Three observations were detected in the photo-electric effect:
The kinetic energy of the ejected electrons doesn't change if you change the intensity of light (instead more electrons are ejected with the same kinetic energy)
There is a certain value of frequency below which no electrons are ejected
The kinetic energy increases if we increase the frequency of light.
The solution to these observations was given by Einstein who said that light consists of a finite number of energy quanta(carriers of energy) which are absorbed or emitted as units. These quanta are called photons and have energy equal to E=hf.
Therefore, if we increase the intensity, more photons are sent, and eventually, more electrons are ejected (since every electron needs a photon to be ejected) with no change in the kinetic energy.
This is what I've understood from the photo-electric effect chapter.
But I still can not understand how can we increase frequency without increasing the energy because I see from E = hf that frequency and energy are proportional.