1
Photons
Quantum theory
describes light as
a particle called a
photon
According to
quantum theory
, a
photon
has an
energy
given by
E = hf
= hc/
h = 6.6x10
-34
[J s] Planck’s constant,
after the scientist Max Planck.
The energy of the light is proportional to the frequency (inversely
proportional to the wavelength) ! The higher the frequency (lower
wavelength) the higher the energy of the photon.
10 photons have an energy equal to ten times a single photon.
Quantum theory describes experiments to astonishing precision
,
whereas the
classical wave description canno
Photoelectric Effect -
Example
Most metals have a work function on the order of several
electron volts.
Copper has a work function of 4.5 eV.
Therefore, the cut-off frequency for light ejecting electrons
from copper is:
hf
cutoff
= 4.5 eV,
or
f
cutoff
= 4.5 x (1.6 x 10
-19
C) x (1 V) / 6.63 x 10
-34
J-sec
= 1.09 x 10
15
Hz
or
cutoff
= c/ f
cutoff
,
or
cutoff
=
(3 x 10
8
m/s) / (1.09 x 10
15
cycles/sec)
=
276 nm
(in the UV range)
Any frequency lower than the cut-off (or any wavelength

This
** preview**
has intentionally

**sections.**

*blurred***to view the full version.**

*Sign up*
This is the end of the preview.
Sign up
to
access the rest of the document.