Major Concepts of Physics PHY102 – Lecture #8 12016Syracuse University

Lecture #8How do we squeeze light through narrow holes? -

Diffraction

February 17th Spring 2016

Prof. Liviu Movileanuhttp://movileanulab.syr.edu/MajorConceptsPhysics2016.html

lmovilea@syr.edu

Room 211, Physics Bldg., 443-8078

Major Concepts of Physics PHY102

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1. Introduction to diffraction. Huygens’s principle Big deal! What is the fundamental condition for wave

optics (interference, diffraction)? is comparable with a (size of obstacles).

2. Single-slit diffraction Experimental hypothesis Single-slit experiment (Experimental evidence) Diffraction patterns (Interpretation) Compare interference with diffraction patterns

3. Lecture demonstration (single-slit interference)

4. Diffraction and the resolution of optical instruments

5. Announcements/Homework

Lecture objectives

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Introduction to Diffraction

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Diffraction – The fundamental condition

a. Pertinent to geometrical optics -- d >> b. Pertinent to wave optics -- d

d

 

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Introduction to Diffractionb. Circular hole --- Spherical waves c. Rectangular slit --- Cylindrical waves

               

Diffraction patterns

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On close examination of the edges, we find: There are alternate regions of darkness and brightness.  First known discovery of diffraction:Observations by Francesco Grimaldi, a Jesuit priest, (1618 – 1663).  He looked at sunlight entering a darkened room through a tiny hole in a screen.  Found: The light spot was enlarged. He examined the border of the image.

Observed: Colored fringes.

Grimaldi’s work was the first experimental evidence that light is a wave.

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Grimaldi’s work

From an experimental point-of-view, there are two signposts of diffraction: 

When light goes through a small opening, it spreads out like a fan.There is no sharp shadow. Instead, the image is blurred at its edges. 

The same statements hold when light encounters an obstacle. Again, there is no sharp shadow.

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Diffraction: a brief review

      

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Announcement

     

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(a/2)sin=(/2) first minimum

     

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ad 5

Double-slit interference and diffraction

d is the distance between slits

a is the slit size

First diffraction minimuma sin =

(d/5) sin =

     

Major Concepts of Physics PHY102 – Lecture #82016Syracuse University

Rayleigh’s criterion

14

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Example 25.8

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Example 25.8 – Continuation

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Example 25.9

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Applications of diffraction: X-ray diffraction

Rosalind Frankin (1953)

James Watson and Francis CrickDNA structure

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1. Reading: Chapter 25th, Sections 25.6, 25.7, 25.8, 25.9 (pages 955-964). Conceptual examples 25.7, 25.8 and 25.9

2. Homework for the next week (on paper): The homework #3 is due on next week’s workshop.

3. Do you want to be challenged? More MCAT preps?Problems 25.7 Diffraction by a single slit

4. This week’s lab: Diffraction

Announcements