ConceptOptics

Physical / wave optics

light as an EM wave (v=f·λ, frequency source-set and unchanged across a boundary), two-source interference conditions (coherence; double-slit, thin-film/oil-film), diffraction (noticeable when aperture ≤ wavelength), distinguishing interference vs diffraction vs polarization (e.g. 3D glasses)

14 ways this goes wrong. Each one is a named misconception the questions are built to catch.

Bright-fringe condition wrongUses a half-wavelength path difference for bright fringes (or a whole-wavelength one for dark), swapping the maxima and minima conditions.→
Coherence requirement ignoredExpects a stable interference pattern from two independent ordinary light bulbs, ignoring that the sources must be coherent.→
Diffraction condition reversedPredicts more spreading as the slit is made much wider than the wavelength, reversing the aperture-vs-wavelength condition.→
Diffraction needs large gapsThinks noticeable diffraction occurs through wide openings, rather than when the aperture is comparable to or smaller than the wavelength.→
Fringe spacing vs slit separation reversedTreats double-slit fringe spacing as proportional to the slit separation d rather than inversely, ignoring w=λ·L/d.→
Interference and diffraction confusedAttributes a two-source fringe pattern to diffraction, or a single-slit spreading effect to two-source interference.→
Interference creates/destroys energyBelieves destructive interference destroys light energy rather than redistributing it into the bright fringes.→
Light frequency changes across boundaryBelieves the frequency of a light wave changes when it enters a new medium, when only speed and wavelength change while f is set by the source.→
Path difference not in wavelengthsCompares the path difference to a raw distance rather than to whole or half multiples of the wavelength.→
Polarization blamed on interferenceExplains 3D-glasses or polaroid effects as interference rather than as polarization of a transverse wave.→
Shorter wavelength widens fringesBelieves blue (shorter-wavelength) light produces wider fringes than red, reversing the wavelength dependence of fringe spacing.→
Sound can be polarizedClaims a longitudinal wave like sound can be polarized, when only transverse waves such as light can be.→
Thin-film phase change ignoredApplies the bare path-difference condition to oil-film or thin-film colors without accounting for the half-wave phase change on reflection.→
v=f·λ misusedRearranges v=f·λ incorrectly, e.g. λ=v·f, or treats frequency as inversely proportional to speed for light.→