Richard Feynman: "I want to emphasize that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you probably learned something about light behaving like waves. I'm telling you the way it does behave - like particles. You might say that it's just the photomultiplier that detects light as particles, but no, every instrument that has been designed to be sensitive enough to detect weak light has always ended up discovering the same thing: light is made of particles." QED: The Strange Theory of Light and Matter p. 15
https://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/0691024170
Feynman's words, if taken at face value, imply that variations of the wavelength of light, as shown e.g. here
https://youtube.com/watch?v=xsVxC_NR64M, are preposterous. The particle model of light is incompatible with the idea of variable wavelength.
The wave model of light is ALSO incompatible with this idea. Variable wavelength of light
https://youtu.be/3mJTRXCMU6o?t=77 violates the principle of relativity. If, as Hawking teaches, the wavelength varied at the emitter, measuring it inside the emitter's spaceship would allow the emitter to know his spaceship's speed without looking outside:
Stephen Hawking, "A Brief History of Time", Chapter 3: "Now imagine a source of light at a constant distance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wavelength of the waves we receive will be the same as the wavelength at which they are emitted (the gravitational field of the galaxy will not be large enough to have a significant effect). Suppose now that the source starts moving toward us. When the source emits the next wave crest it will be nearer to us, so the distance between wave crests will be smaller than when the star was stationary."
http://www.fisica.net/relatividade/stephen_hawking_a_brief_history_of_time.pdf
In future, Einstein-free physics, the wavelength of light will be an invariable proportionality factor in the formula
(speed of light) = (wavelength)(frequency)
Pentcho Valev
https://twitter.com/pentcho_valev