3) Waves Flashcards
Two types of waves
-transverse
-longitudinal
Transverse waves
-waves that vibrate or oscillate perpendicular to the direction of energy transfer (wavy line)
-peak/ crest - highest point
-trough - lowest point
-e.g. ripples on water, vibrations in a guitar string, s-waves, electromagnetic (radio, light, x-rays)
Transverse waves features
-energy transfer is the same direction as the wave motion
-can move in a liquid or solid, not a gas
-some (electromagnetic waves) can move in a vacuum
-constant density, pressure
Longitudinal waves
-Waves where the points along its length vibrate parallel to the direction of energy transfer (like a spring)
-compressions - close together
-rarefactions - space apart
-represented by sets of lines
-e.g. sound waves, p-waves, pressure waves caused by repeated movements in a liquid or gas
Longitudinal waves features
-The energy transfer is in the same direction as the wave motion
-They can move in solids, liquids and gases
-They can not move in a vacuum (since there are no particles)
-changes in density, pressure
Waves & energy
-waves transfer energy and information, not matter
-described as oscillations or vibrations about a fixed point
-all waves can be refracted, reflected
Amplitude
The distance from the undisturbed position to the peak or trough of a wave
-Symbol A
-measured in metres
Wavelength
The distance from one point on the wave to the same point on the next wave
-transverse: measured one peak to the next
-longitudinal: measured from centre of one compression to the centre of the next
-symbol λ (lambda)
-measured in metres
Frequency
The number of waves passing a point in a second
-Symbol f
-measured in Hertz (Hz)
Time period
The time taken for a single wave to pass a point
-Symbol T
-measured in seconds (s)
Calculate wave speed
-wave equation
Wave speed (m/s) = frequency(Hz) x wavelength(m)
v=fλ
Calculate frequency
Frequency (Hz) = 1/Period (s)
f=1/T
The Doppler effect
The apparent change in wavelength and frequency of a wave emitted by a moving source
-when a stationary object emits waves, waves spread out symmetrically
-when object begins to move, the waves can get squashed together at one end, and stretched at the other
-squashed together - wavelength decreases, frequency increases - higher pitch
-stretched out - wavelength increases, frequency decreases - lower pitch
Electromagnetic waves
Transverse waves that transfer energy from the source of the waves to an absorber
-all transverse
-all travel through a vacuum
-all travel at the same speed in a vacuum
Electromagnetic spectrum
Lower energy, long wavelength, low frequency
-radio waves
-microwaves
-infrared
-visible light
-ultraviolet
-x-rays
-gamma rays
Higher energy, short wavelength, high frequency
Raging Martians Invaded Venus Using X-ray Guns
Electromagnetic spectrum - radiation
-higher the frequency, higher energy of radiation
-radiation with higher energy: highly ionising, harmful to cells and tissues causing cancer
-radiation with lower energy: useful for communication, less harmful for humans
Visible light
-the range of wavelengths which are visible to humans (only part of spectrum)
-each colour corresponds to a narrow band of wavelength and frequency
-Red: longest wavelength, lowest frequency and energy
-Violet: shortest wavelength, highest frequency and energy
Radiowaves use
-communication (radio and TV)
-broadcasting
Microwaves use
-heating food
-satellite transmission
-communication (WiFi, phones)
Infrared use
-heaters
-night vision equipment
-remote controls
Visible light use
-optical fibres
-photographs
Ultraviolet use
-fluorescent lamps
-suntan
X-rays use
-observing the internal structure of objects and materials (medical applications, airport security)
Gamma rays use
-sterilizing medical instruments/ food
-treating cancer
Microwaves - danger
-high intensity microwaves can cause heating of internal organs
-as water molecules absorb microwaves strongly
UV waves - danger
-if eyes exposed to high levels - severe eye damage
-ionizing - can kill cells, cause them to malfunction
-premature ageing - skin cancer
Light waves
-visible light is part of the electromagnetic spectrum - transverse wave
-can undergo reflection, refraction
Sound waves
-vibrations of air molecules - longitudinal wave
-When a sound wave comes into contact with a solid those vibrations can be transferred to the solid
Reflection
A wave hits a boundary between two media and does not pass through, but instead stays in the original medium
Refraction
A wave passes a boundary between two different transparent media and undergoes a change in direction
Angle of incidence vs reflection
-angle of incidence: angle of the wave approaching the boundary
-angle of reflection: angle of the wave leaving the boundary
-angles measured between the wave direction and a line 90 degrees to the boundary
The Law of Reflection
Angle of incidence (i) = Angle of reflection
Refraction - density of materials
-from less dense to more dense: light bends towards the normal - rays slow down
-more dense to less dense: light bends away from the normal
-pass along the normal (perpendicular): light doesn’t bend
-frequency of waves do not change
Practical: investigating the refraction of light
- trace around glass object using a pencil
- direct a beam of light at the side face of the block
- mark point on ray close to the block and point on the exit light ray
- mark where the ray enters and exits the block
- connect points
- repeat process for the ray at a different angle
- repeat the procedure for each shape of block (semi-circular, prism)
Snells’ Law
-less dense –> more dense - i>r
-more dense –> less dense - i<r
n = sin i/ sin r
n = refractive index = speed of light in vacuum/ speed of light in material
Critical angle
-the angle of incidence when angle of refraction is exactly 90
-light is refracted along the boundary
Total internal reflection
Occurs when:
-the angle of incidence > the critical angle
The light ray moves from a more dense medium to a less dense medium
Total internal reflection - optical fibres
-consists of very thin core of high purity glass covered by a second layer of less dense high purity glass
-light passing along the core at the angle greater than the critical angle are totally internally reflected
-acts like a mirror, light ray continuously reflected along the length of the optical fibre core
Total internal reflection - prisms
Light is internally reflected by 90 degrees
Calculating critical angle
sin c = 1/n
-large refractive index, smaller critical angle
-light rays inside a material with a high refractive index are more likely to be totally internally reflected
Frequency range for human hearing
20-20000 Hz
Practical: investigate the speed of sound in air
- Have two people 100m between each other
- One person bangs wooden blocks together to generate sound
- Second person have a stopwatch, start when they see person banging blocks, stop when they hear the sound
- Repeat, average taken
- Calculate speed = distance/ time
How oscilloscope and microphone used to display a sound wave
-microphone plugged into oscilloscope
-sound into microphone
-microphone will transfer sound into electrical signal
-oscilloscope displays signal
-x-axis - time
-y-axis - height of wave
Practical: investigate the frequency of a sound wave using an oscilloscope
- Connect microphone to oscilloscope
- adjust time base of oscilloscope until signal fits onto screen
- Strike tuning fork, hold near mic
- Freeze/ take a picture of oscilloscope screen
- Measure, record time period of wave signal by counting number of divisions for one complete wave cycle
- Repeat steps 3-5 with different tuning forks
Pitch relative to frequency
-high frequency, high pitch
-low frequency, low pitch
