Flame tests and their line spectra are a
popular way of showing evidence for the
quantised nature of the atom. But sometimes
you want something even simpler to
demonstrate a complex idea, such as a
coloured LED lighting up or not. This
month we're going to look at what LEDs
can teach us about electronic
transitions.
The equipment you'll need is provided in the accompanying Exhibition
Chemistry article. Before we get onto
that though let's first take a look at
the spectrum from a tungsten bulb for
comparison. I'm using a diffraction
rating here with 600 lines per
millimeter. At low power we get a very
red colour from the bulb, but we still get
a spread of covers in the spectrum,
mostly red of course. As I increase the
power by adjusting the rheostat we get
more and more colours in the spectrum and
the colour of the bulb becomes more
yellow. Now let's repeat the same thing
with a red LED. Make sure you connect the
long leg to the positive terminal as
current can only flow through one way.
Your LED will need a protecting resistor
in place: I'm using a 55 ohm rheostat, but
you might have little kits like
this already made up, or even a dedicated
Planck's constant kit. It's worth
asking somebody in your physics
department because they probably do a
similar experiment with their students.
The first thing I notice is that as the
voltage increases there's no current
flowing initially, unlike the bulb.
Eventually we get some light and the
current increases.
The spectrum this time is much more
focused, more like the line spectra we
see from flame tests, but not perfect in
this regard because we're not dealing
with energy levels in gaseous atoms. But
we can use this led to model an
electronic transition. In the circuit the
rate of flow of electrons through is
shown by the ammeter and the voltmeter
reading shows the energy per
Coulomb the electrons transfer between
the two points where it's connected.
Within the LED there's an energy barrier
similar to the energy levels in atoms. An
electron requires a minimum amount of
energy to overcome it. As electrons cross
the barrier a photon of light is
released of a frequency proportional to
its size. So what's going to happen if I
swap this red LED for a higher frequency
one such as this green LED? With the same
voltage, that is to say energy per unit
of charge, the electrons can't cross the
barrier: no current flows and no light is
released. As you increase the voltage
each electron has more energy and so the
electrons can cross the barrier allowing
a current to flow and causing the higher
frequency LED to illuminate. This
demonstrates that lower frequencies are
associated with less energy and that if
the minimum energy criteria for the
transition isn't met electrons can't
jump the energy barrier. As you increase
the voltage the current increases
proportionally and the LED gets brighter
as more and more electrons possess the
necessary energy to make the jump. I made
a note of the voltages and currents as I
adjusted the variable resistor. Now
obviously we can't see the photon
associated with the jump of a single
electron but we can estimate the voltage
at which that jump becomes possible by
forecasting the linear part of the I/V
graph I just made back to where that
transition occurs at almost zero current.
You can download a copy of this
spreadsheet for use with your classes.
The energy transferred by an electron is
the product of its charge,
1.67x10-19 coulombs
and the potential difference
across which it moves. The value from
this graph corresponds with an energy
value of 3x10-19 joules
which students can convert to a
wavelength of around 650 nanometers: a
14% difference from the advertised 568
nanometers. Here the regression from
multiple measurements reduces random
errors but the value determined suffers
from a systematic error resulting from
the assumption that the LED is 100%
efficient.