OS-4 Diffraction and Interference

Chapter 31

Review from OS 3• Superposition-The adding of waves• Constructive interference-Two crests meet or 2

troughs meet– Increase in amplitude– Anti-nodal– Produces bright bands of light

• Destructive interference-A crest meets a trough– Decrease amplitude or cancellation (NO amplitude)– Nodal– Produces dark bands of light

Antinodal and Nodal

• Antinodal line-Antinodes line up and form a line– Constructive interference

• Nodal line-Nodes line up to form a line– Destructive interference

Review Cont.

• Frequency-How OFTEN waves pass a point (waves/sec)

• Wavelength-The length of one complete crest and trough

• As wavelength increases, frequency decreases (and vice versa)

Diffraction

• Diffraction-The bending of waves around an object or through an opening– Occurs for all waves

Diffraction

• The MOST diffraction takes place when the opening or obstacle is small compared to the wavelength of the wave (ex. Sound passing through a doorway)

• So a longer wavelength=more diffraction• If opening is wide compared to the

wavelength, the wave will NOT diffract much (ex. Light)

How to increase diffraction

• Decrease the size of the opening (for single slit/opening)

• Decrease the distance between openings (for double slit)

• Longer wavelength (Remember as wavelength increases, frequency decreases)

Electromagnetic Waves

• All electromagnetic waves are the same type but vary according to wavelength.

Electromagnetic Spectrum

Young’s Experiment

• 1801-Thomas Young performed Young’s Experiment

• Greatly strengthened the wave theory of light

Young’s Experiment• Young passed light through a pinhole • The spot on the 2nd screen was larger than the original

opening (showed diffraction)• On the 2nd screen he added 2 small openings close

together• On the 3rd screen he saw a series of light and dark

regions (an interference pattern-wavesoverlapping and interfering with each other)

Single Slit vs. Double Slit• Single slit interference patterns have a wide,

bright central band and the side bands are narrower and not as bright

• Double slit interference patterns-all of the bands are equally spaced and equally bright

Simulations for Young’s experiment

• Colorado site http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html

• JAVA http://www.ub.edu/javaoptics/applets/YoungEn.html• Hawaii

http://www.phys.hawaii.edu/~teb/optics/java/slitdiffr/index.html

• Water simulation http://www.acoustics.salford.ac.uk/feschools/waves/diffract3.php

Spectral analysis

• Another use of diffraction to help identify elements present in charged gases

• Colorado explained http://www.colorado.edu/physics/2000/quantumzone/index.html

• Spectral simulation http://phys.educ.ksu.edu/vqm/html/emission.html

• Every elements spectral line http://chemistry.bd.psu.edu/jircitano/periodic4.html

Hologram

• Hologram-Interference pattern that is recorded on film

• Creates a 3-D image when light reflected off an object interferes with incoming light

• Used in forgery prevention, data storage, licenses, passports, money

Hologram

• Holograms are made using laser light. Why?• Laser light is monochromatic and coherent

– Monochromatic-one color– Coherent-light having the same frequency,

wavelength and phase and traveling in the same direction

Normal white light is not monochromatic or coherent!

Examples of Diffraction

• Sound diffracting http://www.acoustics.salford.ac.uk/feschools/waves/diffract.php#diffraction

• Various effects (scrool down to radio waves) http://www.acoustics.salford.ac.uk/feschools/waves/diffract2.php#radiotv

• Acoustic room design http://www.acoustics.salford.ac.uk/feschools/waves/diffract4.php#roomDesign

MATH

• Example: Light having a wavelength of 6.2 X 10-7 meters is projected on a screen. Three bright and three dark bands are observed on a screen 2.0 m away. The 3 pairs of bands measure 4.8 cm. What is the distance between the slits?

MATH

• Required Problems #11• Work on all homework!!!