2.4 Superposition, Interference and Diffraction Flashcards
Principle of superposition
When 2 waves meetThe total resultant displacement of the medium at any point in space is given by the vector sum of the individual displacements
What is a standing wave
Wave in which no energy is transferred
Conditions required to produce standing waves
2 coherent waves
Travelling in opposite directions
Superimpose
Where is a node / anti node located
Node = point where s=0
Anti node = point where s=max
State the relationship between f and lambda
Inversely proportional
What is the fundamental mode of vibration
The lowest possible frequency that gives a pattern involving 2 nodes and an anti node
What is the length of the string in the first mode of vibration in terms of lambda
L = &/2
What does L = in the first mode of vibration for standing waves in air columns in terms of lambda
Same for 2nd and 3rd
Frequency
1/4 & f0
3/4 & 3f0
5/4 & 5f0
Conditions for observable interference
Coherent
Constant phase difference
Similar /same amplitudes
What is monochromatic light
Light of a single wavelength
Describe how to determine the speed of sound in air using a resonance tube
Tuning fork of 250Hz is made to vibrate and then placed above closed end glass tube.
The tube is raised until the sound becomes noticeably higher. This is the fundamental mode of vibration
Measure the distance the tube was raised.
Repeat
Use v = 4Lf to find the speed of sound in air
&=4L
fo = 1/&o
If 2 waves are in phase what is their path difference
Is there constructive or destructive interference
n&
Constructive
If 2 waves are out of phase what is their path difference
(n+0.5)&
Describe Young’s slits experiment to measure the & of monochromatic light (3)
A light source is filtered to give monochromatic light
The single slit diffracts light to make light hitting double slit coherent
The double slit acts as 2 coherent sources of light
When are the best results acquired during young’s slits exp
What values should the slit separation be between
What should the distance between the double slits and the screen be
Low light conditions
0.1-0.5mm
1-4m
Precautions to take when using a laser (2)
Don’t look directly at it
Project onto a screen
Equation for calculating & during young’s double slit exp
& = ay/d
a = slit separation (0.1-0.5mm)
y = interference fringe separation (distance between 2 light patches on screen)
d = distance from double slits to screen (1-4m)
What is diffraction
Spreading of waves when they pass through an aperture/round an obstacle
What 2 conditions make diffraction more profound
& is bigger
Gap width is narrower
When is diffraction at its greatest
& = gap width
What is a diffraction grating
Glass /Perspex material with many parallel equally spaced opaque lines
Describe the use of a diffraction grating and laser to measure & (recall equation)
Set laser behind grating
Turn on laser so diffraction pattern is projected on to screen
Use n& = d sin(theta) to calculate & where;
n = number of orders
*d= slit separation/m (=1/number of slits per m)
Theta = angle from 0 order (middle) to order in question
What should you remember to do when calculating n in the diffraction grating equation (2 things)
Take theta to be 90
Round down
When drawing the single slit diffraction pattern remember…(2)
The centre is by far the largest and is 2x wider than the others
Each outer fringe is the same width
For d in the diffraction grating equation, what do you do if you’re given the no. of slits in 1m
1/ number of slits per meter
Path difference
Difference in path distance travelled by 2 waves which meet at the same point in terms of &
What does collimated mean
Light rays are parallel
What is the time base setting of a CRO
Time /cm or division
What does CRO stand for
Cathode ray oscilloscope
Coherent
Same wavelength, frequency and constant phase difference
