ConceptOptics
← All conceptsGeometric optics — reflection & refraction
law of reflection (θi=θr), Snell's law n1 sin(θ1)=n2 sin(θ2), refractive index n=c/v (higher n = slower light = denser), plane-mirror imaging
12 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Angle measured from surfaceMeasures the angle of incidence or reflection from the mirror surface rather than from the normal, so the two angles do not appear equal.→
- Bending direction reversedBends light toward the normal when it enters a less dense medium, or away from the normal entering a denser one.→
- Light speeds up in denser mediumAssumes light travels faster in the optically denser (higher-n) medium than in the rarer one.→
- Mirror image distance wrongPlaces the plane-mirror image at a distance behind the mirror different from the object's distance in front of it.→
- Plane mirror magnifies imageClaims a plane mirror enlarges or shrinks the image instead of producing one the same size as the object.→
- Plane-mirror image is realLabels the image in a plane mirror as real rather than virtual.→
- Refraction changes frequencySays the frequency of light changes on crossing a boundary, attributing the bending to a frequency change instead of a speed and wavelength change.→
- Snell incident/refracted sides swappedPairs n1 with the refracted-side angle and n2 with the incident-side angle, swapping which medium goes with which angle.→
- Snell's law uses cosPlugs cosine of the angles into n1 sin(θ1)=n2 sin(θ2) instead of sine.→
- Speed unchanged on refractionAssumes the light keeps the same speed in both media and bends only because the surface is tilted.→
- Wavelength unchanged across boundaryKeeps the wavelength fixed when light enters a new medium, changing only the speed and leaving λ=v/f inconsistent.→
- n vs speed relation invertedTreats a higher refractive index as meaning faster light rather than slower, ignoring n=c/v.→
