Chemistry · Ch 4 — Structure of Atom
Particle Nature of Electromagnetic Radiation
Particle Nature of Electromagnetic Radiation
In 1900, Max Planck put forward his quantum theory to explain the spectrum of radiation given off by a heated ('black') body, a phenomenon the wave picture of light could not account for. Planck proposed that the energy of electromagnetic radiation depends only on its frequency, not on its amplitude, and that this energy can only be emitted or absorbed in discrete packets, each called a quantum — the smallest amount of energy that can be exchanged in that form of radiation. The energy E of a single quantum of radiation of frequency is , where h is Planck's constant, found experimentally to equal . In 1905, Albert Einstein used Planck's quantum idea to explain the photoelectric effect — the ejection of electrons from a metal surface when light of sufficiently high frequency strikes it — by treating electromagnetic radiation as a stream of particle-like …
Worked out. Worked example: visible light ranges from 400 nm (violet) to 750 nm (red); 1 nm = 10⁻⁹ m. Using c = νλ, so ν = c/λ. Frequency of violet light = (3×10⁸ m s⁻¹) ÷ (400×10⁻⁹ m) = 7.50×10¹⁴ Hz. Frequency of red light = (3×10⁸ m s⁻¹) ÷ (750×10⁻⁹ m) = 4.00×10¹⁴ Hz. …
Worked out. Worked example: blue light has wavelength 400 nm and red light 750 nm — which has higher energy? Since E = hν and ν = c/λ, a shorter wavelength gives a larger frequency and hence a higher energy. Blue light's shorter wavelength (400 nm, vs red's 750 nm) means blue light carries the higher energy per photon. …