unit three: waves Flashcards
frequency unit
Hertz Hz
wavefront def
a line where all the vibrations are in phase and the same distance from the source
transverse wave def
one that vibrates/oscillates at right angles to the direction in which the energy or wave is moving.
longitudinal wave def
one that vibrates/oscillates along the direction in which the energy is moving
transverse wave example
light waves
longitudinal wave example
sound waves
amplitude def
maximum movement of particles from their resting position
wavelength def
distance between a paticular point on a wave and the same point on the next wave e.g. crest to crest
frequency def
number of waves produced each second / number of waves passing a paticular point each second
frequency (Hz) =
f =
1 / time period (s)
1 / t
wave speed (m/s) = v =
frequency (Hz) x wavelength (m)
f x λ
ripple tank purpose
to study the behaviour of water waves
describe a ripple tank
a tank of water with a woden bar that vibrates when the motor is turned on, creating a series of wavefronts on the surface of the water. when the light above it is turned on you can see the wave pattern shadoes on the floor underneath it
at higher frequencies, the water waves have
shorter wavelengths
at lower frequencies, the water waves have
higher wavelengths
what is a normal
the line drawn at right angles to the surface
what is the angle of incidence
the angle between the direction of the waves as they approach the barrier and the normal
what is the angle of reflection
the angle between the direction of the waves after striking the barrier and the normal.
law of reflection
the angle of incidence is equal to the angle of reflection
the angle of incidence =
the angle of reflection
where do light waves travel slower, air or water
water
which waves can be refracted
all of them
how does refraction occur
the light waves in water travel slower than in air, and this change in speed as they leave the water causes them to change direction, otherwise known as refraction
explain the doppler effect
if there is a stationary siren, then the wavefronts will be equal, creating the same frequency for everyone. if the siren is moving, the wavefronts will bunch up at the front end, so there will be high frequency to the person standing at the front. the wavelengths will become more spaced out at the back of it, a someone standing there will hear a lower frequency
what do all the EM waves have in common
they all transfer energy
they are all transverse waves
they all travel at 300 000 000 m/s, the speed of light in a vaccuum (free space)
they can all be reflected and refracted.
what is the order of colours in the visible part of the electromagnetic spectrum?
red - orange - yellow - green - blue - indigo - violet
uses of radiowaves
communication of information e.g. speech, radio, tv
what waves have the lowest frequencies and the longest wavelength
radio waves
uses of microwaves
heating food and satellite communciation.
uses of infrared
night vision cameras and heating devices.
uses of visible light
humans to see things and photography.
uses of ultraviolet
black lights and sterilising water
uses of x-rays
used to examine the internal structures of the body in medical diagnosis.
uses of gamma rays
sterilising medical equipment and treating cancer.
dangers of microwaves
they can directly heat internal body tissue
dangers of infrared
its readily absorbed by our skin and will cause burns
dangers of ultraviolet
skin cancers and can damage the eyes
dangers of x-rays
cell mutation and cancer
dangers of gamma rays
cell mutation and cancers
which wave has the shortest wavelengths and the highest frequencies
gamma rays
speed of light
300 000 000 m/s
the change in direction of a ray when entering a new material is called
refraction
as a ray enters a block, it
slows down and is refracted towards the normal
as a ray leaves a block, it
speeds up and is refracted away from the normal
if the ray strikes a boundary between two media at 90
the ray continues without a change of direcion
what does n stand for
refractive index
n =
sin i / sin r
practical: investigate the refractive index for glass
shine a ray of light onto one of the sides of the glass block so that it emerges out the opposite side. mark the directions of both of these rays with crosses.
draw around the glass block before removing it
using the crosses draw in the direction of both rays and the one inside the block. draw a normal (90 degrees to the surface) where the ray enters the block. measure i and r. calcuate n
what is the symbol r used for
reflection and refraction
when can total internal reflection occur
when rays of light are travelling towards a boundary with a less optically dense medium (a medium with a lower refractive index)
what is the critical angle (c)
when the angle of refraction hits 90 degrees, and is the smallest possible angle of incidence at which light rays are totally internally reflected
what happens when i is greater than the critical angle (c)
all light is reflected at the boundary - total internal reflection
practical: investigating total internal reflection
use a semi-circular glass box and a ray box. shine a ray of light through the curved side at the centre of the straight side. now carefully increase ad decrease thea ngle to see the smallest angle at which most of the light is reflectd along the edge of the glass block. this is the critcal angle
sin c =
1 / n
practical: investigate TIR In prisms
shine a ray of light into a prism through the centre of a side to hit the centre of the hypotenuse it will reflect through the centre of the last side, at 90 degrees. you can repeat this with half the triangle on both ends to see the ray will come back in the same direction from which it came
a periscope that uses prisms to reflect light is called
a prismatic periscope
what is total internal reflection used in
prismatic periscope
bicycle and car reflectors
optical fibres
endoscope
practical: investigate the speed of sound using echoes
stand 50m away from a large wall or building. bang two pieces of wood together and listen for the echo. bang the pieces of wood together each time you hear the echo. this will create a regular rhythm of claps. ask the friend to time you doing 20 claps, during this time the sound will have travelled, for example, 50m x 20 x 2, (to the wall and back 20 times) and you can divide this distance by the time to work out the speed of sound)
how does echo sounding work
sound waves are emitted from the ship and travel to the seabed.
some of these waves are reflected from the seabed back up to the ship. equipment on the ship detects these sound waves.
the time it takes the waves to make this journey is measured.
knowing theis time, the depth of the sea below the ship can be calculated.
why do some sounds seem louder over water
most of the sound travels to us in a straight line
but some sound travels upwards
if the temp is right, then as the sound waves travel through the air theyre refracted and follow a curved path down. the two sets of soundwaves we now hear seem louder and clearer
what range can a human hear from
20 Hz to 20 000 Hz.
PRACTICAL: investigate the refraction of light using rectangular blocks, semi-circular blocks and triangular prisms
- Connect the ray box to the power supply and insert the single slit slide so that it
produces a clear and thin beam of light. - Place one of the blocks onto the sheet of paper and draw around it.
- Remove the block and then mark the position on the outline that you are going to
shine the light ray at with a cross. - Using a protractor, draw a normal to that point (a perpendicular line).
- Mark on a selection of different angles of incidence by measuring angles from the
normal line. - Replace the block on top of the outline, and then shine the ray of light along each
incident line. For each angle, mark the position on the other side of the block where
the light exits. - Turn off the ray box and remove the block.
- Using a ruler, connect up the entry position and the exit position for each angle of
incidence. - Using a protractor, measure the angles of refraction (the angles that the lines inside
the block outline make with the normal) for the different angles of incidence. - Repeat for the other two shaped blocks and compare results.
PRACTICAL: investigate the refractive index of glass using a glass block
- Set up the experiment in a darkened room.
- Place the glass block on the paper and draw around it to ensure that the block will always
be in the same place even if you remove it and replace it. - Using the protractor, draw a line that is 90º to the surface of the glass block (this is the
‘normal’).
○ It may be easier to move the block and work from the outline on the paper for this
part since you need to continue the line into the glass block outline. - Draw three lines as guides for the angles you are going to direct the light into the block.
○ These will hit the block at the point where you drew the normal.
○ Example angles are 20º, 40º and 60º. - Direct the light along each of these lines in turn and, for each one, make markings where
the light leaves the block on the other side.
○ This can be done by drawing dots or Xs and joining them together with a ruler once
you have moved the block out of the way. - Connect the point of incidence to the point where the light leaves the block for each angle,
which should leave you with something like the diagram below.
○ Ensure that there is a normal line (90º to the surface of the glass as before) at each
point where the light leaves the block. - Use the protractor to measure all the angles of incidence and refraction and mark these on
the paper.
○ The angle of incidence where the light initially hits the block should be equal to, or
very close to, the angle where the light is leaving the block.
○ Always measure angles from the normal to the path of the light. - The refractive index of glass when light enters from air is given by , where ‘i’ is the n = sin(i) / sin(r)
angle of incidence and ‘r’ is the angle of refraction. - Use the angles you measure to calculate the refractive index of the glass block.
○ If the values vary, take the average.
PRACTICAL: investigate the speed of sound in air
- Measure the distance from one side of the open space to the other side using a trundle
wheel or long tape measure, marking with a marker the start and end point of your
measurement. - One of your pair should stand at one marker with the wooden blocks, and the other should
stand at the other marker with a stopwatch. - Once both ready, the block holder should strike the two blocks together.
- At the instant they see the blocks touch, the other person should start the stopwatch.
- At the instant they hear the sound, they should stop the stopwatch.
- This should be repeated 10 times.
- The speed should be calculated using: Speed = distance / time.
- An average speed for all repeats should be calculated.
PRACTICAL: investigate the frequency of a sound wave using an oscilloscope
- Connect the microphone to the oscilloscope and check that waveforms are produced when
sounds are made. You may need to alter the oscilloscope settings to ensure that the wave
forms fill the screen and are clear. - Make a sound using an instrument or by humming a note, and then press the hold button
on the oscilloscope so that the wave-form produced is frozen. - Measure the distance between two peaks, and then by referring to the time base, calculate
the length of time between the peaks. - This time is the time taken for one wave to be produced and is known as the time period.
- To calculate the frequency of the wave, use: f =1/T.
PRACTICAL: investigate thermal energy transfer by conduction
- Set up the equipment as shown in the diagram.
- Using a small amount of petroleum jelly, attach a drawing pin to the end of each of the rods.
○ Try to make this the same amount of petroleum jelly for each rod. - Bring together the other ends of the rods (without the pins) so that they can each be heated
the same amount. - Using the Bunsen burner, begin heating the ends of the rods without the pins and start the
stopwatch. - Record the time taken for the pins to fall off the end of each rod and use this to determine
the order of conductivity of the metals.
○ The first pin to fall will be from the rod that is the best conductor
PRACTICAL: investigate thermal energy transfer by convection
- Fill the beaker with water until it is three-quarters full and place it on top of the tripod and
gauze. - Using the forceps, pick up the crystal and drop it through the glass tube to one side of the
bottom of the beaker. - Cover the top of the tube with your finger and remove the tube carefully.
- Heat the beaker using the Bunsen burner and record observations.
PRACTICAL: investigate thermal energy transfer by radiation
- Pour boiling water into the Leslie cube.
- Align the infrared detector with one side of the Leslie cube, 20cm away from the side, and
take the initial temperature of the surface. - Measure and record the temperature of the surface every 30s for five minutes.
- Rotate the cube and repeat the experiment for a different surface.
- Plot temperature (plot on y-axis, measured in °C) against time (plot on x-axis, measured in
seconds) for each different surface.
what leads to total internal reflection
the angle of incidence is bigger than the critical angle
angle of incidence > critical angle
Explain how the frequency of EM waves in free space differs with increasing wavelength
frequency decreases because speed is constant
explain how microwaves are used to find the distance to an aeroplane (3)
time taken for microwaves to get to plane and return
Distance calculated from speed = distance/time
And is then halved
Suggest why its important for the aerial to rotate through a full circle (1)
updates distance of planes frequently
Suggest why the student uses light of just one colour
refraction id wavelength/colour dependent
how to measure the speed of sound
use two bits of wood and a stopwatch. stand some distance away from a friend and start the stopwatch when you see them bang together the two bits of wood. and stop the stopwatch when you hear the sound. now use the speed = distance/time equation to calulate the speed of sound.
why will you measuring the speed of sound not be very accurate
requires very good eyesight and very fast reactions.
can make it more accurate by using echoes.
how to measure the speed of sound using echoes
both you and friend should stand at least 50m away from a large wall or building.
bang the two pieces together and wait for an echo.
every time you hear an echo, bang together the wood. this will create a regular rhythm of claps.
ask the friend to time you doing 20 claps. during this time the sound will have travelled 40 ways, so divide the distance by the time to work out the speed of sound.
what does sonar stand for
sound, navigation and ranging
can sound waves be refracted
yes
when are sound waves refrated
when some parts are travelling through warm air, they travel quicker, and if travelling though cool air they will travel slower. as a result the direction of the sound wave will change and be refracted,
why do some objects produce higher pitches
if an object vibrates more slowly, it will produce low freq and low pitch waves. it an object vibrates rapidly, it will prodce high freq and high pitch waves.
how can you see a visual representation of a sound wave
connect a microphone to a piece of apparatus called an oscilloscope.
f =
1/T
speed =
v =
frequency x wavelength
f x λ
large amplitude =
loud sound
small amplitude =
quiet sound
