Preview
Waves: Intro
Transverse and longitudinal waves side by side — v = fλ live
Watch a gull riding swells beyond the surf line. The waves march steadily toward the beach — but the gull only bobs up and down, going nowhere. Two facts hide in that picture, and each one breaks a common instinct. First: the water isn't traveling. Each parcel of sea — and each particle in this lab's two chains — oscillates around its own home while the PATTERN sweeps past; what actually travels is energy. Second, and subtler: if the gull starts bobbing twice as fast because the storm offshore drums the water at higher frequency, the waves do NOT arrive any faster. The speed of a wave belongs to the medium — to the water's depth and density, to a string's tension, to the air's temperature — 'the speed of a wave depends on the characteristics of the medium', as the textbook puts it flatly. The source only decides how often it shakes: it owns the frequency. So when frequency doubles at fixed speed, something must give, and it's the spacing: λ = v/f, the wavelength halves so that twice the crests still march at the same pace. In this lab you hold all three knobs separately — the source's frequency, the medium's speed, and the energy's amplitude — over two particle chains running the same wave transverse (blue, swinging sideways) and longitudinal (green, bunching and stretching like sound). Crank each knob and watch which rows move and which refuse: the refusals are the physics.
What you'll be able to do
- Separate the three owners of v = λf on the panel: the source owns f (crank 1.5 → 3.0 Hz and v holds 5.00 m/s while λ halves 3.33 → 1.67 m), the medium owns v (drag it 5 → 10 m/s at fixed f and λ stretches to 6.67 m), and λ is never set directly — it is always the quotient v/f under the amber bar
- Distinguish transverse from longitudinal on one screen: the blue row swings perpendicular to travel while the green row compresses parallel to it (its ticks marking the crowding, ∝ ∂ξ/∂x) — same f, same v, same λ, different oscillation axis; sound is the green row, light the blue
- Put amplitude in its place: sweep A from 0.8 to 2.0 m and watch v, λ, and T refuse to move — amplitude carries the wave's energy (I ∝ A², the sidebar formula), not its speed, wavelength, or clock; the particles just swing wider on the same schedule
Formulas
Make a prediction
Default state: f = 1.50 Hz, wave speed v = 5.00 m/s, so the amber bar spans λ = 3.33 m. Now double the frequency to 3.0 Hz (same medium, same amplitude). What do the speed and wavelength rows do?
No grading here — commit to a guess, then scroll down and test it yourself.
This is a Pro experiment
Upgrade to Pro to access this experiment — or keep learning with one of our free labs.
Check
Did the lab agree with you?
Your prediction
You skipped the prediction — jump back to the preview and commit to a guess first; the comparison is the whole point.
Answer: v stays pinned at 5.00 m/s while λ halves to 1.67 m — the medium sets the speed, the source only sets the frequency, and the spacing gives way: λ = v/f
v = λf is a budget, not a lever: the wave's speed is fixed by the medium — 'the speed of a wave depends on the characteristics of the medium' (OpenStax, verbatim) — and nothing about shaking the source harder or faster changes what the medium can carry. Doubling f to 3.0 Hz leaves v at 5.00 m/s, so the wavelength must halve: λ = 5.00/3.0 = 1.67 m, and the amber bar shrinks to match while twice as many crests march past at the old pace. Option B is the documented instinct this gate targets — it feels right because faster WIGGLING looks like faster MOVING, but watch a single crest: it crosses the screen at exactly the same rate before and after. Option C breaks the budget: if v and λ both held while f doubled, you'd have v ≠ λf — the three are chained, and λ is the link that gives. To genuinely speed the wave up you need the OTHER slider: Wave Speed, the medium's knob — that's the Fast Wave preset's whole lesson.
Quiz (0/3)
A wave has frequency 2Hz and wavelength 3m. What is its speed?
Sound travels at 343 m/s. What wavelength does a 440Hz (A note) have?
Why can you hear a bass drum from far away but not a piccolo flute at the same distance?
You can now
- Separate the three owners of v = λf on the panel: the source owns f (crank 1.5 → 3.0 Hz and v holds 5.00 m/s while λ halves 3.33 → 1.67 m), the medium owns v (drag it 5 → 10 m/s at fixed f and λ stretches to 6.67 m), and λ is never set directly — it is always the quotient v/f under the amber bar
- Distinguish transverse from longitudinal on one screen: the blue row swings perpendicular to travel while the green row compresses parallel to it (its ticks marking the crowding, ∝ ∂ξ/∂x) — same f, same v, same λ, different oscillation axis; sound is the green row, light the blue
- Put amplitude in its place: sweep A from 0.8 to 2.0 m and watch v, λ, and T refuse to move — amplitude carries the wave's energy (I ∝ A², the sidebar formula), not its speed, wavelength, or clock; the particles just swing wider on the same schedule