7. waves Flashcards
Define transverse waves
A wave in which the particles of the medium vibrate perpendicularly to the direction of the wave velocity
Define longitudinal waves
A wave in which the particles of the medium vibrate parallel to the direction of the wave velocity
What are ripple tanks used to demonstrate-
Ripple tanks can be used to demonstrate reflection, refraction and diffraction
Define-
Displacement
Amplitude
Wavelength
- Displacement (x) of a wave is the distance from its equilibrium position. It is a vector quantity; it can be positive or negative
- Amplitude (A) is the maximum displacement of a particle in the wave from its equilibrium position
- Wavelength (λ) is the distance between points on successive oscillations of the wave that are in phase
These are all measured in metres (m)
Define-
Period
Frequency
Speed
- Period (T) or time period, is the time taken for one complete oscillation or cycle of the wave. Measured in seconds (s)
- Frequency (f) is the number of complete oscillations per unit time. Measured in Hertz (Hz) or s-1.
- Speed (v) is the distance travelled by the wave per unit of time. Measured in metres per second (m s-1).
Explain phase
The phase difference tells us how much a point or a wave is in front or behind another
This can be found from the relative positive of the crests or troughs of two different waves of the same frequency
When the crests or troughs are aligned, the waves are in phase
When the crest of one wave aligns with the trough of another, they are in antiphase
Phase difference is measured in fractions of a wavelength, degrees or radians
The phase difference can be calculated from two different points on the same wave or the same point on two different waves
The phase difference between two points:
In phase is 360o or 2π radians
In anti-phase is 180o or π radians
Explain wave energy-
Waves transfer energy between points, without transferring matter
When a wave travels between two points, no matter actually travels with it:
The points on the wave simply vibrate back and forth about fixed positions
Waves that transfer energy are known as progressive waves
Waves that do not transfer energy are known as stationary waves
How to calculate the period and the frequency- from cathode-ray oscilloscope display
The period of the wave can be determined from the time-base This is how many seconds each division represents measured commonly in s div-1 or s cm-1
Use as many wavelengths shown on the screen as possible to reduce uncertainties
Dividing the total time by the number of wavelengths will give the time period T (Time taken for one complete oscillation)
The frequency is then determined through 1/T
How to calculate the speed of the wave-
v=fλ
v= speed
f= frequency
λ/ lamda= wavelength
Name the two waves in terms of transferring energy-
Transverse waves transfer energy
Stationary waves do not transfer energy
Define the term intensity in waves
The amount of energy passing through a unit area per unit time is the intensity of the wave
How do we calculate the intensity of the wave
Intensity= power/ area
intensity= Watt m to the power -2
power= watt
area= m2
The intensity of a progressive wave is also proportional to its amplitude squared and frequency squared
Intensity ∞ Area2
Intensity ∞ frequency 2
Differentiate between transverse and longitudinal waves with examples
Transverse waves- the particles oscillate perpendicular to the direction of the wave velocity.
Transverse waves show areas of troughs and crests.
examples- 1. Electromagnetic waves e.g. radio, visible light, UV
2. Vibrations on a guitar string
transverse waves can be polarised.
Longitudinal waves- the particles oscillate parallel to the direction of the wave velocity.
Longitudinal waves show areas of compression and rarefaction.
examples- sound waves and ultrasound waves
These waves cannot be polarised.
NOTE- When the graph is of displacement and time the wave shown can be either transverse or longitudinal
but when the graph is between displacement and direction of travel then the wave shows can only be transverse.
Define doppler shift-
the change in frequency of a wave observed when the source of the wave is moving towards or away from the observer.
The difference in frequencies when the source moves towards and away from the observer
The frequency is increased when the source is moving towards the observer
The frequency is decreased when the source is moving away from the observer
How to calculate the doppler shift
f0 = fs (V / V +- Vs)
f0= observed frequency
fs=source frequency
V=wave velocity
Vs= source velocity
If the source is moving towards, the denominator is v - vs
If the source is moving away, the denominator is v + vs
Write all the common properties of electromagnetic waves-
They are all transverse waves
They can all travel in a vacuum
They all travel at the same speed in a vacuum (free space) — the speed of light 3 × 108 ms-1
NOTE-
Since they are transverse, all waves in this spectrum can be reflected, refracted, diffracted, polarised and produce interference patterns
Since they are transverse, all waves in this spectrum can be reflected, refracted, diffracted, polarised and produce interference patterns
use of radio waves-
communication -radio and tv
use of microwaves-
heating food
communication- wifi
use of infrared waves-
thermal images- medical
motion sensors
heating food
fibre optic communication
use of visible light-
to see things
taking photographs and videos
use of ultraviolet waves-
fluorescent bulbs
getting a suntan
use of x-rays-
x-ray images- medical, security airport
use of gamma rays-
sterilising medical equipment
treating cancer
Electromagnetic spectrum-
RAdio waves - lower energy, lower frequency, longer wavelengths
MIcro waves
INFrared waves
VIsible light
ULtraviolet
X-rays
GAmma rays - higher energy, higher frequency and shorter wavelengths
Electromagnetic spectrum and their wavelengths values-
look at save my exams and memorise it its important
About polarisation and transverse wavs
Transverse waves can be polarised, this means:
Vibrations are restricted to one direction
These vibrations are still perpendicular to the direction of propagation/energy transfer
A plane polarised waves has oscillations in only one direction.
Why can’t longitudinal waves be polarised?
Longitudinal waves (e.g. sound waves) cannot be polarised since they oscillate parallel to the direction of travel
How can you polarise the waves-
Waves can be polarised through a polariser or polarising filter. This only allows oscillations in a certain plane to be transmitted
note- only unpolarised waves can be polarised- polarised waves cannot be polarised.
What are other ways than polariser and polarising filter that light can be polarised -
Light can also be polarised through reflection, refraction and scattering
What falls by half after the light gets polarised-
intensity of the light
what are the two filters that light passes through when its getting polarised-
First filter is polariser
second is analyser
What are the conditions falling under analyser- to follow malus law
If the analyser has the same orientation as the polariser, the light transmitted by the analyser has the same intensity as the light incident on it
If they have a different orientation, we must used Malus’s law
What is malus law- its equation
Malus’s law states that if the analyser is rotated by an angle θ with respect to the polariser, the intensity of the light transmitted by the analyser is.
I= I0 cos² (θ)
I= intensity of the light transmitted
I0= maximum intensity
θ= angle between polarised light and transmission axis
NOTE- memorize at the table of transmission depending on polariser orientation
