You can find the waves lesson here: https://cognitoedu.org/coursesubtopic...

The Cognito platform just launched. It's totally free, and has been built from ground up to make learning and revision as easy as possible. Some of the main features include:
- Automatic progress tracking of which lessons you've completed, and how well you did.
- Quiz generation on any combination of subtopics you like.
- Signature Cognito videos to teach you new concepts in the shortest possible time.
- An extensive bank of past papers, sorted both by year, but also by topic.