# The
photon

A
quantum of electromagnetic energy is called a photon.

You can think of
photons as 'light particles' - quanta of energy - discrete packages (that
can't be split)...... little packets of wave energy - wavelets!

The image on the right is an artist's view of a photon (a wave packet of energy). Click here to find out more about the artwork.

The energy of a
photon is proportional to its frequency.

**E
= hf**

The constant of
proportionality is called the Planck constant.

It has the symbol 'h'
and is on your data sheet.

It has a value of 6.63 x 10^{ -34}
Js

Now, **for all electromagnetic
radiation the velocity is 'c'** (the speed of light - again, you will find this on your data
sheet as 3.0 x 10^{8} m/s).

Combining this equation
with the wave equation we have:

(always
write it this way - learn to - it saves time!)

### The momentum of a photon

Einstein's equation E = mc^{2 }links mass and energy.

That means that** mass
can be converted into energy and vice versa.**

We
can therefore find the **mass equivalent of a photon** and then work
out its momentum!

E
= hf = mc^{2}

so,

m_{photon}
= hf/c^{2}

and
as the momentum of the photon = mc

p_{photon}
= hf/c

but we know from the wave equation that

f
= c/λ

so,

# p_{photon} = h/λ

This relationship is important as it links the wave and particle properties of the photon. It expresses wave-particle duality.