Rami Arieli: "The
Laser Adventure" Chapter 2.2 page 1

**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):

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

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

The energy is given by:

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:

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.**