Transcript 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
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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
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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 Diffraction
b. Circular hole --- Spherical waves
c. Rectangular slit --- Cylindrical waves
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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.
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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.
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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
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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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