The origin of quantum physics
Every hot body radiates; its spectrum is continuous and its maximum depends on temperature. Wien's displacement law: λ_m·T = b, b = 2.898·10⁻³ m·K; the hotter the body, the more the maximum shifts to short wavelengths (a glowing metal goes from red to white). At the end of the 19th century classical physics could not explain the shape of this spectrum: the Rayleigh–Jeans formula agreed with experiment at long wavelengths but predicted infinite radiation at short ones (the «ultraviolet catastrophe»). In 1900 Planck proposed that radiation and absorption exchange energy not continuously but in portions, quanta; the energy of one quantum is proportional to the frequency: E = hν, with h = 6.63·10⁻³⁴ J·s the Planck constant. Since ν = c/λ, E = hc/λ; in practice hc ≈ 1240 eV·nm is handy (1 eV = 1.6·10⁻¹⁹ J). Planck's hypothesis gave a spectrum that matches experiment and was the beginning of quantum physics.
In your notebook use 1240 eV·nm to calculate the photon energies of red (700 nm), green (550 nm) and violet (400 nm) light in eV and put them in order. Do not heat metal at home to observe it; do the calculation instead.