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March 2009 Physics Masterclass 1
The Michelson interferometer
Alex Pinder
March 2009 Physics Masterclass 2
Intro
• A very simple experiment
• Gave a very unexpected result: light moves at 3×108 m/s always
• Result: Einstein’s special relativity
• Consequences: time moves differently for different people, things look shorter when they move past you at high speed
March 2009 Physics Masterclass 3
First look
March 2009 Physics Masterclass 4
The one you’ll use!
March 2009 Physics Masterclass 5
What you’ll see…
Fringes!
March 2009 Physics Masterclass 6
Why fringes?
Light from two paths interferes constructively at your eye bright fringe
Light from two paths interferes destructively at your eye dark fringe
Path 1
Path 2
March 2009 Physics Masterclass 7
Michelson-Morley experiment (1887)
• Used Michelson interferometer: showed light doesn’t travel through anything (unlike sound)
• If light medium (“aether”) existed, fringe pattern would change with rotation of the apparatus
“Aether” flow
Speed v
c cc + v
c - v
Actual size of interferometer = 11 m
March 2009 Physics Masterclass 8
BUT!!!
• The fringe pattern did not change: N = 0
• Speed of light is fixed…
• …But in a strange way
Sees light coming towards him at 3×108 m/s
Sees same light catching up at 3×108 m/s
LIGHT
March 2009 Physics Masterclass 9
Gravity waves
• Ripples in space and time caused by violent events in the universe
• E.g. a rotating black hole
Could be measured by LISA, a giant Michelson interferometer floating in space!
Gravity waves make interferometer arms expand and contract, then observe the changing fringes
Arms 5 million km long, since changes will be tiny
March 2009 Physics Masterclass 10
Interferometers again:ATLAS experiment
• A different sort of interferometer being used to align the inner particle tracker
• As useful today as it was in 1887…
March 2009 Physics Masterclass 11
Your turn!
• There are several Michelson interferometers in the DWB
• Don’t be afraid: they won’t break!
• There is a question sheet
• Measure wavelength of green light: Michelson is very sensitive (Q1 – 3)
• Then 8 more questions on the MM experiment and the talk
March 2009 Physics Masterclass 12
Some hints…• Moving the mirror by half a wavelength shifts the round-
trip path by one wavelength = one fringe• Distance you move the mirror measured with a
micrometer:
• One rotation moves the screw 0.5 mm (check!)• 50 tickmarks per turn = 0.01 mm per tickmark• But the mirror moves only 1/5 the distance of the screw!
Part you rotate
The fixed line
The tickmarks