Waves

Traps in Waves

• Adds quantities with different units
  Only quantities with the same unit can be added. If the units differ, the formula multiplies or divides them.
• Rearranges a formula wrongly
  Use the triangle or do the algebra: from V = IR, divide both sides by R → I = V/R. Check with units (V/Ω = A).
• Wave equation slips
  v = fλ. Check units: Hz × m = s⁻¹ m = m/s.
• Fails to convert units to SI
  Convert every quantity to SI (m, kg, s) BEFORE substituting. 1 km/h = 1000/3600 m/s; 1 g = 10⁻³ kg; 1 cm² = 10⁻⁴ m².
• 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.
• Forgets that an echo is a return trip
  The sound goes to the reflector and comes back: total path = 2d, so d = vt/2.
• Inverts a formula
  Write the defining equation in words first (density = mass per unit volume), then rearrange algebraically. Check units: kg/m³.
• 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.
• Electromagnetic spectrum order errors
  Increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma. All travel at 3 × 10⁸ m s⁻¹ in vacuum; shorter wavelength = higher frequency and energy.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Sound misconceptions
  Sound is a longitudinal mechanical wave needing a medium. Pitch ↔ frequency; loudness ↔ amplitude; quality (timbre) ↔ overtones.
• Takes part of the condition as the whole answer
  Check whether the option is enough on its own. Equilibrium needs zero resultant force AND zero total moment; a complete explanation names every factor in the stem.