observer's

Meaning: "Observer's" alone is an incomplete phrase, so this covers its most likely MCAT use: the observer's (detector's) perceived frequency in the Doppler effect. When source and observer move relative to each other, wavefronts reach the observer at a different rate than the source emitted them. Closing the distance raises the observed frequency; increasing it lowers the frequency. The wave speed in the medium and the source's emitted frequency do not change. Only the rate of arrival does. A second, less common use is "the observer's frame of reference" in relative-motion problems.

Equation: f' = f (v ± vo)/(v ∓ vs), where f' is the observed frequency (Hz), f is the source frequency (Hz), v is the wave speed in the medium (m/s), vo is the observer's speed (m/s), and vs is the source's speed (m/s). Use + in the numerator and − in the denominator when the two are approaching, and the opposite signs when they are separating.

On the MCAT: Passages use a siren, a bat, or ultrasound reflecting off moving blood. Questions ask whether pitch rises or falls, or ask for f'. The qualitative answer depends only on whether the separation is shrinking or growing. For speeds much less than v, Δf/f ≈ vrel/v.

Common confusion: Students think the source's frequency or the speed of sound changes. Neither does. The observer simply receives a shifted frequency. The pitch is also high during approach and low during recession, so it drops as the source passes, not on approach.