Ether + Michelson-Morley experiment

Question 1

Define absolute rest

Answer 1

At rest relative to the ether

Question 2

Interferometer =

Answer 2

instrument in which the interference of 2 beams of light is employed to make precise measurements = 2 mirrors and a partial reflector

An interferometer detects differences in phase between two light beams

Question 3

Absolute time

Answer 3

Absolute time exists independently of any perceiver and progresses at a consistent pace throughout the universe

Question 4

What was the purpose of the Michelson–Morley experiment?

Answer 4

To measure the motion of the Earth relative to the ether.

Question 5

In the Michelson–Morley experiment, explain the function of the two plane mirrors

Answer 5

To provide an interference pattern.

Question 6

In the Michelson–Morley experiment, explain the function of the compensator glass block

Answer 6

To make two paths equal

Question 7

Explain the formation of a dark fringe in the fringe pattern observed when looking through the telescope in the Michelson–Morley experiment

Answer 7

Path difference of lambda/2,

phase difference pi

Question 8

Describe what observation was expected when the apparatus was rotated through 90°

Answer 8

Shift in interference pattern (~ 0.4 fringe).

Question 9

The Michelson–Morley experiment produced a null result.

a) State what the result was
b) Explain why it was referred to as a null result
c) What was the significance of the null result?

Answer 9

No shift in interference pattern

A result without the expected content.

Evidence against ether.
No change in light detected therefore no absolute reference frame therefore motion can only be detected relative to some other object

Question 10

What kind of frame of reference is the turning roundabout? Explain your answer

Answer 10

It’s an accelerating frame of reference because it’s circular motion and the acceleration is centripetal

Question 11

Phase difference =

Answer 11

delta t/delta period * 2pi

Phase difference if light return times from equally spaced mirrors differs by small amounts.

Question 12

Explain why Michelson and Morley expected that the fringe positions would shift when the apparatus was rotated through 90° -

Answer 12

they expected the time taken for the light to travel in one direction to be different to the other so there would be a phase shift

Question 13

Speed through ether =

Answer 13

circumference of Earth orbit around the sun/time for one orbit (1 year)

Question 14

Although a shift of 0.4 fringes was easily detectable, no shift was observed. Explain what this null result demonstrated and its significance for Einstein in his special theory of relativity. –

Answer 14

Experiment showed speed of light from moving object is same as that from a stationary object/speed of light is invariant which was a postulate of Einstein’s theory of special relativity.

Question 15

Explain why the interference fringes shift their position if the distance from either of the two mirrors to the semi-silvered block is changed. –

Answer 15

Bright fringe is seen where the two beams are in phase; changing the distance to either mirror changes the path difference between the two beams so fringes shift

Question 16

Michelson and Morley predicted that the interference fringes would shift when the apparatus was rotated through 90°. When they tested their prediction, no such fringe shift was observed. Why was it predicted that a shift of the fringes would be observed? –

Answer 16

Speed of light was thought to depend on the speed of the light source (or the speed of the observer). Distance travelled by each beam unchanged by rotation. Time difference between the two beams would change on rotation. Phase difference would therefore change so fringes would shift.

Question 17

Michelson and Morley predicted that the interference fringes would shift when the apparatus was rotated through 90°. What conclusion was drawn from the observation that the fringes did not shift? –

Answer 17

Speed of light is independent of the speed of the light source or the observer / aether hypothesis incorrect

Question 18

Explain why interference fringes were seen

Answer 18

two beams reach the observer, interference takes place between the two beams, bright fringe formed where path difference = whole number of wavelengths / in phase. Dark fringe formed when two beams are out of phase.

Question 19

The interference fringe pattern did not shift when the apparatus was rotated by 90°. Explain the significance of this null observation. –

Answer 19

rotation by 90 degrees realigns beams relative to the direction of the Earth’s motion. No shift means no change in optical path difference between the two beams. The time taken by light to travel to each mirror is unchanged by the rotation. Distance to mirrors is unchanged by rotation. No shift means that the speed of light is unaffected thus disproving theory of aether.

Question 20

Michelson and Morley attempted to detect absolute motion by investigating whether or not the speed of light in a direction parallel to the Earth’s motion differs from the speed of light perpendicular to the Earth’s motion. Discuss what resulted from this experiment and what was concluded. –

Answer 20

no change in the fringe pattern on rotation; the speed of light is the same in the two directions; the speed of light from a light source on Earth is unaffected by the motion of the Earth.

Question 21

Name and explain the purpose of the two metal objects the light meets in the centre of the Michelson Morley interferometer. –

Answer 21

first object it meets is the beam splitter/semi-silvered mirror. – the second object is a compensator (or glass block) which allows for the thickness of the semi-silvered mirror to obtain equal optical path lengths in the two branches of the apparatus.

Question 22

Explain what is observed when the distance from M1 and M2 are equal

Answer 22

concentric rings/an interference pattern

Question 23

Explain what is observed when the distance from M1 is made slightly longer than the distance from M2

Answer 23

fringes shift, half a wavelength longer from M1 gives one complete fringe shift

Question 24

Michelson and Morley used the interferometer to try to detect the motion of the Earth through the hypothetical ether. (i) Outline how the apparatus was used and state what the result was. (ii) Explain the significance of the result.

Answer 24

i) rotate apparatus through ninety degrees, observe the fringes at the same time, observed fringes did not change or shift.
ii) speed of light in free space is invariant.

Question 25

Michelson and Morley attempted to detect absolute motion by investigating whether or not the speed of light in a direction parallel to the Earth’s motion differs from the speed of light perpendicular to the Earth’s motion. Discuss what resulted from this experiment and what was concluded –

Answer 25

no change in fringe pattern on rotation; the speed of light is the same in the two directions; the speed of light from a light source on Earth is unaffected by the motion of the Earth; the laws of dynamics cannot be applied to light; no ether.

Question 26

3 points on the Ether

Answer 26

Introduced by Young

Medium for the propagation of light/EM waves

Observed no sound in vacuum therefore light needed a mediumn

Fixed background all objects and radiation moved with an absolute motion relative to

Question 27

What was the hypothesis regarding the Ether from the Michelson Morley experiment

Answer 27

The postulated motion of the Earth through through the Ether predicted that the resulting ether wind would affect the speed of light depending on whether the light was travelling parallel or perpendicularly to the Earth’s motion by creating interference patterns

Question 28

What was the aim of the experiment

Answer 28

to measure the absolute speed of the Earth through the ether using an interferometer which was designed to detect differences in the speed of light travelling parallel and perpendicular to the Earth’s motion by creating interference patterns.

Question 29

What did they expect from the experiment

Answer 29

they expected the motion of the Earth to affect the speed of light in certain directions

light moving parallel to the Earth’s moion at c+/-V should take longer to travel to the mirror and back than light travelling perpendicularly

If c depended on the Earth’s velocity then if both paths from the plane mirrors to the partial reflector were equal then the time taken for the path lying in the direction of the Earth’s velocity would be greater than the other path

Therefore rotating the apparatus 90 degrees would cause the time difference to revers making the interference pattern shift

Swapping the two beams should cause a small shift the fringe pattern because the speeds would be differed which would confirm the existence of the ether.

Question 30

What was the method

Answer 30

Monochromatic light is sent towards the partial reflector causing light to split into two perpendicular beams

The beams are reflected at the plane mirrors and form an interference pattern back at the partial reflector

The pattern is recorded and then the interferometer is rotated 90 degrees and the experiment is repeated

Question 31

What sort of interference pattern is formed

Answer 31

Concentric rings

Bright fringes in phase
Dark fringes out of phase

Question 32

Name the 5 equipment in the experiment

Answer 32

1) monochromatic light source
2) plane mirror x2
3) Semi-silvered glass block/partial reflector
4) viewing telescope
5) Plane glass block

Question 33

What is the point of the second block - the plane glass block

Answer 33

Ensures both beams of light travel through the same amount of air and glass so they have equal optical path lengths

Question 34

What was the result

Answer 34

After experimental error was accounted for, no shift was detected even after repeating the experiment at different times and points in the year

Question 35

What did the null result show from the experiment

Answer 35

Speed of light was invariant - doesn’t change whether moving relative to the source or not

Cannot detect absolute motion - the ether doesn’t exist