Download Michelson Morley PTK

BODIES IN A STATE OF REST
(MOTION WISE)
Which of the following are bodies in a state of rest?
•
•
•
•
•
•
A carrot in a fridge
A pendulum hanging at equilibrium position
Jens
A geostationary satellite
Venus orbiting the sun
The sun
IT’S ALL RELATIVE!
An introduction to special relativity
1.
To begin to understand what is meant by the term special
relativity
2.
To be able to give examples of and compare different
frames of reference
3.
Be able to describe the Michelson -Morley experiment
and explain its significance
WHAT IS SPECIAL RELATIVITY?
What do you think the term special relativity means?
Answers on a post-it
FRAMES OF REFERENCE
• The motion of a body can only be described relative to something
else - other bodies, observers, or a set of space-time coordinates.
These are called frames of reference.
• Look at the following pictures, what are the possible frames of
reference?
To analyse the motion of train A:
Identify the roles in the frames
The motion of train A:
Possible Observers Relative motion
The motion of one of the books:
Possible Observers Relative motion
The motion of one of the balls:
Possible Observers Relative motion
Key Question: What is the importance of choosing a suitable observer when
analysing motion in different frames of reference?
QUESTIONS
1.
What is a "frame of reference"?
2.
Why is it important to define frames of reference when analysing
motion?
3.
Imagine a ball that is dropped from the top of a ladder on a boat
floating down a river. If you wanted to study the motion of the
ball, from where would you choose to observe it? The top of the
ladder? The deck of the boat? The riverbank? Explain your choice.
4.
Extension – try to come up with a more complex situation of
relative motion and explain what observation you would make
Mr Furneaux’s baguette
Maxwell’s hypothesis
•After Maxwell’s theory of EM waves, many believed the waves were
vibrations in the ‘ether’.
•It should be possible to detect the Earth’s motion through the ether
using experiments involving interference of light.
•Roughly what length should we be able to find information about
using light interference?
Light travel parallel to the motion
Light travel perpendicular to the motion
Michelson-Morley interferometer
Why will interference
fringes appear?
http://galileoandeinstein.physics.vi
rginia.edu/more_stuff/flashlets/m
mexpt6.htm
•Align the apparatus so that one beam is parallel
and one perpendicular to the motion of the Earth
through the ether.
•Turn the apparatus through 90 degrees.
•Fringes should shift by 0.4 of a fringe width.
Apparatus capable of detecting 0.05 fringe shift.
•Absolutely no shift detected – a ‘null result’
Exam question
(a)
(i)
beam splitter [or semi-silvered mirror] (1)
(ii)
a compensator [or a glass block] (1)
allows for the thickness of the (semi-silvered) mirror
to obtain equal optical path lengths in the
two branches of the apparatus) (1)
3
(b)
(i)
concentric rings (1)
an interference pattern (1)
[alt: whole view shows one shade (1) because there is
a constant phase difference(1)]
(ii)
fringes [or rings] shift (1)
05l extra for l1 gives one complete fringe shift
[or fraction of wavelength extra causes noticeable fringe shift
or noticeable change of intensity (if uniform)] (1)
4
(c)
(i)
rotate apparatus through 90° (1)
observe the fringes at the same time (1)
observed fringes did not change [or shift] (1)
(ii)
speed of light in free space is invariant
[or does not depend on motion of source or observer
or no evidence for absolute motion] (1)
max 3
[10]
SPECIAL RELATIVITY IS BASED
ON TWO POSTULATES:
1.
The laws of physics, expressed in equations, have the same form
in all inertial frames of reference.
2.
The speed of light in free space is invariant (ie the same for all
observers regardless of their state of motion and of the speed of
the light source).
THINKING POINT!
•Consider a convertible approaching
you at a speed of 20 m/s. The
passenger pulls out a slingshot and
shoots a rock at you with a speed of
8 m/s .
•Assume the same convertible is
driving towards you at 20 m/s and
it switches its lights on.
•How fast is the light going when
you see it?
•How fast is the rock going?
• How could you test your theory experimentally?
• Come up with an experiment to do in the lab that could test this theory
• Hints are available if you find this tricky
• If you already know of an experiment for this, move on to the calculations
section
The orbital speed of the Earth is 29.8km/s
Some of the available equipment:
Monochromatic light source, mirrors, glass blocks, semi
silvered glass blocks, viewing telescope, screen, accurate
length measuring equipment, mercury
When two sources of light of a constant phase
difference meet they interfere and produce an
interference pattern
If a wave has an increased velocity it will have a shorter
wavelength
What affects the spacing between fringes in an
interference pattern?
What sort of effect could the relative motion of the
Earth have on the speed of light?
The Significance of the Michelson-Morley?
• How did scientists ideas about the
speed of light change after this
experiment had been verified?
• How is this important in developing
a theory of special relativity?
http://webphysics.davidson.edu/physlet_resources/special_relativity
/
INERTIAL FRAMES OF
REFERENCE
There are several ways to describe an inertial frame. Here are a few
descriptions:
An inertial frame of reference is a frame of reference with constant
velocity.
An inertial frame of reference is a non-accelerating frame of reference.
An inertial frame of reference is a frame of reference in which the law
of inertia holds.
An inertial frame of reference is a frame of reference in which
Newton's laws of motion hold.
In an inertial frame of reference no fictitious forces arise.