Light Emitting Diode

When electrons and holes recombine in a direct-band-gap semiconductor, they emit photons. The colour depends on the band gap, which also sets the forward voltage:

Colour	Typical $V_F$
Infrared	1.2–1.5 V
Red	1.8–2.2 V
Yellow/green (old)	2.0–2.2 V
Blue, white, true green	2.8–3.4 V

Series resistor:

$R = \frac{V_{supply} - V_F}{I_F}$

Example: 5 V supply, red LED ($V_F$ = 2 V), 10 mA → $R = 300$ Ω (use 330 Ω). Never connect an LED directly to a voltage source — its current rises exponentially and it will burn out. Long leg = anode (+).