Waves, Sound, and Optics
Domain 3: Waves, Sound, and Optics
This domain tests wave properties, sound, superposition and resonance, reflection and refraction, lenses and mirrors, and interference. Many items are solved by one wave or ray relationship plus a sign convention.
Core rules
- For any periodic wave, v = fλ. Frequency is set by the source and does not change when a wave enters a new medium; speed and wavelength do.
- Sound requires a medium, travels faster in solids than gases, and is a longitudinal pressure wave. Intensity follows an inverse-square law; a 10 dB increase means ten times the intensity.
- The Doppler effect: observed frequency rises when source and observer approach, falls when they separate.
- Reflection: angle of incidence equals angle of reflection. Refraction obeys Snell's law, n1 sin θ1 = n2 sin θ2; light bending toward the normal enters a higher-index medium.
- Thin lens/mirror equation: 1/f = 1/do + 1/di, with magnification m = −di/do. Under the common sign convention used here, a real image has positive di and is inverted.
- Standing waves have nodes and antinodes; resonance occurs at natural frequencies.
Worked example
An object is 30 cm from a converging lens with f = 10 cm. Using 1/di = 1/f − 1/do = 1/10 − 1/30 = 2/30, so di = 15 cm. The image is real and 15 cm on the far side. Magnification m = −15/30 = −0.5: the image is half as tall and inverted.
Traps that cost marks
- Changing frequency instead of wavelength when light enters glass.
- Using degrees and radians interchangeably in Snell's law or interference formulas.
- Forgetting that a virtual image has a negative image distance in the standard convention.
- Assuming sound can travel through a vacuum.
- Confusing nodes (zero displacement) with antinodes (maximum displacement).