Module 4 Basic Wave Properties / Refraction Flashcards
What is a progressive wave?
a wave which transfers energy from a source through the means of oscillations without the transfer of the medium
What are the two types of progressive waves?
Longitudinal and transverse waves
What is a longitudinal wave? Give an example
a wave where the particle vibrations are parallel to the direction of energy transfer
eg sound, ultrasound
What is a transverse wave? Give an example
A wave where the vibrations are at 90 degrees to the energy transfer
eg all electromagnetic waves; water waves
What waves can travel through a vacuum?
Electromagnetic waves
What is displacement?
Distance of a point on a wave above or below the undisturbed position - how much a point has moved from it’s rest position (y-axis)
Amplitude
Maximum displacement of a wave from it’s undisturbed position. Symbol A
What is wavelength?
Minimum distance between two adjacent points in phase
Symbol for wavelength
λ, units in metres
State 2 properties of sound waves
Longitudinal
and
Mechanical (need a medium to travel through)
What is the time period of a wave?
time taken for one complete oscillation or wavelength to pass a point
Symbol for time period
T - unit seconds
What is the frequency of a wave?
The number of wavelengths per unit of time
What are the crest and trough?
crest is the peak and trough is the dip in a snapshot of a transverse wave
What is rarefaction?
part of a longitudinal wave where the particles are the most spaced apart
What does the wave speed depend on?
the medium it is travelling through
Derive the wave equation
Speed = distance/time = λ/T
f=1/T
λx1/T=λf
speed = λf
What is an oscilloscope?
an apparatus which shows the amplitude of a wave on the y and x axis represents time
What is the unit used for the y-axis on an oscilloscope
volts
What does CRO stand for?
cathode ray oscilloscope (usually just called an oscilloscope)
How is amplitude displayed on an oscilloscope?
Biggest value on the y-axis (in volts )
Count number of vertical squares from central line then multiply by voltage setting
How to get time period from oscilloscope
Count the number of squares horizontally for one complete wavelength
Multiply by the timebase
(which is number of seconds per horizontal division)
If the depth in a ripple is reduced what happens to the wave speed?
reduces wave speed
Suggest a method of observing reflection
Visible light and a mirror - a wooden plank in a ripple tank
Definition of intensity
the radiant power passing through a surface per unit area
What is the unit for intensity?
Wm^-2
Equation for intensity?
I=P/A
What properties does intensity have in reference to distance?
- As a wave travels from the source the radiant power spreads out and therefore decreases
- From a point power spreads in all directions
- To find the light intensity at a curtain distance, the area is equal to the area of a sphere
Note: intensity is proportional to one over the square of the distance = inverse proportion - I ∝1/r^2
What is the relationship between amplitude and intensity?
Intensity ∝ Amplitude^2
Definition of refraction
change in speed of a wave as it passes from one medium to another causing a change in direction
What is the refractive index?
the ratio of speed of light in a vacuum over speed of light in a medium
Which direction are lightwaves refracted by when passing through medium when going from less to more dense?
towards the normal (when going from less to more dense)
What happens when light crosses the boundary of two different mediums along the normal?
no change in direction but the light changes speed
What is the relationship between n and the angle of refraction?
greater the index the smaller the angle
What effects are there when two refractive indices are equal?
ray of light continues along it’s original pathway
What stays the same during refraction?
frequency
What happens if the angle of incidence is the critical angle?
refracted light travels along the boundary of the two media (angle of reflection = 90º)
How are wavelength and amount of refraction related?
as wavelength decreases amount of refraction which takes places increases (ie bigger change in direction)
What is Snell’s law?
nsin(θ)=constant
Describe how the refractive index, and thus the critical angle of glass (to air) can be calculated using a RECTANGULAR glass block
- Place the block of glass on a sheet of paper and draw a trace around the block
- Draw a normal at a given point on the side of a block
-Point a laser at this point, and using a protractor, measure the angle of incidence
-Draw a dot where the beam of light exits the glass block and join the dot between entrance and exit
-Measure the angle to the normal inside the block - this is angle of refraction - use nsin(θ)=constant (with n=1 for glass) to find refractive index for glass
- then use sin(C) = 1/n to find critical angle C
How can the critical angle be found using a semicircular glass block
-Place block on a piece of paper and trace
-Shine a laser/light ray at the curved side so that it is at right angles to edge
-vary the angle of incidence by adjusting the light ray until the beam refracts along the straight edge
-Mark the direction, and thus the angle of incidence of the light ray
List wave phenomena for transverse waves
Reflection
Refraction
Interference
Diffraction
Polarisation
List wave phenomena for longitudinal waves
Reflection
Refraction
Interference
Diffraction
If two points are separated by half a wavelength, comment on their displacement
Same magnitude of displacement but travelling in opposite directions
(positive and negative values)
If the phase difference between two points is 270 degrees, how far apart are the points?
3/4 wavelength apart
(270 degrees is 3/4 of 360)
What is phase difference
the angle by which a wave lags or leads another
sometimes used to compare two points on the same wave
what is the phase difference of two points separated by a whole wavelength?
360 degrees or 2pi radians
what is the phase difference of two points separated by a half wavelength?
180 degrees or pi radians
What is a compression on a longitudinal wave?
Region on a longitudinal wave where the particles are closest together (highest pressure)
