Waves, sound, light and optics
Waves, sound, light and optics tests wave quantities, wave behaviour and ray diagrams.
Wave basics
- Transverse waves vibrate perpendicular to travel (light, water surface); longitudinal waves vibrate parallel (sound).
- Amplitude = maximum displacement; wavelength λ = distance between corresponding points; frequency f = waves per second (Hz); period T=1/f.
- Wave equation: v = fλ. Sound needs a medium; speed in air about 340 m/s, faster in solids. Light travels at 3×10^8 m/s in vacuum.
- Sound pitch depends on frequency; loudness on amplitude.
Light and optics
- Reflection: angle of incidence = angle of reflection, measured to the normal.
- Refraction: change of speed at a boundary; frequency stays constant; wavelength and speed change. Light bends towards normal entering denser medium.
- Total internal reflection occurs above critical angle when going from denser to less dense medium.
- Converging lens: parallel rays focus at principal focus; real images can be projected.
Worked example
A sound wave has frequency 170 Hz and speed 340 m/s. λ = v/f = 340/170 = 2.0 m. If the frequency doubles to 340 Hz in the same medium, speed stays 340 m/s, so λ becomes 1.0 m. For a light ray hitting glass at 30° to the normal, the angle of reflection is 30°; the refracted ray bends towards the normal if glass is denser.
Traps
Measuring angles to the surface rather than the normal; thinking frequency changes on refraction; using cm in v=fλ; saying sound travels in vacuum; confusing real and virtual images.