Group vs Phase Velocity
Lesson notes
Overview
Envelopes of wave groups can move at different speeds from the crests inside. This topic is rated intermediate and is number 167 of 227 in Physics. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
A group of waves, such as a pulse, has an envelope that can travel at a different speed from the individual crests inside it. The phase velocity ω/k is the speed of the crests, while the group velocity v_g = dω/dk is the speed at which energy and information travel. In deep water waves the crests move twice as fast as the group, so crests appear at the back of the group and vanish at the front. In dispersive media the two differ.
Reading the relationship
The expression "v_g = dω/dk" is a compact summary of the idea. It uses the symbols v_g, d, ω, k; name what each one stands for before you start. Notice that it contains a ratio, so increasing the denominator makes the result smaller. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.
Try this
Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.
Common pitfalls
Check units and directions first. Many mistakes come from mixing up a scalar with a vector, or from using a formula outside the conditions where it holds.
How it connects
It builds on “Resonance Curve”, the topic before it in Physics. Once you are comfortable, continue with “Lens Maker’s Equation”.
Why it matters
Physics explains how the world moves, heats, shines and interacts, and the same principles power technology around you.
Where you might use it
- Revising “Group vs Phase Velocity” before a class, exam or certification
- Designing vehicles, bridges and devices where motion, forces or waves must be predicted
- Understanding how sensors, lasers and communication links actually work
- Explaining everyday phenomena such as sound, light, heat and orbits
Frequently asked questions
What is Group vs Phase Velocity?
Envelopes of wave groups can move at different speeds from the crests inside. It is an intermediate topic in Physics.
How do I use the Group vs Phase Velocity simulation?
The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression v_g = dω/dk packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Resonance Curve”, the previous topic in Physics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.