"Slowly varying" potential for WKB approximation

[Wrichik Basu]

In order to use WKB approximation, the potential has to be "slowly varying". I learned the method from this video:

Advanced Quantum Mechanics with Applications by Prof. Saurabh Basu (NPTEL):- WKB Approximation, Bohr Sommerfeld quantization condition

• Wrichik Basu
• Dec 1, 2018
• advanced quantum physics

Copyright strictly reserved to Prof. Saurabh Basu and NPTEL, Govt. Of India. Duplication...

But the Professor hasn't mentioned in detail what the measure of "slowly varying" is.

What is the limit beyond which we cannot use the WKB method accurately?

 

[atyy]

There are discussions of how to estimate the error of this approximation at
https://en.wikipedia.org/wiki/WKB_approximationhttp://farside.ph.utexas.edu/teaching/jk1/lectures/node97.html