Waves
8th Grade Science · 8 lessons
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Lessons
- 1Using v = fλ with Real NumbersWave speed is set by the medium, so within one medium frequency and wavelength trade off exactly. Sound travels at about 340 m/s in air, so a 170 Hz note has a 2 m wavelength while a 3,400 Hz note has a 0.1 m one. Move that same sound into water, where the speed is about 1,500 m/s, and the frequency is unchanged — the source still vibrates at the same rate — so the wavelength must grow by more than four times.Experiment
- 2Particle Motion Decides the Wave TypeWhat separates the two wave types is the direction the particles oscillate relative to the direction the energy travels, and that difference has a testable consequence: a transverse wave can be filtered down to one plane of oscillation, a longitudinal one cannot. Sound passes a grille at any orientation, while light through two crossed polarising filters is extinguished. Earthquakes send both — fast longitudinal P waves arrive first, slower transverse S waves follow and cannot cross the liquid outer core at all.Experiment
- 3Refraction Happens Because the Speed ChangesA wave bends at a boundary because its speed changes there while its frequency does not, so the wavelength must change and the wavefronts pivot. Light slows on entering glass and bends toward the normal; it speeds up on leaving and bends away again, which is why a straight ruler in a beaker appears broken at the surface. Reflection is different: no speed change occurs, and the angle of reflection always equals the angle of incidence.Experiment
- 4The Decibel Scale Is LogarithmicHuman hearing responds roughly logarithmically to intensity, so the decibel scale adds 10 dB for every tenfold rise in power per square metre. 90 dB is therefore not slightly louder than 80 dB — it carries ten times the intensity, and 100 dB carries a hundred times. That is why safe exposure time falls so steeply as the number climbs, and why amplitude, which sets intensity, must be kept separate from frequency, which sets pitch.Experiment
- 5The Electromagnetic Spectrum Ordered by Photon EnergyEvery electromagnetic wave travels at 3 × 10⁸ m/s in a vacuum, so v = fλ makes the seven named bands one continuous scale: as frequency rises, wavelength shortens and the energy carried by each photon rises with it. The ionising threshold sits in the ultraviolet. Below it, no increase in brightness will break a chemical bond, because the energy arrives in packets that are individually too small; above it, even a faint beam can damage DNA one photon at a time.Lesson
- 6Echo Location: Timing a Pulse Gives DistanceDistance from a reflected pulse is d = v × t / 2, the halving accounting for the fact that the pulse made a round trip. A sonar ping returning after 0.4 s through water at 1,500 m/s means a seabed 300 m down. The detail such a system can resolve is set by the wavelength — nothing much smaller than one wavelength shows up — which is why medical scanners run at megahertz frequencies rather than at the kilohertz a ship's echo sounder uses.Experiment
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