What is meant by the frequency of a single photon?

[nabeel17]

When I think of frequency, I think of something happening per second or per unit time. When light is a wave, I can understand that it has a frequency, but when it is a particle like a photon, what is the interpretation of its frequency? What is occurring per unit time? Or do we associate a frequency with it when it exhibits wave properties instead of particle? Or just based on its energy we say it has frequency related by E=hf?

 

[jambaugh]

For an operational meaning matching your sense of "cycles per unit time" you can observe the frequency of a single photon by filtering it with a grating of a given spatial frequency (cycles per unit distance) which we can equate to time frequency using speed of light.