State the main features of the photon picture of electromagnetic radiation. Find the energy of a photon of wavelength 663 nm.

1. 1 · Introduce the photon. When radiation interacts with matter, it behaves as if made of particles called photons.
2. 2 · Energy and momentum. E = hν and p = h/λ; photons move at speed c.
3. 3 · Intensity. At one frequency, every photon has the same energy; brighter light has more photons per second.
4. 4 · Charge and conservation. Photons are neutral and not deflected; energy and momentum are conserved in collisions, but photon number is not.
5. 5 · Do the numerical.
   • E = hc/λ = (6.63 × 10⁻³⁴ × 3 × 10⁸) ÷ (6.63 × 10⁻⁷) J
   • E = 3 × 10⁻¹⁹ J ≈ 1.88 eV
6. 6 · Real life. Ultraviolet photons can cause sunburn; red photons cannot.
7. 7 · Close in one line. So, light carries energy in packets of hν.

Scoring tip: Write E = hν and p = h/λ first, then the numbers with units.