Wave period stays constant across media while wavelength and speed change

Evidence

The period must be the same inside the block as it is outside. (Whether you are inside or outside the block, a pulse sent at 1 Hz would have to be received at 1 Hz.) Since the period is the same but the speed is slower, λ = c/f implies a shorter wavelength, as seen here. — Eric J. Heller, Why You Hear What You Hear- An…, 216

Categorization

• DEFINE: Clarifies the key relation λ = c/f and the invariant period across a medium boundary — a foundational definition for why wavelength shortens in slower media.
• XREF: connects to: Total daily travel time stays constant regardless of transport speed — Hartmut Rosa, Social Acceleration: A New Theory of…, 241