Light

Traps in Light

• Lens/mirror formula slips
  1/f = 1/u + 1/v (real-is-positive). Work with reciprocals, then invert at the end. Magnification m = v/u.
• Forgets to invert the reciprocal at the end
  With 1/R = 1/R₁ + 1/R₂ (or 1/v), the sum gives 1/R. Finish with R = 1 ÷ (sum). Check: parallel R must be smaller than the smallest resistor.
• Uses the wrong arithmetic operation
  Write the formula symbolically and substitute only at the end. Check units: if the units of your answer are wrong, so is the operation.
• Inverts a formula
  Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
• Wrong trig function for critical angle
  At the critical angle the refracted ray grazes the surface (r = 90°). Snell's law gives n sin c = sin 90° = 1, so sin c = 1/n.
• Adds an unjustified step (halving)
  Follow the formula exactly; there are no extra factors. sin c = 1/n gives c directly.
• Uses everyday intuition instead of physics
  Name the law that applies (Newton's first law, Archimedes' principle, kinetic theory) and reason from it. If the option only 'feels' right, test it against the law.
• Mixes up a pair of similar physics terms
  Make a two-column table for each pair with one defining feature and an example. Underline the defining word in the stem before you look at the options.
• Reverses a relationship or direction
  Write the relationship with an arrow or proportionality sign first (E ∝ f; R ∝ L; cooling a metal → R falls). Then check the option follows the same direction.
• Picks a related law, device or effect with a different job
  Learn laws and devices as 'name → what it tells you' cards. Contrast neighbours: left-hand rule = motor force, right-hand rule = dynamo current, grip rule = field around a wire.
• Basic optics misconceptions
  Reflection: angle i = angle r. Refraction: bending when speed changes between media. Dispersion: splitting of white light into colours (prism). Total internal reflection needs dense → less dense and i > c.
• Over-generalises a rule
  Every rule has conditions. Before choosing, ask which waves, which media, which angles or which components the rule really applies to.
• Chooses a true but irrelevant statement
  Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.