I've been thinking about this for quite some time, and from looking online I haven't found a satisfying answer.
Lots of photons, such as visible-light photons have very small wavelength (which from my understanding of basic physics is the distance between two crests/troughs), but I also know that some EM waves have wavelengths a few metres or even kilometres long e.g radio waves.
What keeps me up at night is the question "How can a photon have a wavelength of a few kilometres and yet still be thought of as a particle?"
Does this mean that one individual photon is several kilometres long? If so, wouldn't it be subject to so many variations between the beginning of the wave and its end?
I realise that matter is also wave-like, where it's uncertainty in position is given by its De Broglie wavelength. Does this apply to the photon?
In other words, is the wavelength of a photon simply the uncertainty in its position?