(The Dual Nature of Electromagnetic Radiation; Plank’s Constant) Thursday September 22, 2011 (The Dual Nature of Electromagnetic Radiation; Plank’s Constant)

What is electromagnetic radiation? Bell Ringer Thursday, 9-22-11 What is electromagnetic radiation? Explain why passing electricity though a tube filled with gas causes the gas to glow. Introduction of electrical energy causes the electrons in the gas atoms to jump up to higher energy levels. When these “excited” electrons fall back down to their original “ground state” energy levels, they emit the extra energy in the form of photons of light at a certain frequency and color, causing the glow. The entire range of radiation including, in order of decreasing frequency, gamma rays, x-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.

Assignment Currently Open Date of Notes on Website Notebook Page Date of Notes on Website Date Issued Date Due Late Date to Remediate Lab – Properties of Matter: Density of Metals TBD 9/7 9/9 Lab – Identifying Substances and Mixtures 9/13 Lab – Physical Properties of Matter 9/12 9/14 Lab – Chemical Properties of Matter 9/16 WS – Categories, Properties, and Phases of Matter 9/15 9/20 Test 1 9/30 WS – What’s in the Atom 9/19 Project – Atomic Structure N/A 9/26

Announcements Do you need to remediate Test 1? Are you working on your Atomic Structure project?

The Particle Description of Light A German physicist named Max Planck was studying the emission of light by hot objects. He proposed that a hot object does not emit electromagnetic energy continuously, as would be expected if the energy emitted were in the form of waves. Instead, Planck suggested that the object emits energy in small, specific amounts called “quanta.” A quantum is the minimum quantity of energy that can be lost or gained by an atom.

The Particle Description of Light Planck proposed the following relationship between a quantum of energy and the frequency of radiation. E = hν In the equation, E is the energy, in joules, of a quantum of radiation, v is the frequency of the radiation emitted, and h is a fundamental physical constant now known as Planck’s constant h = 6.626 × 10−34 J¢s.

The Particle Description of Light In 1905, Albert Einstein expanded on Planck’s theory by introducing the radical idea that electromagnetic radiation has a dual wave-particle nature. While light exhibits many wavelike properties, it can also be thought of as a stream of particles. Each particle of light carries a quantum of energy. Einstein called these particles “photons.”

The Particle Description of Light A photon is a particle of electromagnetic radiation having zero mass and carrying a quantum of energy. The energy of a particular photon depends on the frequency of the radiation. Ephoton = hv Einstein explained the photoelectric effect by proposing that electromagnetic radiation is absorbed by matter only in whole numbers of photons.

The Particle Description of Light In order for an electron to be ejected from a metal surface, the electron must be struck by a single photon possessing at least the minimum energy required to knock the electron loose. According to the equation Ephoton = hv, this minimum energy corresponds to a minimum frequency. If a photon’s frequency is below the minimum, then the electron remains bound to the metal surface. Electrons in different metals are bound more or less tightly, so different metals require different minimum frequencies to exhibit the photoelectric effect.

The Least You Need to Know Light (all EM radiation) has a dual nature. Light can travel as a wave or a particle. The wave and particle natures are related by Plank’s formula, E = hv.

… continue working on your Atomic Structure project with your partner. Homework … continue working on your Atomic Structure project with your partner.