Waves
Waves: Wave Behavior, Sound, and Light
Wave questions test the relationship among speed, frequency, and wavelength and how waves behave at boundaries. The universal wave equation is v = fλ. Frequency is set by the source and does not change when a wave enters a new medium; speed and wavelength do change.
Know the two wave types: transverse (disturbance perpendicular to propagation, like light and waves on a string) and longitudinal (disturbance parallel, like sound). Mechanical waves need a medium; electromagnetic waves do not.
Key behaviors: reflection (angle of incidence equals angle of reflection), refraction (bending due to speed change, with light bending toward the normal when slowing), diffraction (spreading around obstacles, most noticeable when wavelength is comparable to the opening), and interference (constructive when waves arrive in phase, destructive when out of phase). Standing waves form at fixed frequencies with nodes and antinodes.
The Doppler effect shifts observed frequency when source and observer move relative to each other-higher frequency approaching, lower receding. For light, this appears as blueshift and redshift.
Worked example: A sound wave of frequency 500 Hz travels at 340 m/s in air. Its wavelength is λ = v/f = 340/500 = 0.68 m. If it enters water where speed is higher, the frequency stays 500 Hz but the wavelength increases.
Traps: changing frequency across a boundary (wrong-frequency is fixed by the source); confusing period and frequency (they are reciprocals); assuming louder sound means higher pitch; and forgetting that diffraction is greatest when the wavelength is large relative to the aperture. Always check whether the question asks about the medium or the source.
Rule of thumb: for any wave problem, write v = fλ and mark which two quantities are fixed before solving for the third.