Ripples cross a lake toward a beach. At the line where the bottom turns shallow, the ripples slow down.
Nothing about the source changed. So what happens to the wavelength as the ripples cross that line?
Start with what cannot change. The source made these waves, and crests arrive at the shallow line at the source's rate.
Every crest that arrives moves on into the shallow water. None vanish, and none pile up forever. So crests set out across the shallow side exactly as often as they arrive: the frequency is the same on both sides of the line.
What does change is the speed. Shallow water is a different medium for a ripple, and ripples travel more slowly in it.
Now put the two together. Each crest crawls away more slowly, but the next crest arrives just as soon as before. The new crest closes in on the slow one ahead, so the gap between them shrinks.
The wavelength shortens in the slower medium: the crests bunch up.
Crossing the other way, into a faster medium, the gaps stretch: the wavelength lengthens. Either way the frequency holds; the wavelength adjusts.
Ripples travel at 3 m/s in deep water, with 2 crests arriving at a shallow shelf each second. On the shelf they travel at 2.4 m/s. What is the wavelength on each side?
The frequency is the same on both sides: 2 Hz.
Deep side: write down the values and the arrangement, and calculate.
λ = v ÷ f = 3 ÷ 2 = 1.5, so 1.5 m.
Shallow side: the same arrangement.
λ = 2.4 ÷ 2 = 1.2
The wavelength shortens from 1.5 m in the deep water to 1.2 m on the shelf; the frequency stays 2 Hz.
A wave on a light cord passes onto a heavier rope, where waves travel more slowly. What happens to the wave's frequency and wavelength?
The source's rate is unchanged, and crests cross the joint as often as they arrive: the frequency is the same on both sides.
The heavy rope is a slower medium, so each crest crawls away from the joint more slowly while the next arrives on time.
The frequency is unchanged; the wavelength is shorter on the heavy rope.
Ripples slow down as they cross onto a shallow shelf. What happens to the distance between their crests?
Ripples cross from shallow water into deep water, where they travel faster. What happens to their frequency?
Eirik says: 'When a wave moves into a medium where it travels faster, its wavelength gets longer.' Is Eirik correct?
A wave crosses from a thin cord onto a thick rope, where it travels more slowly. Which pair describes the wave on the thick rope?
Why does the frequency stay the same when ripples cross onto a shallow shelf?
Ripples travel at 2 m/s in deep water. On a shallow shelf their speed is half of that. The frequency is unchanged. What happens to the wavelength?