Refraction and diffraction Flashcards
Define monochromatic
Single wavelength/frequency
Wave fronts are
lines of constant phase which are perpendicular to the direction of wave travel
Describe how diffraction pattern changes with longer wavelength
maxima further apart
Why wasn’t Young’s theory originally accepted
Because Newtons ideas were accepted due to him being respected but repetitive experiments eventually disproved his theories
State and explain one advantage associated with a smaller diameter core
Less light is lost so better quality signal
Less refraction out of core improved data transfer
State two requirements for two light sources to be coherent
Same wavelength
Constant phase difference
Define dispersive medium
One which the speed of the wave depend so its frequency
Why do the double slits emit wave fronts with a constant phase difference (coherent)
Each wave crest or wave trough from the single slit always passes through one of the double slits a fixed time after it passes through the other slit. The double slits therefore emit wave-fronts with a constant phase difference.
Why does a glass prism cause white light to disperse
White light is composed of light with a continuous range of wavelengths and so the glass prism refracts light by different amounts depending on its wavelength. The shorter the wavelength in air, the greater the amount of refraction so each colour is refracted by a different amount. The dispersive effect occurs because the speed of light in glass depends on wavelength (violet light travels more slowly than red light in glass so the refractive index of violet light is greater than the refractive index)
To measure the fringe separation w…
measure across several fringes from the centre of a dark fringe to the centre of another dark fringe as they are easier to locate compared to bright fringes and divide by the number of measured fringers
Define coherent
Waves have constant phase difference
Diffracted waves spread out more if
the gap is made narrower or the wavelength is made larger
Define coherent
constant phase difference and the same frequency
Fringe separation is the
distance from the centre of a bright fringe to the centre of the next bright fringe
Peak intensity of diffraction… related to
temperature
Critical angle equation
At critical angle, angle of refraction is 90 so sinic = n2/n1
Equation linking refractive index’s to speeds
n2/n1 (where n1 =1 as vacuum or air)
= c1/c2
The maximum number of orders is given by
the value d/lambda rounded down to the nearest whole number
1nm =
10^-9m
When light enters a prism North east from bottom left where does refracted ray finish
forms volcano shape and normal’s form upside down triangle
Explain why optical fibres used for communications need to have cladding.
Keeps signals secure, maintains quality, prevents breakages
No refraction occurs when
incident light ray is along the normal
fringe seperation condition
w<
Uses of optical fibres
endoscopes and communications (carry light signals)
Name the 2 parts of optical fibre
central core with small diameter with cladding on the outside
The number of slits per metre in the grating, N =
1/d (grating spacing)
What is monochromatic light
light of a single wavelength
State one application of optical fibres and explain how this has benefited society
Endoscope - improve medical diagnosis
Communications - improve data transmission
Fringe pattern of white light
the central fringe is white because every colour contributes at the centre of the pattern. The inner fringes are tinged with blue n the inner side and red on the outer side as red fringes are more spaced out than blue fringes and the two patterns don’t overlap exactly. The outer fringes merge into an indistinct background of white light becoming fainter with increasing distance from the centre. This is because, where the fringes merge, different colours reinforce and therefore overlap.
When does total internal reflection occur Total internal reflection occurs
if the incident substance has a larger refractive index than the other substance and the angle of incidence exceeds the critical angle.
if the angle of incidence exceeds the critical angle
and
goes from more refractive index to less refractive index
Explain spectral/chromatic dispersion of white light
Different wavelengths Refract differently So travel at different speeds Duration of travel lengthened Peak intensity reduced Come out of fibre at different times Weakening signal
Light from a star is passed through a grating
Explain how the appearance of the first-order beam can be used to deduce one piece of information about the gases that make up the outer layers of the star.
Dark lines reveal the composition of the light (star’s atmosphere)
The peak of intensity is related to the temprature
State and explain two physical properties of the light produced by a laser which makes
it different from the light produced by a filament lamp.
Monochromatic - waves of single frequency/wavelength
Collimated - produces an approximately parallel beam
Coherent - waves produced are in constant phase
Polarised - vibrations in single plane
If the single slit pattern is observed: using different sources of monochromatic light in turn, the observations show that…
the greater the wavelength the wider the fringes
State and explain what happens to the value of angle θ for the first order beam if the
wavelength of the monochromatic light decreases.
Angle gets smaller because path difference gets smaller as d is constant
What happens to fringe width as slit space decreases and/or wavelength increases
Increases
Explain how the glass cladding around the optical fibre’s core improves the security
of data being transmitted through it and give a reason why this is important
Light does not enter the cladding so cannot pass across from one fibre to a neighbouring fibre. Personal data must be transmitted along fibres from which there is no danger of leakage of light resulting in a breach of security
The width of the central diffraction fringe, W =
2lambdaD(slit-screen distance)/a
The fringe spacing of the interference fringes, w =
lambda * D/s
For interference to occur source must be
Coherent
Wavelength > gap Wavelength < gap Wavelength = gap
Large diffraction (lower amplitude as waves more spread out) Small diffraction Maximum diffraction
A laser is replaced by another laser emitting visible light with a shorter wavelength.
State and explain how this will affect the spacing of the maxima on the screen.
Maxima closer together because:
w=lambda*D/s
D and s are constant and so w, fringe spacing, is proportional to the wavelength therefore as the wavelength decreases the fringe spacing decreases
Constructive inteference means
Constant phase
Peak or trough meets peak or trough
The medical endoscope contains
two bundles of fibres (must be coherent bundle so fibre ends at each end in same relative position)
Function of cladding
to prevent light from crossing from one fibre to another which would cause the signals to not be secure
The angle between the incident ray and the plane mirror is
equal to the angle between the reflected ray and the mirror
How are refractive index and speed related
As refractive index increases, density increases so the speed of light decreases
Where a bright fringe is formed…
the light from one slit reinforces the light from the other slit - the light waves from each slit arrive in phase with each other
A laser is a _____ source of light
A non-laser is a _____ source of light
coherent
incoherent
Structure of optical fibre
Each fibre consists of a very narrow core surrounded by a layer of cladding of lower refractive index
Fringes become more widely spaced if
the distance D from the slits to the screen is increased
the wavelength of the light used is increased
the slit spacing is reduced
The longer the wavelength of the light used the _____ the fringe separation
greater
Snells law =
n1sini = n2sinr
Diffraction is the
spreading of waves when they pass through a gap or by an edge
Continuous spectra
the spectrum of light from a filament lamp is a continuous spectrum of colour from deep violet to red. Intensity depends on temperature. The hotter the light source, the shorter the wavelength of the brightest part of the spectrum. By measuring the wavelength of the brightest part of a continuous spectrum we can measure the temperature of the light source
Laser light is ______ so can…
monochromatic so can specify wavelength accurately
Function of cladding
to prevent light from crossing from one fibre to another which would cause the signals to not be secure
Refractive index of air =
1
The pattern from double slit diffraction may be represented as a graph of intensity against position on
the screen. What does the graph look like
Highest maxima at 0
Maxima of similar intensity to central maximum but decreasing in intensity away from centre
All fringes same width as central fringe
Why are fringes formed
due to interference of light
The central bright fringe of the single slit pattern is _____ as wide as the others and
twice
much brighter than the rest
Define refraction
The change of direction that occurs when light passes non-normally from one medium into another changes speed
Young’s fringes are produced on the screen from the monochromatic source
Explain how when a monochromatic source of light passes through a single slit and then a double slit it produces interference fringes on the screen. In your
answer, explain why slit S should be narrow and why slits S1 and S2 act as coherent sources.
Narrow single slit gives wide diffraction
To ensure that both S1 and S2 are illuminated
Slit S acts as a point of source
Narrow single slit ensures it provides coherent sources of light at double slits
Double slits are illuminated by same source giving same wavelength
Paths to double slits are of constant length giving constant phase difference
Light is diffracted as it passes through double slits and the diffracted waves overlap and interfere
Where the path lengths from s1 and s2 to the screen differ by whole numbers, n of wavelengths, constructive interference occurs producing a bright fringe on the screen
Where the path lengths differ by (n+0.5) wavelengths, destructive interference occurs producing a dark fringe on the screen
Where a dark fringe is formed…
the light from one slit cancels the light from the other slit - the light waves from the two slits arrive 180 degrees out of phase
Light ray bends _____ the normal if it passes into a ______ refractive substance
bends towards normal if passes into more refractive
bends away from normal if passes into less refractive
The smaller the speed of light is in a substance…
the greater is the refractive index of the substance
Explain why the final pulse has a lower amplitude and is longer than the initial pulse when travelling across an optical fibre
reduced amplitude due to energy loss within the fibre
Pulse broadening caused by multipath dispersion
Why must the core of an optical fibre be very narrow
To prevent multipath/spectral dispersion and prevent pulses from merging (can also occur if white light is used rather than monochromatic light)
For reinforcement at a point on the screen, the path difference =
Line from slit 1 to point on screen - line from slit 2 to same point on screen = lambda*m where m is a whole number
Therefore light emitted simultaneously from each slit arrives in phase at the point on the screen if reinforcement occurs
When angle of incidence=critical angle
one ray refracted along boundary another reflected
Why does multipath dispersion occur
A signal pulse will become distorted due to the fact that not all paths through the fibre are the same length which affects the time travelled thus broadening the wave and reduce peak intensity
1 m in mm
1m in nm
x10^3
x10^9
vertical screen is placed several metres beyond a vertical double slit arrangement
illuminated by a laser. The diagram below shows a full-size tracing of the pattern of spots
obtained on this screen. The black patches represent red light whilst the spaces between them
are dark.
(a) Using the wave theory, explain how the pattern of bright and dark patches is formed.
You may be awarded marks for the quality of written communication provided in your
answer.
slits act as coherent sources
light diffract at slits
waves superpose
bright patches: constructive interference so waves in phase and reinforce each other
dark patches:
destructive interference so waves out of phase and cancel each other
Why are vapour lamp discharge tubes a monochromatic light source
produces light with a dominant colour
For diffraction to occur what must be true
each slit must be narrow enough for sufficient diffraction; the slits must be close enough so the diffracted waves overlap.
Why is no light energy lost in optical fibres
Because angle of incidence > than critical angle so total internal reflection occurs
Diffraction grating applications
used in spectrometer to study the spectrum of light from any light source and measure wavelengths accurately
Fringe separation, w =
lamdaD/s
if w «_space;D
D = distance from the slits to the screen
s = distance between the centres of the slits
lambda = wavelength of light
For a given order and wavelength, the smaller the value of d
the greater the angle of diffraction (the larger the number of slits per metre the bigger the angle of diffraction)
When angle of incidence>critical angle
total internal reflection so completely reflected only
Angle of refraction of emerging light = angle of incidence of light ray entering block when
two sides of block at which refraction occurs are parallel to each other
To find the maximum number of orders produced
substitute X=90 and calculate n using n=d/lambda
A diffraction grating consists of…
a plate with many closely spaced parallel slits ruled on it
Just over two hundred years ago Thomas Young demonstrated the interference of light by
illuminating two closely spaced narrow slits with light from a single light source.
What did this suggest to Young about the nature of light?
Wave nature of light
Describe the necessary procedure in order to obtain an accurate and reliable value
for the wavelength of the laser light.
Your answer should include details of all the measurements and necessary
calculations
nlambda=dsinX
(for single slit, d=width of slit)
Measure angle between more than one order beam by tanX = x/D
Repeat for different distances
Use large distance to screen
d=1/lines per mm
State two ways in which the appearance of the fringes would change if the slit was made narrower.
wider spread and lower intensity
If the slits are very far apart then what happens and why
no interference pattern is observed because diffracted light doesn’t overlap
State how speed, frequency and wavelength changes in refraction from air to glass
c = decreases f = constant lambda = decreases
Diffraction of light by a single slit can be demonstrated by directing a parallel beam of light at the slit. The diffracted light forms a pattern that can be observed on a white screen. The pattern shows…
a central fringe with further fringes either side of the central fringe. The intensity of the fringes is greatest at the centre of the central fringe. At the point directly in front of the laser the intensity is at its highest.
dsinX =
Where d is slit spacing
n*lambda
At nth order maxima
Derivation
Fringe spacing is proportional to
wavelength/single slit width
lambda/s
Explain why the total internal reflection will not occur when the ray travels from water to glass.
only when ray travels from higher n to lower n or (water to glass) is
lower n to higher n
Line absorption spectra
A line absorption spectrum is a continuous spectrum with narrow dark lines at certain wavelengths e.g. if the spectrum of light from a filament lamp is observed after passing it through a glowing gas, thin dark vertical lines are observed superimposed on the continuous spectrum. The part of the dark lines is due to the elements in the glowing gas. These elements absorb light of the same wavelengths they can emit at so the transmitted light is missing these wavelengths. The atoms of the glowing gas that absorb light then emit the light subsequently.
Name the part of the optical fibre outside of the core
cladding
What changes and doesn’t change in refraction
speed
wavelength
not frequency
A single slit diffraction pattern is produced on a screen using a laser. Sketch how the intensity varies across the screen to the right of the central maximum.
One large maxima with peak at 0
2nd maxima in between first and 2nd minima
etc..
each maxima is lower done
When demonstrating double slit interference what needs to happen to a laser or non-laser
Laser is source of coherent light so doesn’t need to pass through narrow single slit like a non-laser when demonstrating double slit interference
As slit number increases what happens
Fringe width decreases and intensity of maxima decreases
In double slit diffraction light passes through the two slits and produces
an interference pattern
How is multipath disperson prevented
Fibres kept thin and within maximum useable length
Too much signal disperson results in
Pulses merging and becoming unreadable
The angle of diffraction between each transmitted beam and the central beam increases if
light of a longer wavelength is used, or if a grating with closer slits is used.
What has narrower fringes blue light or red light
blue light because has a smaller wavelength
Line emission spectra
A glowing gas in a vapour lamp emits light at specific wavelengths so its spectrum consists of narrow vertical lines of different colours. The wavelengths of the lines are characteristic of the chemical element that produced the light. If a glowing gas contains more than one element, the elements in the gas can be identified by observing its line spectrum.
For cancellation at a point on the screen, the path difference =
Line from slit 1 to point on screen - line from slit 2 to same point on screen = lambda*(m+0,5) where m is a whole number
Therefore light emitted simultaneously from each slit arrives out of phase by 180 degrees at the point of the screen if cancellation occurs
w, fringe spacing =
Lambda*D/s
D= distance between slits and screen
s = slit spacing
Why cant light from light bulbs form interference patterns
Light from the nearby lamps bulbs could not form an interference pattern because not coherent so the two light sources emit light waves at random. The points of cancellation and reinforcement would change at random so no interference pattern is possible
Slit density units for diffraction grating
xlines/mm
Path difference =
distance between where each slit reaches a point on the screen
State and explain one reason why part B of the optical fibre is made as narrow as
possible
To reduce multipath dispersion which would cause light travelling at difference angles to arrive at different times
To prevent data loss which would cause a weaker signal
Why are the double slits described to be coherent sources and how
The double slits are described as coherent sources because they emit light waves with a constant phase difference provided we illuminate the double slits with laser light, or light from a narrow single slit, if we are using non-laser light.
If the single slit pattern is observed: using an adjustable slit, the observations show that…
making the slit narrower makes the fringes wider
State one safety precaution when using a laser
Wear safety goggles and don’t look directly into laser
For constructive interference path difference =
nlamda = S1A - S2A
