Which property of a wave remains constant when the wave travels from one medium into another?

Step 1: Analyze the question We want the property that does NOT change when a wave crosses the boundary between two media (for example, light going from air into glass, or sound going from air into water).

Step 2: The concept The wave equation is:

v = fλ

where v is velocity, f is frequency and λ is wavelength.

• The frequency is set by the source that produces the wave. The medium does not control how many waves the source sends out per second. At the boundary, the number of wave crests arriving per second must equal the number leaving per second, otherwise crests would pile up or vanish at the boundary. So frequency stays the same.
• The velocity depends on the properties of the medium (density, elasticity, refractive index and so on), so it changes.
• Since v = fλ and f is constant, a change in v means the wavelength changes too (v ∝ λ).
• The amplitude changes as well, because some energy is reflected at the boundary and some is absorbed, so the transmitted wave has a different amplitude.

Step 3: Check each option

• A. Amplitude: changes (energy is split between reflected and transmitted waves).
• B. Wavelength: changes (λ = v/f, with v changing).
• C. Velocity: changes (depends on the medium).
• D. Frequency: remains constant (determined by the source).

Example: Light moving from air (v ≈ 3.0 × 10⁸ m/s) into glass (v ≈ 2.0 × 10⁸ m/s) slows down and its wavelength shortens, but its frequency is unchanged. This is why the colour of light does not change when it enters glass or water.

Answer: D. Frequency

Exam tip: Remember "Frequency is Fixed by the source." Whenever a question asks what stays the same across a boundary (refraction, change of medium), the answer is frequency. Velocity and wavelength change together, since v = fλ.