Hello! This is a revision podcast
for Excel I G C S E and this is on topic 3
waves. There are two kinds of waves:
transverse and longitudinal waves. Longitudinal waves vibrate
back and forth in the direction in which the wave travels
this could be a slinky spring
here are areas where the spring has been compressed. The spring will pass on that
vibration to the adjacent particle.
That particle will move forwards and then move back into position.
The wave moves across to the right but the particles themselves move forwards and
backwards.
Areas where the waves are close together
are compressions. And areas where they are spread apart
are called rarefactions. Transverse waves
vibrate up and down across the direction of travel
in this example the wave is moving to the right
but the water (if it was a water wave) or the particles
would move up and then back down again. The wavelength is the distance from one
peak
to the next and the amplitude from the center of a wave
to the top. Both of these kinds of wave transfer
energy although neither of them transfer
any matter. The time period
is the time it takes to make one complete oscillation
so that will be measured in seconds which are the standard units for time.
the frequency is the number of vibrations made in a second
and that has standard units hertz (Hz) which means
the number per second. In this
graph you can see that as the time period reduces
the frequency increases. In fact the relationship between frequency and time
period is reciprocal
frequency is one over the time period. You are given this equation
on the Excel paper. Here is an example using that equation:
if a jigsaw blade vibrates up and down
at a frequency of 80 hertz, how long does it take to make one complete
vibration? That's the time period. You rearrange this formula to make T the
subject
that will be T equals 1 over f. Substitute the data:
80 Hertz. 1 over 80 is 0.0125 seconds
that's the time period for the jigsaw blade. The wave equation
relates the wave speed to the wave's frequency
and its wavelength. The letter lambda
is a Greek L and that represents the wave length
as the frequency of a wave
decreases the wavelength increases
for example
if the waves at the shore for beach arrive at a frequency of 0.4 hertz
and are eight meters apart, calculate the wave speed.
so you'd use this equation in this form
wave speed would be the frequency
times by the wavelength gives 3.2 meters per second
note the the units here are standard so the output units
are also standard units for speed - or meters per second
Any wave involving
electromagnetic waves would travel at the speed of light
so if a question asked you: Radio one is broadcasting a radio wave
97.9 megahertz what would be
its wavelength? You'd rearrange this equation
to make a lambda the subject. So if you use a triangle
cover up this term and that leaves v divided by f.
Lambda is speed over frequency
so 3 x 10^8 over 97.9 x 10^6
that's 97.9 million gives 3.06 meters
When waves pass through a gap they spread out slightly
this process is called diffraction
diffraction is greatest when the wavelength of the wave
that's the distance from one peak to the next is about the same size as the width
of the gap
and destroying the show's about maximum diffraction
was the wavelength here is about the same size as the gap
when you draw these diagrams note that the wavelength remains constant
throughout distance here is about the same
as the distance here
diffraction happens when there's only one edge
in this case the gap is considered very large and so the diffraction is greatest
when the wavelength is very large
I E a very long distance gap so a very long way
will def racked the most
always to fract including lights but we don't observe like defecting very much
because its wavelength is so small
you'd need a very small gap to see its diffraction
you need to know about electromagnetic waves which are oscillations electric
and magnetic fields
that may sound confusing but the part of a family of well-known waves
three from long wave radio waves to very short wave gamma rays
all electromagnetic waves carry energy
but only the shortest wavelength ones are ionizing
gamma rays a very ionizing and ultraviolet least ionizing
all electromagnetic waves travel at the same speed
that's the speed of light
here's a new moniker might help you learn the spectrum
in terms of frequency amp it starts with gamma rays
which are the lead shortest wavelength and goes to radio waves
which are the longest wavelength I can't claim credit for this demonic
it was invented by one of my students
generally the higher the frequency the more dangerous the electromagnetic wave
gamma X rays and ultraviolet are all ionizing
it's not about some uses and some dangers electromagnetic waves
here some statements taken from the syllabus ready way to use for
broadcasting
Mike Rice for cooking infrared for heating
visible light ultraviolet X-rays
and gamma rays all have uses and hear some dangers
microwaves will he to internal body tissue
they don't ionized a infrared can cause skin burns
it's also not ionizing ultraviolet
does damage to the skin surface cells and blind and can cause blindness
and gamma rays can cause cancer and can mutate cells
mirrors will reflect light according to the law
of reflection that is that the angle of incidence
is equal to the angle of reflection
you measure angles to the normal line a normal line
is drawn at 90 degrees to the surface
of the mirror you need to build to construct a diagram to show
the location of on image
so here is an object that we're looking at
and the image is formed the same distance away
inside the mirror so after all on
the image location equally distant and level
with the object here
if an observer looks at this object
they will see the image inside the mirror
this ray diagram shows you how it forms have the image forms here
to start off with your ruler at the position of the image
and you draw or brave that goes to the mirror and continues to the eye
that ray would've come from the in the object over here
and it reflects at the mirror surface
I've repeated that process for a second ray of light
and you can see that diverging spreading out
if you put your pencil at the object position
and join a lineup with where the ray hits the mirror
you will get an accurate reflection
these rays reflect of the mirror go to the RI
these rays aren't real they're called Virtual Race
and they should be door drawn dotted
now the diagrams complete we haven't image
that's inside the mirror the same distance away
but laterally inverted that is swapped left to right
and that's virtual the image can't be formed on the screen
from these rays nope really travel there is no light
here virtual raise to show where the image
appears you can see reflections
mirrored surfaces and also obviously at waters edge is
refraction is what happens when a wave changes speed
here waves kept to a boundary and they travel slower
in this medium one side of the way travels faster than the other side of
the way
and so it overtakes and they travel in a different direction once they go into
the other medium
note that the wavelength is also shorter in the slower medium
this happens to light when it travels into glass
the angle of incidence is greater than the angle of refraction
as the light bends towards the normal
a normal line here is at 90 degrees to the surface
as it was for a merit roaring
an analogy to explain it would be a car driving from
tarmac into mud
as the car drives towards the mud
eventually one wheel gets to the mud before the other we'll
this wheel will travel slower and this will foster
so the car will change direction one side will be traveling faster
and it will turn
here the car is now change direction
and once it's entered the mud it will continue going to straight-line
when it leaves the mud on the other side the opposite will happen
and this we'll we'll get traction first and the car will move back
to its original path the mathematics of refraction
are that n the refracted index of the material
is equal to Sinai over synar and that could be put into a triangle
to help you manipulate the formula the opposite of the sine function
is signed to the -1 he normally press SHIFT and then sign on your calculator
you'll need that in the examples so much in light
enters glass at an angle of 45 degrees to the normal
and refracts 29 of 30 degrees calculate the refracted index
formula here is used in this form
Sinai over synar which comes out to be 1.41
that's the refracted index at the glass and it doesn't have
units it's a ratio in this example
we want to know what angle light will refract T when it enters diamond
at an angle diamonds refracted indexes 2.4
because we want to know what angle it refracts to you
we need to cap Hill 8 are of formula
rearranges to give us sign a bar sign a bar
Sinai over N which comes out to be not
point 319 that isn't
the angle of refraction bio to find that we need to use the inverse sine function
are is the inverse sin of the art so we just got
which gives 18.62 grease worth checking when you've got an answer
but a look sensible compared to the original angle of incidence
let's consider what happens to light when it tries to leave
glass or more optically dense material if it travels at a fairly shallow angle
it will refract and leave the class it bans away from the normal of course
if you increase the angle of incidence at the surface
eventually the lights will graze along the glass surface
edge it does that at an angle called the critical angle
so acts the critical angle light
bends as far as it possibly can along the surface of the glass
beyond to the critical angle
litres got nowhere left to refract and so
it reflects inside to block all
the light energy reflects none comes out and so this is called
total internal reflection the relationship between two critical
Anglesey
and to refract if index have the glass is this
sign see equals 1 over N and you could put that
into a triangle to help you manipulate it again to calculate the critical angle
for perspex
if you know it's reflective indexes 1.4 again you start
by calculating sign of the critical angle which is 1 I've Iran
with that number we then need to use the inverse sine function again
to calculate the critical angle which comes out to be close to 45 or 46
degrees
a prism can disperse light
into its constituent colors here white lights
refracts when enters and refract again as it leaves the prism
but the refractive index of blue lights
is slightly more the refracted index for red light
so it bends slightly more and the white light is split
into its constituent colors
when light hits this prism its arrives at the surface
at the normal line and so it doesn't refract
and hits this surface at an angle a 45 degrees
to the normal which is above the critical angle
and so it reflects the same process happens again here
and we've used these prisons to create a periscope
SLR cameras use presumes to reflects light
down through the lens of a camera so that you see a picture of what you're
about to take
like can be transmitted along optic fibers as a digital signal
the light flashes on and off quickly to represent am code of ones and zeros
which is a binary code light
hits the edge at an angle greater than the critical angle between the two
materials
and so always totally internally reflects
analog signals take any shape or size but digital signals
are either on or of when you transmit
signals along any in any medium noise can distort them
let's have a look at the distorted signals
so he's the signals with some noise added to them
you can see the analog signal has lost some of its original integrity
we could clear this up but it would be less like the original shape
whereas a digital signal you can still
interpreters ones and zeros you still know if the voltage
is high or low or in this case if the light intensity
is bright or dim so digital signals
are less prone to noise by interference and the original can be cleared up
sound waves along tattooed no vibrations a family kills
the air from a loudspeaker moves back and forwards
and Knox into the molecules of its neighbors creating a compression wave
like other way a sound can be reflected at san ako
it can be refracted and it can be diffracted
you need to know experiment to measure the speed of sound
to do so you need some people a large distance away
from a loud sound
you use a gun you by the guy
and you'd watch for the smoke the people would start the stopwatch when they saw
the smoke
and stop it when they hear the bank they do see question speed equals distance
over time
to calculate that speed but the sound to take and travel
there are other methods for working at the speed of sound
other methods may be more accurate than this method but this method works
if you connect a microphone to an oscilloscope you can create a trace that
represents the sound
the trace would be voltage produced by the microphone
against time but the voltage represents the position or movement of the
microphone had
the time period can be measured from the time taken to from one wave
to the next and you could use the frequency former
remember that was given on the pegs am paper to calculate the frequency of the
wave
if you knew its time period higher sounds
have higher frequencies
when object vibrates the Soundwave a crate on this telescope
will look different depending on the sound is producing
the app to cheat and the wavelength represents
the loudness on the pitch can you work out which one of these waves his loudest
and which one is highest
the higher the pitch the shorter the period over wave
so this one represents the highest pitch sound
because it has the highest frequency and therefore shortest
time period the Bourne object vibrates the greater the empty will be
so be represents the quietest wave
and the shape of a wave indicates what was vibrating
so love these were the same instrument and the
was slightly different instruments it vibrates in a slightly different way
humans here sounds from about 20 to about 20,000 hurts
although as you get older this high-end tends to drop off