Download Major Concepts of Physics PHY102 – Lecture #5

Major Concepts of Physics PHY102
Lecture #5
How do we squeeze light through
narrow holes? - Diffraction
January 28th
Spring 2015
Prof. Liviu Movileanu
www.physics.syr.edu/MajorConceptsPhysics2015.html
[email protected]
Room 211, Physics Bldg., 443-8078
Major Concepts of Physics PHY102 – Lecture #5
2015Syracuse University
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Lecture objectives
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
Major Concepts of Physics PHY102 – Lecture #5
2015Syracuse University
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Introduction to Diffraction
Major Concepts of Physics PHY102 – Lecture #5
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Diffraction – The fundamental condition
a. Pertinent to geometrical optics -- d >> 
b. Pertinent to wave optics
--   d
d
Major Concepts of Physics PHY102 – Lecture #5
2015Syracuse University
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Introduction to Diffraction
b. Circular hole --- Spherical waves
c. Rectangular slit --- Cylindrical waves
Major Concepts of Physics PHY102 – Lecture #5
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Diffraction patterns
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Grimaldi’s work
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 spot was enlarged, relative to expectations from
a particle model of light.
He examined the border of the image.
Observed: Colored fringes.
Grimaldi’s work was the first experimental evidence that light
is a wave.
Major Concepts of Physics PHY102 – Lecture #5
2015Syracuse University
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Diffraction: a brief review
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.
Major Concepts of Physics PHY102 – Lecture #5
2015Syracuse University
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Major Concepts of Physics PHY102 – Lecture #5
2015Syracuse University
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Major Concepts of Physics PHY102 – Lecture #5
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Announcement
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(a/2)sin=m(/2) first minimum
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d  5a
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Rayleigh’s criterion
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Observation of diffraction
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Example 25.8 – Continuation
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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 Crick
Major Concepts of Physics PHY102 – Lecture #5
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Announcements
1. Reading: Chapter 25th, Sections 25.6, 25.7, 25.8, 25.9
(pages 962-971). Conceptual examples 25.7, 25.8 and
25.9
2. Homework for the next week (on paper): Please pick up
the print outs today (or see the class website). The
homework #2 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
Problems 25.8 Diffraction and the resolution of optical
instruments
4. This week’s lab: Interference
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