principle of superposition
when 2 or more waves of the same type meet at a point at the same time, the displacement of the resultant wave is the vector sum of displacements of individual waves at that point at that time.
interference
specific result of the superposition of coherent waves, where the combination produces a pattern of alternating constructive and destructive regions
coherence, for 2 coherent waves what must be the same
property of 2 waves with constant phase difference, frequencies of 2 coherent waves must be the same
path difference
difference between distance travelled by 2 waves meeting at a point
(single slit)diffraction
spreading of waves through an aperture or bending of wave around an obstacle
what characteristics of the waves change and remain unchanged after diffraction
changed: velocity and intensity of waves
unchanged: wavelength and speed of waves
what is extent of diffraction dependent on, and when is diffraction significant
dependent on the relative sizes of the aperture or obstacle to the wavelength of the wave
diffraction significant only if wavelength of wave is of similar order to aperture or obstacle it is passing through
conditions for 2 source interference fringes to be observable in an experiment
sources must be coherent
sources must have approximately the same amplitude
For transverse waves, sources must be unpolarised or polarised in the same plane
For electromagnetic waves, the wavelengths used should be in the visible range (400nm - 700nm)
2 differences between double slit interference and multiple-slit interference
centre to centre separation between adjacent maxima or minima:
double slit: constant, multiple slit: not constant, unless angle of diffraction is so small that small angle approximation can be used
number of maxima (for a monochromatic wave): double slit: infinte, multiple slit: 2n+1
rayleigh criterion
for 2 diffraction patterns to be just distinguishable, the central maximum of one diffraction pattern must lie on the first minimum of the other diffraction pattern
why is the use of diffraction grating to measure wavelength more precise than 2-slit system
as no. of slits increase, each additional slit provides an additinal point source of light for constructive interference and intensity of principal maximum increases. since a diffraction grating produces maxima that are much sharper than a 2-slit system, more precise measurement.
