Derive Group Velocity: vg=vp+k(dvp/dk) and vg=vp-λ(dvp/dλ) Explained

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SUMMARY

The discussion focuses on deriving the relationship between group velocity (vg) and phase velocity (vp) using the equations vg=vp+k(dvp/dk) and vg=vp-λ(dvp/dλ). Participants clarify that k is defined as (2π)/λ and ω as 2πf. The challenge presented involves using calculus to relate the derivatives dvp/dk and dvp/dλ, which is essential for transforming the initial equation into the desired form.

PREREQUISITES
  • Understanding of group velocity and phase velocity concepts
  • Familiarity with calculus, specifically derivatives
  • Knowledge of wave properties, including wavelength (λ) and wave number (k)
  • Basic grasp of angular frequency (ω) and its relation to wave parameters
NEXT STEPS
  • Study the relationship between derivatives dvp/dk and dvp/dλ in wave mechanics
  • Explore the implications of group and phase velocity in different media
  • Learn about the applications of these concepts in advanced physics problems
  • Investigate the mathematical techniques for transforming wave equations
USEFUL FOR

Students and professionals in physics, particularly those studying wave mechanics, as well as educators seeking to explain the concepts of group and phase velocity in a clear manner.

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Homework Statement



Starting from: vg=vp+k(dvp/dk) show that the group velocity can also be expressed by vg=vp-λ(dvp/dλ).

Homework Equations



I'm told that vg=vp+k(dvp/dk) with vg=group velocity and vp=phase velocity. I also know that k=(2π)/λ and ω=2πf.

The Attempt at a Solution



I tried to start with the equation ω=kvp and take the derivative with respect to λ but it doesn't work out.
 
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What rule from calculus would allow you to relate

dvp/dk & dvp/dλ?
 

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