Energia do Fóton (E=hf)

Photon energy from frequency: E = h·f

Frequency alone sets a photon's energy: E = h·f = h·c/λ, where h = 6.626·10⁻³⁴ J·s is Planck's constant and c = 3·10⁸ m/s. To go from joules to electronvolts, divide by 1.602·10⁻¹⁹. Take a visible photon at f = 5.45·10¹⁴ Hz (λ ≈ 550 nm, green); it carries E ≈ 3.6·10⁻¹⁹ J ≈ 2.25 eV. Push the frequency up into the UV and the photons get more energetic, enough to ionize matter. Drop it into the infrared and they carry less, mostly just heating things. Back in 1905 Einstein explained the photoelectric effect (which won him the Nobel in 1921) by showing that only photons above the work function knock electrons loose, and that light energy comes in quanta.

Applications

Solar panels, where silicon's 1.1 eV band gap pins the cutoff wavelength at roughly 1100 nm. Atomic and molecular spectroscopy. LEDs, whose emitted color tracks the semiconductor's band gap. You also find it behind CCD and CMOS sensors, lasers, photomultipliers and ionizing-radiation dosimetry.

FAQ

What's a typical visible-photon energy? It runs from about 1.7 eV (red, λ ≈ 700 nm) up to 3.1 eV (violet, λ ≈ 400 nm). Green at 550 nm lands near 2.25 eV.

Does a brighter beam mean each photon has more energy? No. Brightness only controls how many photons arrive per second. What each one carries still comes down to frequency, and that was Einstein's key insight.

Why does silicon's 1.1 eV band gap matter for solar cells? Any photon below 1.1 eV (infrared past 1100 nm) can't lift an electron across the gap, so its energy is simply lost as heat. That's what caps the efficiency of single-junction silicon.