.
Michelson’s Interferometer consists of two plane mirrors M1
and M2 arranged perpendicular to create coherent sources by division of
amplitude. The light from the sources goes to the beam splitter P and then
divides itself to go to M1 and M2 separately. The reflected light
mirror M1 and M2 is allowed to interfere and
interference fringes are observed.
The two mirrors M1
and M2 are placed at equal distance from the plate. It is assumed that the earth
is moving through the ether with velocity along the direction of the incident light.
Due to the motion of the apparatus with the motion of earth,
the time taken by ray 1 travelling to mirror M1 and ray 2 travelling
to mirror M2 would not be the same.
If c is the velocity of light through
ether, then using Galilean Transformation, the velocity of light along the
direction of incident light is c-v for light from P to M2 and c+v for light
from M2 to P also
The whole apparatus was rotated by 90
degree and the fringe pattern was observed to measure the amount of fringe
shift.
Due to rotation of apparatus,
The expected fringe shift was evaluated
using the formula in Equ ..(6) and was calculated to be 0.4 but actually even
though the experiment was done very precisely and number of times, no fringe
shift was observed. This null result could not be explained by Michelson and
Morley at that point of time.
EXPLANATION OF NEGATIVE RESULT
1. ETHER-DRAG HYPOTHESIS: The
hypothesis assumed that ether moved along with the earth while the later moved
through space. Thus there was no relative motion between the earth and ether.
This implied that the fringe shift goes to zero.
.This hypothesis was rejected because it
was purely mathematical and had no experimental evidence to support itself.
3. CONSTANCY OF THE SPEED OF LIGHT: In order to preserve
the ether hypothesis it was proposed that light travel with a constant speed
with respect to the source. This was in contradiction to the wave theory, thus
a negative result of the Michelson-Morley experiment could not be explained
through the proposed hypothesis at that time.
This is the most easiest way to explain this experiment!! Thankyou Very much
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