Concept · Optics

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)

Switches you to Physics

Named traps

14 ways this goes wrong.

Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.

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.→

Key formulas

No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.

Shaky underneath?

If the concept underneath still feels loose, patch it here before coming back to CSCA questions.

FoundationsPhysics 2 breadth