Rami Arieli: "The Laser Adventure" Chapter 2.2 page 1
2.2 Photons and the energy diagrams

Electromagnetic radiation has, in addition to its wave nature (described in Chapter 1.1), some aspects of "particle like behavior".
In certain cases, the electromagnetic radiation behaves as an ensemble of discrete units of energy that have momentum. These discrete units (quanta) of electromagnetic radiation are called "Photons".

The relation between the amount of energy (E) carried by the photon, and its frequency (n), is determined by the formula (first given by Einstein):

E = hn

The proportionality constant in this formula is Planck's constant (h):

h = 6.626*10-34 [Joule-sec]

Sometimes angular frequency (w) is used instead of frequency (n), so a corrected constant h(bar) is used:

h(bar) = h/2p = 1.054*10-34 [Joule-sec]

The energy is given by:

E = hn = h(bar)w

This formula shows that the frequency of the radiation (n), uniquely determines the energy of each photon in this radiation.

This formula can be expressed in different form, by using the relation between the frequency (n) and the wavelength: c = l*n
to get:

E = h * c/l

This formula shows that the energy of each photon is inversely proportional to its wavelength. This means that each photon of shorter wavelength (such as violet light) carries more energy than a photon of longer wavelength (such as red light).

Since h and c are universal constants, so either wavelength or frequency is enough to fully describe the photon.