Waves and modern · AP Physics 2 · 7 min
Physics 2 often tests whether you know which representation fits the situation: ray, wavefront, interference, or photon.
01 The trap
Wrong path
Students use a memorized equation before deciding whether the problem is geometric optics, physical optics, or quantum energy.
Why it feels right
The units share vocabulary like wavelength, frequency, energy, and intensity, so problems can look more similar than they are.
02 Correct model
Use ray diagrams for image formation, path difference for interference, wave speed relations for media changes, and E = hf for photon energy.
03 Mini-example
Prompt
Light enters glass from air. What happens to frequency?
Common wrong answer
Frequency decreases because the light slows down.
Correct reasoning
Frequency stays fixed across the boundary; speed and wavelength change in the new medium.
Diagnostic cue
If the question mixes wavelength, medium, and energy, decide whether it is asking about a wave property or a photon property first.
04 Guided practice
Warm-up isolates the principle. Bridge changes the context. Contest-style requires a complete setup on less familiar geometry.
A light wave enters glass from air and slows down. What happens to its frequency?
Use v = fλ, but decide which quantity stays fixed.
The frequency stays the same. The speed decreases in glass, so the wavelength decreases.
Trap: Saying frequency decreases because the wave speed decreases.
In a double-slit experiment, what happens to fringe spacing if the slit separation d is increased while wavelength and screen distance stay fixed?
Fringe spacing is inversely proportional to slit separation.
The fringe spacing decreases. Since Δy is proportional to 1/d, increasing d makes the bright fringes closer together.
Trap: Thinking wider slit separation spreads the pattern out because the slits are farther apart.
Light below the threshold frequency shines on a metal. If intensity is doubled but frequency stays below threshold, are electrons emitted?
Intensity changes the number of photons per second, not the energy of each photon.
No electrons are emitted. Doubling intensity sends more photons, but each photon still has energy below the work function.
Trap: Treating higher intensity as enough energy per electron.
Light of wavelength 600 nm passes through slits separated by 0.40 mm. A screen is 2.0 m away. What is the spacing between adjacent bright fringes in the small-angle approximation?
600 nm = 6.00 × 10⁻⁷ m and 0.40 mm = 4.0 × 10⁻⁴ m.
Δy = (6.00 × 10⁻⁷)(2.0)/(4.0 × 10⁻⁴) = 3.0 × 10⁻³ m = 3.0 mm.
Trap: Substituting nanometers and millimeters without converting them to the same unit.