M4 Waves - Part 2 Flashcards
superposition, young's slits, path difference, diffraction gratings
What is the principle of superposition of waves
when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements
what is constructive interference
when 2 waves meet, if their displacements are in the same direction, they combine to give a bigger displacement
the points are known as maxima
what is destructive interference
if a wave with a positive displacement meets a wave with a negative displacement, they cancel each other out
when it’s equal and opposite displacements they completely cancel out (total destructive interference)
the points are known as minima
what is coherence
waves are emitted from 2 sources with a constant phase difference
what does it mean if waves are coherent
a constant phase difference/relationship
same frequency and wavelength
what do the waves have to be for interference occur
when two waves are coherent
what are examples of interference
water waves sound waves ( 2 speakers connected to same signal generator)
what is the equation for path difference in constructive interference
path difference =nλ
what is the equation for path difference in destructive interference
path difference =(n+0.5)λ
in interference, what is it called when the path difference is 0
central maxima
in interference, when there is phase difference of 2π, what is the path difference
1 wavelength
What is observed in Young’s Slits Experiment
Light from a single source directed to a double slit arrangement
Where the light from these slits overlaps, interference occurs and a pattern of bright and dark fringes is observed
In young slits experiment, why are the two light sources derived from a single laser
To ensure the two interfering sources of light are coherent
In Young’s slits experiment, how do we ensure that maximum diffraction occurs at the slits
Make the size of the slit in the same order of magnitude as the wavelength of the wave
Light is 10^-7 m
In Young’s slits experiment, explain why there is a bright fringe at the centre of the pattern
The light waves are in phase with each other as they leave the source
The waves travel the same distance so constructive interference occurs and a bright fringe results
In the equation
λ = ax / D
What does it all mean
In Young’s double slit experiment
Wavelength = ( separation of the slits X fringe separation) / slit to screen distance
In Young’s double slit experiment, what happens to the fringe spacing as
A) wavelength increases
B) separation between slits increases
C) distance between slit and screen increases
A) increases
B) decreases
C) increases
How does the fringe pattern change as you cover the first slit
And interference pattern is formed
How does the fringe pattern change if there was red light at the first slit and blue light at the second slit
The frequencies aren’t the same and therefore the sources aren’t coherent as wavelength is different
Therefore no interference pattern occurs
What is the equation to find the critical angle from one substance to the next
sinC = n2 / n1
N2 is the less dense substance
N1 is the more dense substance
what is diffraction
when waves spread around obstacles or through gaps
what do you observe when a light from a laser is directed to a single slit of suitable width
a bright central band that extended well into the shadow region of the slit - central maximum
the bright band is bordered by regions of darkness - minima
moving out past the dark regions, the bright regions reduced in intensity - subsidiary maxima
describe interference with sound waves
there is a loud sound at the centre of the interference pattern because the waves are in phase with each other
they travel exactly the same distance to the point at the pattern’s centre and so are still in phase with each other
constructive interference occurs so a loud sound is detected
explain the meaning of interference
when 2 (or more) waves meet and there is a change in overall displacement
what does the equation n=c/v mean
refractive index of the material = speed of light / speed of light in the material
what’s the refractive index of air
1
what’s snell’s law
apply it to nsin(θ) = constant
the angle that incoming light makes to the normal is called the angle of incidence
the angle the refracted ray makes with the normal is the angle of refraction
so
n1sin(θ1) = n2sin(θ2)
what vital evidence did Young’s double slit experiment provide about diffraction and interference?
light could both diffract (through two narrow slits) and interfere ( to form the interference pattern on the screen)
define path difference
the amount by which the path travelled by one wave is longer than the path travelled by the other wave
Two coherent sources of sound waves each with a wavelength of 1.5m are set up so that they produce interference fringes. When the path difference is 3.75m, would you expect constructive or destructive interference?
find out how many times the wavelength goes into the path difference:
3.75 / 1.5 = 2.5
so the path difference is 2.5 X λ so the interference is destructive
how can you demonstrate the diffraction of light
shining a monochromatic, coherent light through a very narrow slit onto a screen
define monochromatic light
light of a single wavelength (so a single colour)
if you narrow the slit, what happens to the diffraction pattern
becomes wider
define diffraction grating
lots of equally spaced slits very close together
what happens when a monochromatic light passes through a diffraction grating? why?
the interference pattern is really sharp because there are so many beams reinforcing the pattern
in a diffraction grating pattern, what is the line of maximum brightness called
what are the lines called next to it
zero order line
the lines either side are called first order lines, then second order lines and so on
how can you calculate the wavelength of light (using a diffraction grating)
- pass the light through the grating
- use the zero order and first order lines
- measure the distance between the lines and the distance from the grating to the screen
- work out the angle between them by using tan^-1(x / D)
- work out the spacing between the slits in the diffraction grating
- use the equation dsin(angle) = n X wavelength (n=1 and d is the slit spacing)
in diffraction gratings what does it mean if:
a) the wavelength increases
b) split spacing increases
a) wavelength increasing means the angle gets bigger so the pattern will spread out
b) d increasing means the angle is smaller so the pattern comes closer together
what happens when you diffract white light
zero order maximum is white
all the other orders form spectrums with red on the outside and violet on the inside
what other factor do coherent waves have to have for interference to be noticeable during an experiment
the waves have to have approximately the same amplitude
