2. The Quantum Theory
There are five major ideas represented in
the Quantum Theory:
Energy is not continuous, but comes in small but
discrete units.
The elementary particles behave both like particles
and like waves.
The movement of these particles is inherently random.
It is physically impossible to know both the position
and the momentum of a particle at the same time. The
more precisely one is known, the less precise the
measurement of the other is.
The atomic world is nothing like the world we live in.
3. Planck’s Equation
E=hc/λ=eV0
Where:
E is the energy of the photon (Joules)
λ is the wavelength of light (nanometers)
8
c is the speed of light (2.998 x10 meters per
second)
h is Planck’s constant (6.626 x 10-34 J-s)
e is the charge of an electron (1.6022 x10-19
Coulomb)
V0 is the threshold voltage for the LED (Volts)
Linear equation (y=mx+b): V0 = (hc/e)(1/ λ)
4. The Methods
Method One is looking at the LED for the first sign of light.
Turning the potentiometer gradually increases the voltage
supplied to the LED. The threshold voltage is the drop in
voltage across the LED when light first becomes visible.
Method Two measures the threshold voltage when current
begins to flow through the LED. The supplied voltage is
increased gradually until current begins to flow, which is
measured by the voltage drop across a resistor. The
threshold voltage is the drop in voltage across the LED when
voltage is greater than zero. I will use a value of 0.3
millivolts.
In Method Three, I will graph voltage drop across the entire
range of the supplied voltage. The threshold voltage will be
measured by drawing a straight line through the last few
points and continuing the line until it crosses the x-axis.
5. Description of the Three
Methods
3500
3000
Y-axis is the voltage
measured at point a. This
is proportional to the
current flow through the
LED.
2500
Voltage a (Millivolts)
2000
X-axis is the voltage
between point b and
point a. This is the
voltage drop across the
LED.
1500
Method One: Measure
diode voltage when the
light is first visible.
Method Three: Draw a
straight line between
the last four points, and
continue the line to the
x-intercept.
1000
Method Two: Measure
diode voltage when Va
equals 0.3 millivolts.
500
0
1
-500
1.1
1.2
1.3
1.4
1.5
1.6
Diode Voltage, Vd (Volts)
1.7
1.8
1.9
2
6. Purpose/Variables
The purpose of this experiment is to measure
Planck’s constant and to compare the
accuracies of three methods used to measure it.
Variables:
Independent: wavelength of LED and supplied
voltage
Constant: wavelength of the LED
Dependent: voltage drop across the LED and
voltage drop across the resistor
7. Hypothesis
I hypothesize that Method Two will be the
most accurate for measuring Planck’s constant.
All the data required will come directly from the
two multimeters, so human error should not
present a problem. I think that Method One will
be less accurate because it calls for human
judgment as to when the LED first begins to emit
light. In Method Three, a line needs to be made
from the last few points of the Va and Vb-Va
graph, which I believe will be curved like an
exponential graph. The curve will make choosing
points to make a line difficult and likely cause this
method to be less accurate.
8. Materials
Wood base
Battery holder
D-cell battery
Wire
Wire strippers
Solder
Soldering iron
Hot glue gun
Potentiometer
Switch
Multimeters
Banana clip
Wire staples
Resistors
LEDs with various
wavelengths
LED mount
Microsoft Excel
Blue: 468 nm
Green: 574 nm
Yellow: 588 nm
Amber Yellow: 595 nm
Yellow-Orange: 611 nm
Red-Orange: 621 nm
Red: 632 nm
Super Red: 639 nm
Infrared: 940 nm
Infrared: 880 nm (not
shown)
