Waves

waves

oscillations or vibrations about a fixed point

transfer energy and information without transferring matter

amplitude

The distance from the undisturbed position to the peak or trough of a wave

wavelength

The distance from one point on the wave to the same point on the next wave

frequency

The number of waves passing a point in a second(hz)

time period

The time taken for a single wave to pass a point(s)

equation relating frequency and time period

f = 1/T

wave speed

The distance travelled by a wave each second

How are wavefronts used to represent wavelength

Distance between wavefronts represents wavelength

2 types of waves

• Transverse

• Longitudinal

transverse wave

Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer

properties of transverse waves

• The energy transfer is perpendicular to wave motion

• They transfer energy, but not the particles of the medium

• They can move in solids and on the surfaces of liquids but not inside liquids or gases

• Some transverse waves (electromagnetic waves) can move in solids, liquids and gases and in a vacuum

• Constant pressure and density

examples of transverse waves

• Ripples on the surface of water

• Vibrations in a guitar string

• S-waves (a type of seismic wave)

• Electromagnetic waves (such as radio, light, X-rays etc)

representing transverse waves

• Transverse waves are drawn as a single continuous line, usually with a central line showing the undisturbed positions

• The curves are drawn so that they are perpendicular to the direction of energy transfer

  • These represent the peaks and troughs

longitudinal waves

Waves where the points along its length vibrate parallel to the direction of energy transfer

properties of longitudinal waves

• The energy transfer is in the same direction as the wave motion

• They transfer energy, but not the particles of the medium

• They can move in solids, liquids and gases

• They can not move in a vacuum (since there are no particles)

• Pressure and density in the wave can change

compressions

points in a longitudinal wave that are close together

rarefactions

points in a longitudinal wave that are spaced apart

examples of longitudinal waves

Sound waves

P-waves (a type of seismic wave)

Pressure waves caused by repeated movements in a liquid or gas

representing longitudinal waves on wavefront

Longitudinal waves are usually drawn as several lines to show that the wave is moving parallel to the direction of energy transfer

Drawing the lines closer together represents the compressions

Drawing the lines further apart represents the rarefactions

Visualising transverse and longitudinal waves

transverse wave - shaking a rope

longitudinal wave - shaking a spring/coil

wave equation(2)

wave speed = distance travelled by wave/time

or

v = fλ

v = wave speed

f = frequency(hz)

λ = wavelength(m)

3 experiments to determine wave speed

Measuring sound from 2 points

Measuring echo

using an oscilloscope

How to measure sound from 2 points(experiment)

1. Two people stand a distance of around 100 m apart

2. The distance between them is measured using a trundle wheel

3. One person has two wooden blocks, which they bang together above their head

4. The second person has a stopwatch which they start when they see the first person banging the blocks together and stops when they hear the sound

5. This is then repeated several times and an average value is taken for the time

6. The speed of sound can then be calculated using the equation: speed = d/t

How to measure sound from echoes(experiment)

1. A person stands about 50 m away from a wall (or cliff) using a trundle wheel to measure this distance

2. The person claps two wooden blocks together and listens for the echo

3. The person then starts to clap the blocks together repeatedly, in rhythm with the echoes

4. A second person has a stopwatch and starts timing when they hear one of the claps and stops timing 20 claps later

5. The process is then repeated and an average time calculated

6. The distance travelled by the sound between each clap and echo will be (2 × 50) m

7. The total distance travelled by sound during the 20 claps will be (20 × 2 × 50) m

8. The speed of sound can be calculated from this distance and the time using the equation: speed = 2 x distance/time(echo means the wave travels 2 times forth and back)

How to measure sound from oscilloscopes(experiment)

1. Two microphones are connected to an oscilloscope and placed about 5 m apart using a tape measure to measure the distance

2. The oscilloscope is set up so that it triggers when the first microphone detects a sound, and the time base is adjusted so that the sound arriving at both microphones can be seen on the screen

3. Two wooden blocks are used to make a large clap next to the first microphone

4. The oscilloscope is then used to determine the time at which the clap reaches each microphone and the time difference between them

5. This is repeated several times and an average time difference calculated

6. The speed can then be calculated using the equation:  speed = distance between microphones/time between peaks

How to measure the speed of ripples on water surfaces

1. Choose a calm flat water surface such as a lake or a swimming pool

2. Two people stand a few metres apart using a tape measure to measure this distance

3. One person counts down from three and then disturbs the water surface (using their hand, for example) to create a ripple

4. The second person then starts a stopwatch to time how long it takes for the first ripple to get to them

5. The experiment is then repeated 10 times and an average value for the time is calculated

6. The average time and distance can then be used to calculate the wave speed using the equation: average speed = distance/time

How does echo sounding work to detect objects underwater

The sound wave is reflected off the ocean bottom

The time it takes for the sound wave to return is used to calculate the depth of the water

The distance the wave travels is twice the depth of the ocean

This is the distance to the ocean floor plus the distance for the wave to return

When doing equations what do you have to remember about echoes?

Account for the distance travelled as it could be doubled due to it returning

Therefore when calculating depth don’t forget to account for this as you will need to either halve the time at start or halve the distance at the end

4 different wave interactions

• Reflected

• Refracted

• Transmitted

• Absorbed

reflection

A wave hits a boundary between two media and does not pass through, but instead stays in the original medium

What surfaces are the most reflective?

smooth surfaces

What surfaces are the least reflective?

rough

opaque surface interaction with light

will reflect light which is not absorbed by the material

The electrons will absorb the light energy, then reemit it as a reflected wave

refraction

A wave changes speed at the boundary between two materials of different densities

This can result in a change in direction.

transmission

A wave passes through a substance

For light waves, what characteristic of the material influences its transmission?

The transparency of the material

More transparent = more transmission

Condition for transmission

the wave must pass through the material and emerge from the other side

differences between original and transmitted wave

• The transmitted wave may have a lower amplitude because of some absorption

  • For example, sound waves are quieter after they pass through a wall

absorption

Energy is transferred from the wave into the particles of a substance

What can be said about the absorption and reflection if an object appears red?

• Only red light has been reflected

• All the other frequencies of visible light have been absorbed

acronym for refraction

Faster

Away

Slower

Towards

When would light change speed but not direction in refraction?

If it passed through the normal(perpendicular)

When would light bend to the normal?

Entering a medium of higher density

Due to decrease in speed

When would light away from the normal?

entering a medium of lower density

due to increase in speed

What properties of waves change and stay the same in refraction?

Change - wavelength and wave speed

stay the same - frequency (thus we don’t perceive a colour change)

How can wavefronts represent refraction?

If speed increases(lower density):

show a change in the direction(away from normal)

show a decrease in wavelength

If speed decreases(higher density):

show a change in the direction(towards the normal)

show an increase in wavelength

What is the factor of waves that determines their interactions with different materials?

wavelengths

whilst the wavelength of some waves may be transmitted, other wavelengths may be reflected, absorbed or refracted

Core Practical: Ripple Tank to measure frequency wavelength and wave speed

Use a ripple tank to generate waves in water

light source is needed to create shadows

metre ruler needed to measure distance

a paper is needed to act as a screen for the shadows of the wavefront

1. Turn on the power supply and the light source to produce a wave pattern on the screen

2. The wavelength of the waves can be determined by using a ruler to measure the length of the screen and dividing this distance by the number of wavefronts(5 for example)

3. The frequency can be determined by timing how long it takes for a given number of waves(20) to pass a particular point and dividing the number of wavefronts by the time taken

4. Record the frequency and wavelength in a table and repeat the measurements

5. A mobile phone could be used to improve data observation

6. Find speed: v = fλ

Explain natural frequency of objects and the result of it.

• Different solids have a tendency to vibrate at different frequencies 

• As a result, sound waves with a frequency that is close to a particular solid's natural frequency will cause larger vibrations than for sound waves with frequencies much larger or smaller than the solid's natural frequency

• This means some frequencies of sound are transferred much more efficiently to the solid than others

range of frequency a human can hear

20-20000 hz

How does someone detect a sound

Sound waves travel into the pinna/ear flap and enters the ear/auditory canal

Then it reaches the eardrum and the vibration pattern of the sound waves creates the same vibration pattern of the ear drum

The vibration is then amplified via 3 ear bones, the hammer, the anvil and the stirrup and transfers the vibrations to the liquid in the cochlea in the inner ear

Tiny hairs inside the cochlea detect the vibrations and create electrical impulses that travel along neurones in the auditory nerve to the brain giving the sensation of sound

ultrasound

Sound waves with a frequency above the human hearing range of 20 000 Hz

infrasound

Sound waves with a frequency below the human hearing range of 20 Hz

Uses of ultrasound and infrasound(3)

• Sonar(ultra)

• Foetal scanning(ultra)

• Exploration of the Earth's core(infra)

How does sonar use ultrasound

Sonar uses ultrasound to detect objects underwater

The sound wave is reflected off the ocean bottom

The time it takes for the sound wave to return is used to calculate the depth of the water

The distance the wave travels is twice the depth of the ocean

This is the distance to the ocean floor plus the distance for the wave to return

How is ultrasound used in foetal scanning?

• In medicine, ultrasound can be used to construct images of a foetus in the womb

  • An ultrasound detector is made up of a transducer that produces and detects a beam of ultrasound waves into the body

  • The ultrasound waves are reflected back to the transducer by different boundaries between tissues in the path of the beam

  • For example, the boundary between fluid and soft tissue or tissue and bone

• When these echoes hit the transducer, they generate electrical signals that are sent to the ultrasound scanner

• Using the speed of sound and the time of each echo’s return, the detector calculates the distance from the transducer to the tissue boundary

• By taking a series of ultrasound measurements, sweeping across an area, the time measurements may be used to build up an image

• Unlike many other medical imaging techniques, ultrasound is non-invasive and is believed to be harmless

P waves

primary waves, named so because they travel faster and so these waves are felt first in an earthquake

S waves

secondary waves, named so because these travel slower and so these waves are felt second in an earthquake

properties of P waves

longitudinal

faster than S waves

can pass through solids and liquids

infrasound

properties of S waves

transverse

slower than P waves

can only travel through solids

How is the outer core being liquid proven through seismic waves

The detection of P waves but not S waves on the opposite side of the earth to an earthquake - S waves can’t travel through liquid so this proves that there must be a liquid outer core that hinders passing the S waves.

How is the inner core being solid proven through seismic waves

refractions between layers causes 2 shadow zones where no P waves are detected.

Thus, the inner core is solid due to the positions and sizes of these shadow zones indicating large refraction taking place

How do sound waves transfer energy

They transfer energy by the molecules vibrating and knocking into neighbouring molecules

Which state of matter is sound waves fastest and slowest in?

fastest in solids

slowest in gases

What properties of a soundwave change upon entering a different medium

Wave speed

Wavelength