6.1 The Dual Nature of Light Chemistry Ms. Pollock

Introduction Understanding of behavior of electrons in electron cloud required revision of thinking about matter and energy Mechanics study of motions of objects under influence of forces Based on work of Isaac Newton in 1600s Quantum mechanics – study of behavior of matter at atomic and subatomic level; based on work of many scientists

Properties of Waves Different experiments with light = different indications about nature of light Energy waveforms (water/sound) exhibit certain characteristics ▪ Diffraction – bending of waves around corners ▪ Interference – adding or subtracting of energies when waves overlap

Properties of Waves

Interference ▪ Overlapping waves create new patterns ▪ Crests lining up or troughs lining up – amplitude increased; constructive interference ▪ Crests lining up with troughs – amplitude decreased; destructive interference

Light as a Wave Light able to undergo diffraction and interference Properties demonstrated with double slit experiment

Light as a Wave Light behaved like particles – slits to back of box as two bright spots Light behaved like waves – slits to back of box as bright spots with areas of constructive and destructive interference Light behaving like a wave

Light as a Particle Planck studying black body radiation Black body object that absorbs all light that falls on it – reflects no radiation and appears perfectly black Black body radiation – energy that would be emitted by ideal black body

Light as a Particle 1900 Planck published paper on electromagnetic radiation emitted from heated black object Radiation emitted in discrete bundles of energy called quanta Quantum – small unit into which certain forms of energy are divided

Light as a Particle Energy of quanta related to frequencies Energy calculated with Planck’s constant E = hƒ Photon – particle of light In discussion of light, photon and quantum are used interchangably. What is the frequency of a photon of light whose energy is 3.00 X joules? ƒ = E = 3.00 X J = h 6.63 X J ⋅ s E = 4.52 X Hz

Light as a Particle What is the energy of a photon whose frequency is 2.00 X s -1 ? E = hƒ E = (6.62 X J ⋅ s) ⋅ (2.00 X s -1 ) E = 1.33 X J Planck’s work basis for quantum theory Energy only able to exist in discrete amounts (quanta) Hard to measure since molecules are so small Observable in bond length expansion and contraction – only certain vibration levels available to molecules

Light as a Particle Photoelectric effect – phenomenon in which electrons are emitted from the surface of a material after the absorption of energy Light strikes metal surface with enough energy to knock off electrons

Light as a Particle Low frequency light unable to knock off electrons High frequency light able to knock off many electrons Need specific energy to remove electrons, since energy is quantized Source of Einstein’s Nobel Prize

Wave-Particle Duality Some evidence light is wave Some evidence light is particle Led to development of wave-particle duality of light Wave-particle duality – light travels as wave and interacts with matter like a particle Light traveling exhibits wavelength and frequency Light interacting exhibits quanta