Waves, Sound and Light
Wave properties
A wave transfers energy without transferring matter. Know amplitude, wavelength, frequency, period, wave speed and the equation v = fλ. Transverse waves (light, water surface) oscillate perpendicular to travel; longitudinal waves (sound) oscillate parallel, forming compressions and rarefactions. Frequency is the number of oscillations per second in hertz, and period is the reciprocal of frequency. A common trap is mixing wavelength with amplitude, or forgetting to convert cm to m.
Sound
Sound is a longitudinal wave requiring a medium, so it cannot travel through a vacuum. Its speed is about 340 m/s in air, faster in liquids and fastest in solids. Pitch depends on frequency and loudness on amplitude. Echoes obey the reflection rule, and ultrasound has frequencies above human hearing (about 20 kHz), used in medical scanning and sonar.
Light and optics
Light travels in straight lines; know reflection (angle of incidence equals angle of reflection, measured from the normal) and refraction (bending due to speed change, with Snell's law n = sin i/sin r). Total internal reflection occurs above the critical angle. Lenses: converging lenses focus parallel rays at the focal point, and 1/f = 1/u + 1/v gives image position. Dispersion splits white light because different colours refract by different amounts.
Worked example
A sound wave has frequency 500 Hz and wavelength 0.68 m. Speed v = fλ = 500 × 0.68 = 340 m/s. If an echo returns after 0.4 s, the reflecting surface is d = vt/2 = 340 × 0.4/2 = 68 m away. The division by two is essential because the sound travels there and back.