Concept · Optics

Geometric optics — reflection & refraction

law of reflection (θ_i=θ_r), Snell's law n1 sin(θ₁)=n2 sin(θ₂), refractive index n=c/v (higher n = slower light = denser), plane-mirror imaging

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Named traps

12 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.

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.→

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