The Physics Aviary
This site is home to an extensive collection of cross-platform interactive labs and tools for introductory physics with the aim of helping students "anywhere in the world" master big ideas in physics. NOTE: Click "Search" and then click "Waves and Optics under the "Unit" heading. This will generate 28 virtual labs, 3 digital tools, 2 games, and 17 sets of homework exercises. Topics include wave properties, wave interference, standing waves, mirrors, lenses and refraction, diffraction, oscillators and pendula, color, and more.

Physics Classroom Concept Builders: Waves & Sound
A direct link to The Physics Classroom's Concept Builders for the topic of Waves and Sound. This resource set gauges mastery of  focused concepts through sequenced questions requiring students to demonstrate mastery of fundamentals before progressing to more sophisticated ideas. Trophies are awarded for each level mastered. There are 8 Concept Builders in the Waves & Sound collection, covering wave properties, sound intensity, and harmonics in strings and air columns.

Physics Classroom Concept Builders: Light and Color
Here you'll find five Concept Builders, designed to gauge student mastery of the electromagnetic spectrum, light intensity, color subtraction, color pigments, and filters.

AAPT Lessons: Modeling Rosalind Franklin's Discovery with a Pen Spring
Here's a diffraction lab students can do at home, using a ballpoint pen spring and a laser pointer. The purpose of the lab is twofold: 1) To investigate the light diffraction phenomenon, and 2) To gain insight into how English chemist Rosalind Franklin used x-ray crystallography in her work to unlock the structure of DNA. Teachers: the light projected from a laser pointer through a pen spring produces a unique x-shaped pattern, similar to Franklin's famous "Photo 51".