Special Relativity Flashcards
Why was it assumed that light travelled through a medium
At the time, all other known waves such as sound and water travelled through a medium, so it was assumed that light did as well
What was the medium for light waves called
Luminiferous aether/ aether
What was the aim of the Michelson-Morley experiment
To use the difference in speeds of light to determine the absolute motion of the Earth relative to the aether and therefore prove the existence of the aether
What was the interferometer
A device consisting of two arms of identical lengths at right angles to each other, with mirrors at each end and a beam splitter at their intersection
What is a beam splitter
A semi-silvered mirror which allows some light to pass through and reflects some light
Why is a pane of glass used in the Michelson-Morley experiment
To ensure both beams of light travelled the same distance of glass and air
To make time a controlled variable
To obtain equal optical path lengths
What happened to the light when it reached the beam splitter
Some light was reflected and some was transmitted, forming two beams that were initially coherent
What does the observer see through the telescope in the Michelson-Morley experiment when the distances are equal
An interference pattern formed of concentric circles
Outline how the Michelson-Morley interferometer apparatus was used and state the result and its significance
Rotate the apparatus through 90 degrees.
Observe the fringes at the same time.
Observed fringes did not change or shift.
This means speed of light in free space is invariant and there is no aether
The interference fringe pattern did not shift when the apparatus was rotated by 90 degrees. Explain the significance of this null observation
Speed of light is invariant.
Rotation by 90 degrees realigns beams relative to direction of Earth’s motion.
No shift means no change in optical path difference between the 2 beams.
Time taken by light to travel to each mirror unchanged by rotation.
Distance to mirrors unchanged by rotation.
No shift means speed of light is unaffected
Einstein postulated that the speed of light in free space is invariant. Explain what is meant by this postulate
Speed of light does not depend on the motion of the light source or that of the observer
What conclusions were made from the null result of the Michelson-Morley experiment
The aether does not exist therefore light can travel without a medium
Speed of light is unchanged by the Earth’s motion - it is invariant
Explain why Michelson and Morley predicted the fringes in the interference pattern would shift when the interferometer was rotated 90 degrees
They predicted the speed of light depended on the motion of the Earth, relative to the aether.
Therefore, the time difference would change between the two beams when they were rotated.
So there would be a change in phase difference, shifting the fringes
PHASE DIFFERENCE, NOT PATH DIFFERENCE AS LENGTHS REMAIN THE SAME
What does invariance of light mean
When in a vacuum, the speed of light is always ‘c’ for evert observer
What is an inertial frame of reference
A reference frame that is non-accelerating.
All inertial reference frames are moving at constant velocity with respect to each other
