Instructor Outline:
UM Physics Demo Lab 07/2013
The Wave Nature of Light: Interference and Diffraction
Lab length: 70 minutes
Lab objective: To instruct the students about the interference of light and the use of
diffraction as an experimental tool.
Materials
40-Watt “candle” lamp on wooden base
Power strip
Red Laser Levels – Carpenter Levels
Standard diffraction grating (d=1788 nm) white border
Bi-directional diffraction grating (colored border)
Calculator
2 Meter sticks
4 Medium binder clips (support legs for diffraction gratings)
Microscope (shared by class)
Introduction: 5 minutes - Lecture
The principles of linear superposition and interference are reviewed and the
diffraction through a grating is introduced.
Exploration: 15 minutes - Group Lab Work
The students measure the spacing of the diffraction grating using a LASER.
Application: 25 minutes – Group Lab Work
The students predict which color should be diffracted the most and then observe the
spectrum of a glowing filament through the grating. Next, they observe the
Hydrogen H-а spectral line and compute its wavelength based on their calibration of
the grating spacing. They then predict the pattern of diffracted LASER light produced
when the grating is oriented horizontally. They then predict the diffraction pattern
for LASER light passing through two crossed gratings and based on these
observations deduces the pattern of lines for the bi-directional grating.
Application: 25 minutes – Class Activity
The diffraction pattern from a helical lamp filament is projected for the class and
compared to the X-Ray diffraction pattern generated by DNA. The instructor leads a
discussion regarding the possible shape of the projected unknown, based on what
the class has learned regarding the orientation of grating lines and the resulting
LASER diffraction pattern. After predicting the structure of the unknown object, the
students observe the lamp filament under the microscope to see if their proposed
shape was correct. The relevance to the structure of DNA is discussed.
Suggested Demos:
Single slit diffraction slide for use with 532 nm laser projector.
Slinky (helical structure)
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
Concepts developed:
1. Light exhibits both wave-like and particle-like behaviors.
2. The principle of linear superposition says that the amplitudes of two or more
waves arriving at a point in space add at every instant of time. This gives
rise to interference, where two or more arriving waves add up to larger
amplitude, smaller amplitude or zero.
3. Waves which encounter structures small compared to their wavelengths
undergo diffraction whereby they bend around corners or obstacles.
4. Light waves projected through a slit narrower than the wavelength will
interfere to produce a pattern of alternately light and dark fringes.
5. A diffraction grating consists of many narrow parallel slits and disperses light
into its component wavelengths (colors). Each wavelength of light will diffract
to a specific angle after passing through the grating.
6. A diffraction grating can be used to measure the wavelength of atomic
emission lines. This application of diffraction is called spectroscopy.
7. Conversely, the diffraction of a monochromatic (one color—one wavelength)
beam of LASER light can be used to measure the spacing of a diffraction
grating.
8. Diffraction of monochromatic light from a LASER or X-Ray source can be used
to infer the size and structure of small objects, crystals and molecules. Small
structures produce large diffraction angles; large structures produce small
diffraction angles.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109