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Light and sound
Jim Reed/Digital Vision WW (NT)
How is light reflected from a surface?
• Angles of incidence, reflection and refraction are always
measured between the ray and the normal.
• The normal at a point on a mirror is perpendicular to the
mirror.
• For a light ray reflected by a mirror:
the angle of incidence = the angle of reflection.
• Real images are formed where rays of light cross so they
can be produced on a screen.
• Virtual images cannot be produced on a screen – they
are where rays of light appear to have come from.
Image is virtual, same size as object, same distance behind mirror as object is in
front,
Upright and laterally inverted (back to front).
What happens with a curved mirror?
• The principal focus of a concave mirror is
the point where parallel rays of light are
focused to by the mirror.
• A concave mirror forms:
– a real image if the object is
beyond the principal focus,
– a virtual image if the object is
between the mirror and the
principal focus.
• A convex mirror always forms
Photodisc 46 (NT)
a virtual image of an object.
Principal Rays in mirrors
1. The ray to the centre of the mirror is reflected
according to i=r.
2. The ray parallel to the optical axis is refracted
through (or appearing to come from if
diverging) the Focal point.
3. The ray through (or heading towards if
diverging) the Focal point emerges parallel.
Converging mirrors
Concave mirror forming real inverted smaller image.
Object is outside focal length.
Concave mirror where object is between focus and mirror
Image is virtual and upright.
Diverging mirror
The image formed is virtual and minified
What is refraction?
• Refraction of light is the change of
direction of a light ray when it
crosses a boundary between two
transparent substances.
• If the speed is reduced, refraction
is towards the normal (e.g. air to
glass).
• If the speed is increased,
reflection is away from the normal
(e.g. glass to air).
SPL: Alfred Pasieka
What are converging and diverging
lenses?
• A converging lens makes parallel rays of light
converge to a focus. The point where they are
focused is the principal focus of the lens.
• A diverging lens makes
parallel rays of light diverge
(spread out). The point
where the rays appear to
come from is the principal
focus of the lens.
Corbis V257 (NT)
What type of images do the lenses
form?
• A real image is formed by a converging
lens if the object is between its principal
focus and infinity (magnified if between F
and 2F, minified if beyond 2F).
• A virtual image is always formed by a
diverging lens, and by a converging
lens if the object is between the principal
focus and the lens.
How is a converging lens used
in a camera?
• A camera contains
a converging lens
that is used to form
a real image of an
object.
Principal rays for a converging lens
For an object outside the focal point, a real inverted image will be
formed.
How is a converging lens used
in a magnifying glass?
• A magnifying glass
is a converging
lens that is used to
form a virtual
image of an object.
Photo: S. Meltzer/Photolink/Photodisc 24 (NT)
For an object inside the focal point, a virtual erect image will be
formed.
Principal rays for a diverging lens
Concave lenses are used to correct short sight. They
always produce upright virtual images
What is sound?
Sound waves:
• are longitudinal waves.
• can travel through liquids and gases and
in solids.
• cannot travel in a vacuum.
• can be reflected (echoes) and refracted.
How do we make musical sounds?
• We change the volume – the
loudness of a note depends
on the amplitude of the sound
waves.
• We make the notes higher or
lower – the pitch of the note
depends on the frequency of
the sound waves.
Photo: Doug Menuez/Photodisc 45 (NT)
note
noise
ULTRASOUND
Ultrasound waves are sound waves that have a
frequency above the threshold of hearing 20kHz.
The frequency can be varied to give a range of
wavelengths when scanning a foetus to achieve
clear imaging of the skeleton/organs etc.
High frequencies are used to resonate kidney
stones to shatter them.
Ultrasound reflections are used to detect flaws in
metals and identify metal fatigue.
Electromagnetism
SPL: R. Maisonneuve, Publiphoto
Diffusion
What is the motor effect?
• When a current is passed along a wire in a magnetic
field, a force may be exerted on the wire – this is the
motor effect.
• The size of the force is increased if the current or
the strength of the magnetic field is increased.
• The direction of the force is reversed if the direction
of the current or the magnetic field is reversed.
• The direction of the force is at right angles to the
direction of the magnetic field and the wire.
FLEMINGS LEFT HAND RULE
YOU DRIVE ON THE LEFT IN A MOTOR
THE ELECTRIC MOTOR
+
-
How does electromagnetic
induction work?
• When a wire cuts the lines of a
magnetic field, a potential
difference (p.d.) is induced in a
wire.
• If the wire is part of a complete
circuit, the induced p.d. causes a
current in the circuit.
• The current is increased if the
wire moves faster or a stronger
magnet is used.
FLEMINGS RIGHT HAND RULE - GENERATOR RULE
How do transformers work?
• A transformer
consists of two coils of
insulated wire (a
primary coil and a
secondary coil)
wrapped on the same
iron core.
• Transformers only
work using
alternating current.
Why are transformers used in the
National Grid?
• The National Grid supplies energy from distant power
stations to our homes.
• For maximum efficiency and minimum energy wastage, the
grid p.d. is at least 132 000 V. In your homes the p.d. is
230 V.
• Transformers are used to step the voltages up or down.
What is the
transformer equation?
Higher
p.d. across primary, VP
number of turns on primary, NP
p.d. across secondary, VS = number of turns on secondary, NS
=
SPL: R. Maisonneuve, Publiphoto
Diffusion
A transformer is used to step a p.d. of 230 V down to 10 V.
The secondary coil has 60 turns. How many turns are
there in the primary coil?
Use the transformer equation:
VP NP

VS NS
VP = 230 V, VS = 10 V, NS = 60 turns
230
10
NP
=
=
NP
60
230  60
10
= 1380 turns
Turning forces
What is a moment?
• A moment is the turning effect of a force.
• Moment = force  perpendicular distance
from the pivot to the line of action of the force
=Fd
• F is the force in newtons.
• d is the perpendicular distance from the pivot
in metres.
• The unit of a moment is newton metres (Nm).
Higher
What can you say about the moments
of the forces acting on an object in
equilibrium (it isn’t turning)?
The sum of the anticlockwise moments
about any point
=
the sum of the clockwise moments
about that same point.
Using levers
• Calculating moments is
important when you use levers.
• The weight is called the load.
• The force you apply to the
crowbar is the effort.
• The point about which the
crowbar turns is the pivot.
• Levers enable you to lift heavy
loads with little effort.
What is the centre of mass of an
object?
• The centre of mass of an object is the
point where its mass may be thought to be
concentrated.
• When a suspended object is in
equilibrium, its centre of mass is directly
beneath the point of suspension.
• The centre of mass of a symmetrical
object is along the axis of symmetry.
Tilt or topple?
Higher
• An object will tend to topple over if the line of
action of its weight is outside its base so …
• … bodies with a low centre of mass and a
broad base are more stable than bodies with a
high centre of mass and a narrow base.
• You can increase the stability of an object by
making its base wider and its centre of mass
as low as possible.
How can a body moving at a
steady speed be accelerating?
•
•
•
When it’s moving in a circle at constant
speed!
The object accelerates continuously
towards the centre of the circle.
The centripetal force on it increases as:
– the mass of the object increases,
– the speed of the object increases,
– the radius of the circle decreases.
The car is travelling at a steady speed but the direction is
always changing – so the velocity is changing.
Acceleration is the change of velocity per sec - so the
car is accelerating. The friction between the tyres and
the road provide the centrepetal force to keep the car
travelling in a circular path.
What is the force of gravity?
• The Earth exerts a force of gravity on all of us – and
we exert one on the Earth!
• The force of gravity between
two objects:
– is an attractive force,
– is bigger the greater the
mass of each object is,
– is smaller the greater the
distance between the two
Digital Vision 7 (NT)
objects is.
What keeps the Earth orbiting
the Sun?
• The force of gravity provides the
centripetal force needed to keep
one smaller object in orbit around
another larger object (e.g. the Earth
around the Sun).
• To stay in orbit at a particular
distance, a small body must move
at a particular speed around a larger
body.
• The larger the orbit is, the longer the
orbiting body takes to go round it.
The elliptical planetary orbits have the Sun as one foci of
the ellipse and the gravitational pull of other bodies as
the other focus (p227) The centrepetal force is created
by the gravitational attraction of the Sun on the other
planets.
What is the period of the orbit of a
satellite?
• The period of a satellite is the time it takes to make
one complete orbit.
• The Moon is the Earth’s only natural satellite.
• A satellite in geostationary orbit (e.g. a
communications satellite) has a period of 24 hours
and stays at the same position above the equator.
• Monitoring satellites in low polar orbits have a
period of 2–3 hours.
Stars and space
Corel 768 (NT)
What are galaxies?
• The Big Bang which created the
universe was about
13 thousand million years ago.
• As the Universe expanded, it
cooled and uncharged atoms
formed.
• The force of gravity pulled
matter into galaxies and stars.
SPL NOAO/AURA/NSF
How are stars born …?
• Stars form out of clouds of dust and gas.
• Particles gather under gravity to form a
protostar.
• The protostar becomes denser and
hotter. If it reaches a point where
hydrogen and other atoms fuse – huge
amounts of energy (including light) are
released and a star is born!
Image from: www.star.ucl.ac.uk/groups/hotstar
UCL Hot Star Research Group
Horsehead nebula where stars are born
NASA/ESA/STScI
… And how do stars die?
• When the supply of hydrogen nuclei runs out,
the star swells.
• As it swells, its surface temperature cools and
turns red – a red giant.
• When all the light elements in the core have
fused, fusion stops. The star collapses on itself
and heats up to become a white dwarf.
• Smaller stars (like our Sun) then fade out and go
cold.
What happens to bigger stars?
• High-mass stars continue to collapse beyond the white
dwarf stage until there is a massive explosion outwards
again – a supernova.
• The supernova
compresses
the core of the star
into a very dense
neutron star.
• If the neutron star is
massive enough it
becomes a black hole.
How were the chemical
elements formed?
Higher
• Light elements – up to and including iron
– are formed as a result of the fusion of
nuclei in stars.
• Heavy elements are formed when a
massive, ageing star collapses and then
explodes as a supernova.