Acknowledgement
I would like to express my heartfelt gratitude to my Physics teacher, Mrs. Jayalakshmi, for her valuable
guidance, constant encouragement, and support throughout the completion of this project. I am also
thankful to the Principal and the school administration of Army Public School Bolarum for providing me
with the necessary resources and facilities to carry out this investigation successfully.
Aim of the Project
To determine Planck’s constant (h) using the threshold voltages of Light Emitting Diodes (LEDs) of
different colors and relate their electrical and optical characteristics.
Theory
When an LED just begins to emit visible light, each electron crossing the p–n junction loses energy equal
to that of one photon of light:
eV = hf
where e is the electron charge, V is the threshold voltage of the LED, h is Planck’s constant, and f is the
frequency of emitted light given by f = c/λ, where c is the speed of light.
Rewriting, V = (h/e)f + V₀, where V₀ is a small constant representing contact potential. A graph between
V (on the y-axis) and f (on the x-axis) is a straight line with slope h/e, so h = e × slope.
Each LED emits light when electrons recombine with holes across its band gap Eg, such that Eg = eV = hf.
The band gap is larger for higher-frequency (blue/violet) LEDs and smaller for red/yellow ones.
