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Introduction
This is a reproduction of an actual lab experiment used to obtain Planck's constant using light emitting diodes (LED). It is based on the semiconducting properties of diodes, which involves the quantization of energy. When a external electric potential provides electrons with a certain energy, they jump from the valence band, in the n- region of the diode, to the conduction band and over to the p-region of the diode. Following this, they tend to go back to a lower energy level, thus emitting the excess energy as photons. This certain amount of energy is provided by the potential threshold. LEDs have because of this a definite wavelength for the light they emit. And, with this wavelength and the threshold voltage, one may use modern physics relations to obtain the Planck's constant.
Procedures
Turn on main switch (above). Use individual LED switch (below).Use the potentiometer to regulate the voltage feeding the LED (middle left). Voltage and current readings will be displayed near the selected LED.
From I-V curve, get V0 (V threshold). Or, roughly, just get the potential with which the led begins to emit. It's tricky to decide when it really starts to glow in here.
One should get close to
h=6.63 . 10^-34 J.s
Then use simplified form (deduction below)
h = (5.35 . 10^-36) . ( V0 . lambda)
Where
V0 : V threshold (in Volts)
lambda : LED wavelength (in nm, already converted to m in the above formula)
e: electron charge (C)
lambda: led wavelength (m^-1)
c: speed of light ( m/s)
nm = 10^-9 m | X nm = X . 10^-9 m
e = 1.60217 . 10^-19 C
c = 2.99795 . 10^8 m/s
E = h.f ; E = e.V0 ; f = c / lambda
therefore,
eV0 = h.c/lambda
h = e .V0 . lambda / c
Eg: For VO=0.847 V and lambda=645 nm
h = (5.35 . 10^-36) . (0.85 . 645)
h = (5.35 . 10^-36) . (5.48 *10^2)
h = 5.48 * 10^-34 J.s
%error = 100 - (5.48/6.63)*100
%error = 17.5%
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