Download department of physics - Bishopston Comprehensive School Moodle

P1 PHYSICS
BISHOPSTON COMPREHENSIVE
SCHOOL
PHYSICS DEPARTMENT
P1 PHYSICS REVISION
BOOK 1 – THE UNIVERSE
Name: _______________ Form: ___ Teacher: ______________
BOOK 1 – THE UNIVERSE
P1 PHYSICS
THE DOPPLER EFFECT
If you’ve ever stood still and listened to an ambulance pass quickly by you with it’s siren
blaring, then you would here the pitch of the siren change as the ambulance moves past.
Don’t worry, you’re not going nuts, what’s happening is called the Doppler Effect.

As the ambulance moves away from you the sound waves are stretched.

This means that the wavelength will be longer.

This means the frequency will be less.

This means the pitch will be lower.
This works in reverse as well, let’s say the ambulance is moving towards you:

As the ambulance moves towards you the sound waves are squeezed.

This means that the wavelength will be smaller.

This means the frequency will be higher.

This means the pitch will be higher.
Look at these three diagrams:
Ambulance Still - Wavelength Normal - Sound Normal
Ambulance
Ambulance Moving Forward - Wavelength Squeezed - Sound Higher Pitched
Ambulance
Ambulance Moving Away - Wavelength Stretched - Sound Lower Pitched
Ambulance
BOOK 1 – THE UNIVERSE – PAGE - 1
P1 PHYSICS
RED SHIFT
Now we’ve just discussed the Doppler effect using sound waves, but the Doppler effect
applies to ALL waves not just sound waves. So for example if we looked at stars from a
distant galaxy and the light wave received on earth was slightly stretched, then that would
mean that the galaxy must be moving away from us (just like the ambulance on the
previous page).
Now how do we know that the distant star is moving away from us?
Well, we can’t hear a difference in pitch as with the ambulance, but the light received from
the star will be shifted towards the red end of the visible spectrum. This is because red light
has the longest wavelength of the coloured light within the visible spectrum. See below:
Star Stationary - Light Wave Not Stretched - No Red Shift
Star Moving Away From Earth - Light Wave Stretched - Red Shift
Because the light from every star is shifted towards the red end of the spectrum, we know
that every star in every galaxy is moving away from us. So the universe is expanding as
we speak !
The further away a galaxy is - the bigger the red shift !!
Question
If the light from distant stars was shifted to the blue end of the spectrum, what would that tell
us about the universe?
……………………………………………………………………………………………………………
BOOK 1 – THE UNIVERSE – PAGE - 2
P1 PHYSICS
THE BIG BANG THEORY
Due to red shift from distant galaxies, scientists have come up with the following theory:


The whole universe is expanding, and
The whole universe may have started about 15 billion years ago from one small place
with an explosion called the ‘BIG BANG’.
BOOK 1 – THE UNIVERSE – PAGE - 3