4.4 Waves Flashcards
What is a progressive wave
An oscillation that travels through matter or a vacuum, transferring energy (not matter) from one place to another
What happens to the particles in progressive waves
They vibrate as the wave passes through them but don’t move along with the wave
What are the two types of progressive waves
Transverse and longitudinal
What is a transverse wave and give an example
Where oscillations are perpendicular to direction of energy transfer
Eg EM waves
What does a transverse wave look like
They have peaks / troughs at the maximum / minimum points of displacement
What are longitudinal waves and give an example
Where particles oscillate parallel to direction of energy transfer
Eg sound waves
What do longitudinal waves look like
Areas of compressions where particles are close together and areas of rarefaction where particles are spread out
What is the displacement of a wave and state its units
The distance form the equilibrium position to the given point in a particular direction
Metres (m)
What is the amplitude of a wave and state its units
The maximum displacement from its equilibrium point
Metres (m)
What is wavelength (λ) and state its unit
The distance from peak to peak on a wave oscillating in phase
Metres (m)
What is the time period (T) and state its units
The time taken for a full oscillation of one wavelength to pass a given point
Seconds (s)
What is the frequency (f) and state its units
The number of complete oscillations passing a point per unit time
Hertz (Hz)
What is wave speed (v) and state its units
The distance travelled by a wave per unit time
Metres per second (ms -1)
What is phase difference and state its units
The difference in displacement of particles along a wave or two waves
Radians
What is in phase
When the particles oscillate in sync with each other
Phase difference will be a multiple of 2π
What is antiphase
When the particles are oscillating with a phase difference of an odd multiple of π
They are half a wavelength apart
What is out of phase
When the particles are not moving in phase or in antiphase
What is the equation for frequency (f)
Frequency (Hz) = 1 / time period (T)
f = 1/T
What is the equation for wave speed (v)
Wave speed (ms -1) = frequency (Hz) x wavelength (m)
V = f λ
What is reflection
When a wave changes direction at a boundary between two media, remaining in the original medium
What is the relationships between the angles in reflection
Angle of incidence = angle of reflection
What happens to the wavelength / frequency / wave speed in reflection
All stay the same
What is refraction
When the wave changes direction due to a change in speed when it enters a new medium
What happens to the wavelength / frequency and/ wave speed in a denser material in refraction
Wave speed - longitudinal speed up, transverse slow down
Frequency - stays the same
Wavelength - changes proportionally to wave speed
What is diffraction
The spreading out of a wave for t as it passes through a gap
How does diffraction vary with the size of the gap
Smaller than wavelength - no diffraction as wave doesn’t pass though
Equal to wavelength - maximum diffraction
Bigger than wavelength - diffraction will decrease gradually
What happens to the wavelength / frequency / wave speed in diffraction
All stay the same
What is polarisation
Unique to transverse waves
When oscillation of the wave is restricted to only one plane (plane polarised)
How are light waves and microwaves polarised
Polarising filters
- when two polarising filters are perpedicular to each other they completely block light
Metal grilles
- free electrons in the metal bar cancel out the electric field around the grille and absorb it
- the horizontal electric field passes as if the grilles are positioned vertically and vies versa
What is intensity (I) of a progressive wave and state its units
The radiant power passing at a right angle per unit area
Watts per metre squared (Wm -2)
What are the equations for intensity (I)
Intensity Wm-2) = power (W) / area (m2)
I = P/A
Intensity (Wm-2) ∝ Amplitude2 (m)
I ∝ A2
What are electromagnetic (EM) waves
Transverse waves
Travel in a vacuum
All travel at the speed of light in a vacuum
3.0x108 ms-1
Name the EM waves in order from lowest frequency to highest
Radio waves
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays
What happens to the energy of the waves as frequency increases
It increased meaning the aves become more ionising and harmful to cells
This is why UV, X-rays and gamma rays can cause cancer
What is the main use of radio waves
Communications (radio and TV)
What are the main uses of microwaves
Heating food
Communications (Wi-Fi, mobile phones, satellites)
What are the main uses of infrared
- remote controls
- fibre optic communications
- thermal imaging (medicine and industry)
- night vision
- heating and cooking
- motion sensors
What is the main use of visible light
Seeing things
Taking photos or videos
What are the main uses of UV
- Security marking (fluorescence)
- fluorescent bulbs
- tanning
What is the main use of X-rays
X-ray images (medicine, industry, airport security)
What is the main use of gamma rays
- sterilising medical instruments
- cancer treatment
What is the principle of superposition
When two or more waves with the same frequency arrive at a point, the resultant displacement in the sum of the displacements of each wave
What is constructive interference
Where both displacements are in the same direction and add together
What is destructive interference
Where individual displacements are in opposite directions and counteract each other leaving a flat line
When are waves coherent
When they are emitted with a constant and unchanging phase difference
What is interference
The superposition occurring between two coherent waves when phase difference is an event multiple of pi so the crests of the wave combine
When does minimum resultant displacement occur
When phase difference is an odd multiple of pi so one crest and one trough act to cancel earth other out
What is the equation for wavelength used in Young’s double-slit experiment
Wavelength (m) = ( distance between slits (m) x distance between maxima (m) ) / distance between double slits and the screen (m)
λ = ax/D
What is the equation for refractive index (n)
Refractive index = speed of light (ms-1) / speed of light in medium (ms-1)
n = c/v
What is total internal reflection
When no refraction occurs
Light must be travelling from a medium with a higher refractive index to a lower refractive index
Angle of incidence must be above the critical angle
What is the equation to work out the critical angle (C)
sinC = n2/n1
What is a stationary wave
A wave that stores energy
It appears to stay in place
How do stationary waves form
When two or more progressive waves superpose
What conditions are needed to form stationary waves
- two progressive waves travelling in opposite directions
- same frequency
- almost the same amplitude
- same speed
What is a node
The region of the stationary wave with the displacement of zero
What is an anode
The region of the stationary wave at maximum displacement
what is the separation between adjacent nodes
half the wavelength
λ/2
What is the equation used in polarisation
Final intensity (Wm-2) = original intensity (Wm-2) x cos2(θ)
I = I0cos2(θ)
