Electromagnetic Radiation Glenn V. Lo Department of Physical Sciences Nicholls State University

Light Experiments dealing with light were critical to the development of modern atomic theory. Blackbody radiation, photoelectric effect, Compton effect Light is also known as “electromagnetic radiation”

Wave Model Behavior of light (reflection, refraction, etc.) are well-explained by thinking of light as an electromagnetic wave. Wave = a repeating disturbance that travels through space; transports energy from one place to another Light waves travel at 300,000 km/s, or 3x108 m/s.

EM wave What’s traveling? Electric field and magnetic field; affects charged particles. E, electric field = electric force (per unit charge) that a charged particle would “feel” at a given location; the “thing” that is oscillating B, magnetic field, the other thing that oscillates perpendicular to E. http://en.wikipedia.org/wiki/Image:Onde_electromagnetique.svg

EM wave Speed = 300,000 km/s, or 3x108 m/s = “c” Wavelength (represented by Greek letter lambda) = distance traveled per cycle = “color” Frequency (represented by Greek letter nu) = number of cycles passing through a point per second Amplitude = “Eo”, maximum value of electric field, is related to the intensity of the radiation; Intensity=energy delivered per unit area Eo

Electromagnetic Spectrum Visible light: 400-800 nm 1 nm = 10-9 m x-rays, ultraviolet infrared, microwave, radiowave Longer wavelength Lower frequency

The Particulate Model The notion of light as an electromagnetic wave is referred to as a “wave model” Problem: Towards the end of 19th century, several experimental phenomena were discovered that could not be explained by the wave model. Solution: imagine light as a stream of particles, called “photons”

Photons Energy of photon = proportional to frequency of the light. Proportionality constant (h) is called “Planck’s constant” h = 6.63x10-34 J s Higher frequency  More energetic photons

Effects of Light Absorption Light absorption leads to increase in energy of atom or molecule X-rays: photons energetic enough to remove electrons from atom (“ionizing radiation”) Ultraviolet: photons energetic enough to “excite” electrons. Infrared: photons too weak to affect electrons, but energetic enough to stimulate vibration of atoms in a molecule. Increased vibration  increase temperature.

Effects of Light Absorption Light absorption leads to increase in energy of atom or molecule Microwave: photons too weak to even stimulate vibration, but energetic enough to stimulate rotation of molecules. Microwave ovens work well on food with high water content; tuned to stimulate rotation of water molecules. Radiowaves: used to transmit radio, television, and cell phone signals. Also used in magnetic resonance imaging “MRI” machines. Very weak photons. In MRI machines, photons cause nucleus to change “spin”

Web Resources http://antoine.frostburg.edu/chem/senese/101/quantum/index.shtml#Lecture%20outline http://www.colorado.edu/physics/2000/waves_particles/index.html