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 108 m/s).

Combining this equation with the wave equation we have:

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

Try this question:

A 50W lamp radiates in the visible region of the spectrum (400 to 700 nm). Estimate the number of photons produces each second by the lamp.

The momentum of a photon

Einstein's equation E = mc2 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 = mc2

so,

mphoton = hf/c2

and as the momentum of the photon = mc

pphoton = hf/c

but we know from the wave equation that

f = c/λ

so,

pphoton = h/λ

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