Lecture 17 Diffraction Chapter 24.6  24.9 Outline Diffraction and Diffraction Grating Polarization of Light

Diffraction Diffraction is deviation of light from a straight-line path. Diffraction occurs when light pass through small openings, bends around obstacles, meets sharp edges. Two major types of diffraction: Fresnel and Fraunhofer. Fraunhofer diffraction requires a large distance between the slit(s) and the viewing screen as well as parallel sources of light (special case).

Single Slit Diffraction Demonstration L a  x =  sin  =  2 2 Condition for a dark fringe Destructive interference: sin d = m/a Width of the central maximum: 2L/a

Diffraction Gratings The diffraction grating is a multiple-slit (equally-spaced) system to analyze light. Gratings are used in spectroscopy, CD tracking, etc. A typical grating contains thousands of lines (slits) per millimeter. d sin br = m  For bright fringes

Problem with Grating Problem: A diffraction grating with 104 lines/cm exhibits the 1st order maximum for =510 nm at what angle? 1 nm = 109 m d = 104 cm m = 1 d sin br = m  sin br = 0.51  br = 30.7 degrees

Polarization of Light Individual atoms produce their own orientation of the electric field (E) vector in space. The resultant EM wave from a multiple-atom source is a superposition of individual waves with a random orientation of E vector. Such light is called unpolarized. Light is linearly polarized if the resultant electric field vibrates only in one direction. Three processes to polarize light: selective absorption, reflection, and scattering.

Summary Diffraction is the property of EM waves to bend around obstacles. Unpolarized light has the electric field vectors vibrating in any direction, while linearly polarized light has the electric field vector vibrating only in one direction.