Light, Sound, Waves GCSE Introduction Questions Why do we put sound and light together? They are so different. Why do we call them “waves”? What do we.
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Slide 1
Slide 2
Light, Sound, Waves GCSE Introduction
Slide 3
Questions Why do we put sound and light together? They are so
different. Why do we call them waves? What do we know about them?
What are they made of?
Slide 4
How does light behave? It radiates out from a source In
straight lines unless it gets reflected or there is a change of
material It seems to move so fast it arrives instantaneously and
measuring its speed is difficult (but we can: 3 x 10 8 m/s) A
source is often so hot it glows and emits light in all directions
(there are cold sources like LEDs, glow-worms, fireflies,
etc.)
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How do we draw it? With beamsor with rays Source Barrier beam
Source Barrier rays
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Filament Bulbs and point sources Filament bulbsAre too spread
out
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Spread out sources
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Compact sources From above a vertical filament looks like a
point source of light and gives clear rays.
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Ray boxes From above the filament gives a compact source of
light and clear rays The lamp slides forwards and backwards
Different shapes of glass/plastic can be put in front to
investigate the effect on rays.
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Using ray boxes Connect the bulb to 12 V dc or ac and check it
works. Use a triple slit in the slot at the front, put the ray box
on a sheet of white paper, slide the bulb forwards and backwards.
Stand a cylindrical lens on the paper and note the effect the lens
has on the rays. What makes you think light travels in straight
lines?
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Things to try
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We need to think in terms of rays to see what is happening. The
image is opposite the pinhole on the screen It is upside down. Why?
Because light travels in a straight line from top of object to top
of image, etc. Pin hole Cameras
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Images Each image is opposite one pinhole so light travels
straight there. A pinhole camera shows what a lens does to
light
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Rays forming images A lens brings all the rays from one point
on the source to a single point in the image Converging rays form a
real image A real image is one that can be formed on a screen
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Reflection
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Reflection and images A plane mirror reflects regularly A rough
surface scatters the rays and produces diffuse reflection no
image
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Its the Law The angle of incidence equals the angle of
reflection Incident ray, normal and reflected ray all lie in the
same plane.
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Does it always work elsewhere?
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Rays and images Converging rays form an image with lenses What
about rays after reflection? What sort of image do we see? Is it
real? Follow the instructions carefully.... an accurate drawing of
rays will be obvious in the result!
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Real Rays Reflected Fold the A3 sheet in half and set up the
mirror on the fold. Mark its position. Set up a ray box (dont use a
lens) and triple slit so that the rays diverge within the area of
the paper. Make sure the lamp is over the sheet of paper and direct
three rays onto the mirror. Mark on the paper along the centre of
each incident and reflected ray. Draw in a pencil line for each
ray. Where do they cross? Look back along the rays; where does it
look like the raybox is?
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Finding the Image behind the mirror Look back along the
reflected rays to see where they apparently come from. Is each one
straight or curved or crooked? Draw behind the mirror with a dotted
line where each ray appears to come from. Where do the dotted lines
cross? Draw a line from Object position to Image position. What is
the angle between this line and the mirror? What is the mirror to
object distance and the mirror to image distance? How does the
image compare to the object?
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Virtual images - an illusion Rays seeming to come from an
object behind a mirror are an illusion: a virtual image forms from
diverging rays. The image is the same distance directly behind the
mirror as the object is in front
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Hotel pool with glass side = endless fun. Allegedly. Distorts
!
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Refraction C B A 90 Set up a single ray going through a
parallel sided glass block. Look back along the emergent ray,
through the block. What is strange about it? Try different angles
of incidence, including at 90 to the side of the block i.e. 0 to
the normal.
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Try these!
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Refraction The pattern is that a ray entering an optically more
dense material is bent towards the normal, and vice versa. Special
case: a parallel sided block causes the emergent ray to be parallel
to the incident ray.
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Why Refraction? Water waves slow down in shallower water Light
waves slow down in denser materials
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Refraction: Why? Because waves slow down in denser materials
(like glass), just as sea waves slow down in shallow water
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The part of a wavefront entering the glass first slows down
first and the rest of the wave follows.
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Refraction and images The bending of the light at the water air
surface means the image you see under water is an illusion!
How?
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Refraction and image formation Looking back along the rays,
they seem straight so they seem to come from one place, whereas in
fact they come from another creating an illusion, a virtual image.
Real images can be formed on a screen, virtual images cannot.
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A Sound Study Good Vibrations
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What is sound Something our ears respond to! Seems to spread
out from anything that oscillates fast enough Seems to be
mechanical Travels through the air Travels under water Travels
through solid objects
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Surround Sound It spreads out and travels behind obstacles
(unlike light and shadows) diffracts It is reflected by large flat
surfaces - the Abbey, a canyon wall, the seabed for sonar It
changes direction in denser air refracts so sometime bends upwards
and cannot be heard on the ground. Is it like light or not?
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So what is sound? Pumping the air out of a bell jar shows that
sound cannot exist in a vacuum We can see the bell vibrating, but
cannot hear it. We can see it. So is sound like light or not? By
the way, dont buy comics on the moon...
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I said, what is it? If you watch a loud speaker, the paper cone
vibrates back and forth It pushes the air forward, then pulls it
back. This pressure wave travels out through the ear and is still
strong enough to make eardrums vibrate What about tuning
forks?
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So it is waves of pressure travelling out ? Yes. They are
longitudinal waves Longitudinal waves are when the displacement is
parallel to the direction of the wave
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You need to be able to sketch and label something like this
diagram! Animated version!
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How do we hear?
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From there to ear
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How does sound sound? Frequency (f ) this is a count of how
many waves pass by every second and is measured in Hertz (Hz) The
time for just ONE wave to pass is the Time Period of the wave; T =
1/f. The sensation of Pitch increases as the frequency increases
high notes, high frequency. More singers! Proper wine glass
video
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Sounds interesting... Sounds in the range 2020 000 Hz (approx.)
can be detected by the human ear. < 20 Hz is infrasound
(elephants, pigeons,...) > 20 kHz is ultrasound (dogs,
bats,...)
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How else does sound sound? The amplitude of a wave is the
maximum movement of a vibration from the midpoint Loudness
increases as the amplitude of the wave increases. Wavelength is the
distance between two compressions or peaks
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Wave Speed, frequency and wavelength A.If a wave starts out at
f=2Hz, it completes 2 wavelengths per second B.If f= 4Hz, it
completes 4 wavelengths per second. If we know the wavelength = 3m,
then A travels at 2 x 3 m/s B travels at 4 x 3 m/s Speed =
wavelength x frequency
Slide 50
1)A water wave has a frequency of 2 Hz and a wavelength of 0.3
m. How fast is it moving? 2)A water wave travels through a pond
with a speed of 1m/s and a frequency of 5 Hz. What is the
wavelength of the waves? 3)The speed of sound is 330 m/s (in air).
When Dave hears this sound his ear vibrates 660 times a second.
What was the wavelength of the sound? 4)Purple light has a
wavelength of around 6x10 -7 m and a frequency of 5x10 14 Hz. What
is the speed of purple light? Some example wave equation questions
0.2 m 0.5 m 0.6 m/s 3x10 8 m/s
Slide 51
Homework Research one group of instruments of the orchestra,
and explain how their sounds are different in pitch, loudness and
quality ( i.e. what they sound like and why they sound different)
Choose one from strings, or woodwind, or percussion (inc piano:
why?)
Slide 52
Thats not all about how sound sounds Sounds can seem as pure as
a choir boys top note Or as screechy as a learner on the violin.
Listening to someone on the phone, you know who it is as soon as
they say its me Mmmmm.
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Waveform or Quality (timbre) We need to have a way of watching
the pressure waves as they go past An Oscilloscope can do this for
us, drawing a real time graph of pressure against time
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The horizontal scale (or time base) is marked in the time each
division represents, hence time/div Usually there is a grid of 1 cm
squares, so a time/div of 0.5 ms means each square represents 0.005
s
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NB You must be able to: compare amplitudes and frequencies of
sounds from diagrams of oscilloscope traces.
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Compare these oscilloscope traces.. If the time/div is 0.2 ms
what frequencies are these traces? time A C B D F E
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The Speed of Sound
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Speed is Distance travelled time taken The trouble with sound
is It goes so fast! It gets more spread out and weaker as it
travels Wonderful students
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How can we measure it? Crash some cymbals 100m away and time
how long after we see the clash until we hear it because light
travels so much faster than sound. Problem: It is still a very
short time and hard for mere humans to measure! Solution: send the
cymbals further away Problem: The sound gets fainter as it spreads
out Solution: make a louder sound blow something up! Problem: get
arrested. Solution: use electronic timing...
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If we know how fast sound travels we can work out how far away
a thunder storm is. How exactly?
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Echoes Sound reflects best from large flat surfaces Can use
this to measure speed of sound
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Stand in front of a large building Measure distance to building
Clap rapidly but in time with the echo Time a number, say 20
echoes, Calculate (20 x distance there and back)/time