Waves and Optics Flashcards
Effect of using laser with shorter wavelength in Young’s Double Slit Experiment
- quote formula
- fringe separation is proportional to wavelength
- shorter wavelength means spacing between maxima decreases
- as path difference is smaller
Monochromatic
single wavelength
Coherent
- same frequency/wavelength
- constant phase difference
- NOT IN PHASE
Effect of narrower slit in Young’s Double Slit Experiment
- wider fringe separation
- lower intensity
Pattern on screen for white light in Young’s Double Slit Experiment
- distinct fringes shown with subsidiary maxima
- entral fringe white
- blue on inner edge
- red on outer edge
Describe and explain stationary wave experiment
- waves reflected
- pass through each other in opposite directions
- have same wavelength/frequency + similar amplitude - superposition of waves results in cancellation at nodes
- reinforcement at antinodes
- energy is not transferred along string
- nodes have no displacement/zero amplitude
- antinodes have maximum amplitude
- between node and antinode amplitude increases
- particles with an odd no. of nodes between them are 180 degrees out of phase
- particles with an even no. of nodes between them are in phase
Explain why amplitude of waves is reduced during transmission
- absorbed by medium
- scattering
- reflected at different angles by medium
Describe and explain Young’s double slit experiment
- measure w (fringe separation) using vernier calliper
- measure D (distance between laser and screen) using metre rule
- ensuring large enough value for D, e.g. 2m
- use fringe separation formula to calculate wavelength
- measure across several fringes from CENTRE
- carried out in dark room (for clearer observation)
- repeats
Young’s Fringes white light vs red laser light
- different colours vs monochromatic
- less intense vs more intense
- maxima closer together for white compared to red laser
- fringes for white compared to dots for laser
Describe diffraction grating experiment and suggest ways to reduce uncertainties
- measure angle of first order beam using a protractor
or tan(angle)= w/D as it is easier to measure fringe separation with vernier calliper - d = 1/lines per mm
- measure angle for more than one order (NOT AVERAGE ANGLE but average wavelength)
- repeat for different/larger distances from screen
- protractor with 0.5 degrees intervals or less
- plot n against sin(angle) where gradient =d/wavelength
Describe motion of particles on standing wave (not including at nodes)
- oscillate continuously up and down
- about equilibrium position
Explain importance of correctly aligning aerial of a TV to ensure strongest signal
- transmitted radio waves are often polarised
- aerials must be aligned in same plane of polarisation on waves
Applications of diffraction gratings
- spectrometers (analysis of gas compositions)
- analysis of chemical composition of stars (produces light spectrum)
Purpose of narrow single slit in Young’s Double Slit Experiment
- narrow slit gives wide diffraction
- ensures both slits are illuminated
- provides coherent source of light (same wavelength and constant phase difference)
Describe how to obtain an accurate value for wavelength in Young’s Double Slit Experiment
- increase D
- as this increases w
- measure across more than 2 maxima
Describe how to determine the maximum number of orders for a transmission diffraction grating
- substitute 90 degree into n x wavelength=dsin(angle)
- n=d/wavelength
Transverse
Waves with a direction of oscillation perpendicular to direction of propagation
Longitudinal
Waves with a direction of oscillation parallel to the direction of propagation
Polarised vs unpolarised light
Photons oscillate in one plane only vs all planes
Describe how to calculate width of central fringe in Young’s Double Slit Experiment
- 2 x wavelength x D / a
- D (distance between slit and screen)
- a (slit width)
Waves
Periodic disturbances in a material or space
Mechanical waves
Waves which travel through a physical medium by means of vibrations
EM waves
A photon consisting of electric and magnetic waves in phase travelling at right angles
Polarisation
When transverse waves are made to oscillate in one plane only after travelling through a polaroid filter
Displacement
Distance and direction of oscillating particle from its equilibrium
Amplitude
Maximum displacement from equilibrium position
Wavelength
Least distance between two successive oscillating particles that are in phase
Period
Time for one complete wave to pass a point
Frequency
Number of complete waves passing a point each second
Phase difference
Fraction of a cycle between two oscillating particles measured in radians or degrees
Wavefront
Lines of constant phase eg crests
Ways to increase diffraction
- narrower gap
- longer wavelength
Superposition
Effect of adding together two waves together - total displacement equal sum of individual displacements
Interference
Formation of points of cancellation and reinforcement where two coherent waves meet
Explain why intensity of fringes decreases further away from central fringe
- intensity is inversely proportional to distance between slits and screen
- light travels longer distance further away from central fringe
Suggest why you would use monochromatic light in an optical fibre
- prevent spectral dispersion
- causing signals to merge
- since different wavelengths at different speeds
Stationary Waves
- waves produced when two or more waves
- similar amplitude and same frequency/wavelength
- pass through each other in opposite directions
Progressive Waves
Waves which travel through a substance or space
Compare the nature of stationary and progressive waves
Stationary
- all particles except those at nodes vibrate at same frequency
- amplitudes varies from zero to maximum at antinodes
Progressive
- all particles vibrate at same frequency
- amplitude is constant
Suggest why waves that produce a standing wave do not cancel each other out completely and how complete cancellation could be achieved
- not always 180 degrees out of phase
- travel in opposite directions in standing wave
- cancellation only at nodes
- if waves travelled in the same direction with a phase difference of 180 degrees they would cancel out
Suggest how the amplitude of the wave that produces a standing wave compares to amplitude at antinode
Amplitude at antinode is double original amplitude since complete superposition occurs
Suggest how the properties of an EM wave change when it travels through a denser medium
- frequency constant always
- wavelength and speed change
Explain how fringes are formed in Young’s Double Slit Experiment
- slits act as coherent sources of light
- waves diffract at slits
- waves overlap and interfere
- bright patches = reinforcement when the path diff is a whole number of wavelengths
- dark patches = cancellation when the path diff is an odd number of half wavelengths
Differences between transverse and longitudinal waves
- difference between direction of oscillation and propagation
- longitudinal all require a medium to travel through
- only transverse can be polarised
Suggest why longitudinal waves cannot be polarised
Particles oscillate in the same direction as wave propagates
Examples of transverse and longitudinal waves
Transverse - EM waves - waves on string - water waves - secondary seismic waves Longitudinal - sound waves - primary seismic waves
Suggest why TIR does not take place when light travels from water (n=1.33) to glass (n=1.47)
TIR only takes place when light travels from a material with higher to lower refractive index
Suggest why cladding is used in optical fibres
- lower refractive index (step index) so ensures TIR reduces refraction out of core (signal loss)
- prevents scratching of core / breakage
- prevents crossover of signals to other fibres
- increases tensile strength since core needs to be thin
Suggest why stationary waves only occur at specific frequencies
- at frequencies other than a harmonic frequency
- interference of reflected and incident waves
- results in irregular and non-repeating disturbance in medium
Node
no displacement/zero amplitude
Antinode
maximum amplitude
Path Difference
difference in distance travelled by two coherent waves from two different sources at a particular point (given in metres or in terms of wavelength)
Diffraction
spreading of waves on passing through a gap or near an edge
Describe the relationship between width of central fringe and slit width/distance between slit and screen in Young’s Double Slit Experiment
- directly proportional to distance between slit and screen
- inversely proportional to slit width
Explain why light transmitted by diffraction grating is in certain directions only
- light passing through slit is diffracted
- diffracted waves from adjacent slit reinforce each other in certain directions only
- cancel out in all other directions
Suggest why diffraction grating is useful in separating colours in incident light
- different wavelengths diffracted at different angles
- according to n x wavelength=dsin(angle)
Suggest how to calculate grating spacing given lines per mm
d = 1/lines per mm
convert to m from mm
Explain the derivation of diffraction grating formula given by n x wavelength=dsin(angle)
- wavefront emerging from slit P reinforces wavefront from adjacent slit Q
- earlier wavefront from Q must have travelled n wavelengths from slit
- hence distance QY from slit to wavefront is n x wavelength
- d is distance PQ
- angle between wavefront and plane of slits given by sin(angle)=QY/QP
- hence sin(angle) = (n x wavelength) / d
Approximate refraction index of air
1
Harmonics
Integer (whole number) multiples of fundamental frequency
fn = nf, where n is harmonic number and f is fundamental frequency (wavelength/2)
Fundamental Frequency
- lowest frequency of periodic waveform
- longest wavelength
- wavelength/2
Conditions for total internal reflection
- refractive index of first medium is greater than refractive index of second medium
- angle of incidence is greater than critical angle
Modal/Multipath Dispersion
- lengthening of light signal pulse as it travels along optical fibre
- light enters optical fibre at different angles so different TIR
- rays that repeatedly undergo total internal reflection having to travel longer distances compared to those that undergo fewer
=pulse broadening
Material Dispersion
- lengthening of a light signal as it travels along an optical fibre
- monochromatic light contains a continuum of wavelengths within a small range
- wavelengths with a higher refractive index are totally internally reflected more so pulse take longer to travel down fibre
= pulse broadening
Pulse broadening
- spreading of light pulses as they travel down optical fibres
- merging of signals
- poor quality signal/signal loss
- slower transmission
Suggest why a narrow core is used in optical fibres
- increased probability of TIR so less light lost by refraction out of core
- reduces number of waves entering fibre at high angle of incidence to reduce multipath dispersion
- waves totally internally reflected less so less energy loss
- signals straighter and shorter so faster transmission
Order of electromagnetic wave spectrum
real monkeys insist very useful xmas gifts
radio microwave infrared visible ultraviolet x-ray gamma
Describe how to draw refracted ray when refractive index of both mediums is known
- decrease in refractive index means refract away from normal
- increase in refractive index means refract towards normal
Uses of optical fibres
- endoscopes in medical diagnosis
- communication for faster transmission
Describe how to draw diagram when incident ray between two mediums equals critical angle
- light ray travels across boundary
- weak internally reflected ray (dashed)
Suggest safety measures taken when using lasers
- avoid shining at a person
- LASER safety goggles
- avoid reflections
