Chapter 19Chapter 19

• Interference and DiffractionInterference and Diffraction

Diffraction is…Diffraction is…

• the bending of waves, esp. sound and the bending of waves, esp. sound and light waves, around obstacles in their light waves, around obstacles in their path.path.

Interference is…Interference is…

• electromagnetic waves of the same electromagnetic waves of the same frequency combine to reinforce or frequency combine to reinforce or cancel each othercancel each other

• the amplitude of the resulting the amplitude of the resulting wave being equal to the sum of the wave being equal to the sum of the amplitudes of the combining wavesamplitudes of the combining waves

DiffractionDiffraction

• The Dutch scientist Christian Huygens The Dutch scientist Christian Huygens (1629-1695) proposed a wave model to (1629-1695) proposed a wave model to explain diffraction. These wavelets explain diffraction. These wavelets expand in every direction and are in expand in every direction and are in step with one another. Light sources step with one another. Light sources consist of an infinite number of points, consist of an infinite number of points, which generate a plane wave front.which generate a plane wave front.

http://demonstrations.wolfram.com/HuygenssPrinciple/

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• Young could measure light's Young could measure light's wavelengths and provided additional wavelengths and provided additional evidence of the waves nature of light. evidence of the waves nature of light. By directing the beam of light at two By directing the beam of light at two narrow, closely spaced slits in barrier, narrow, closely spaced slits in barrier, light diffracted and the two rays light diffracted and the two rays overlapped. This created light and dark overlapped. This created light and dark bands.bands.

Young’s two-slit Young’s two-slit experimentexperiment

Cheng, Poon S., Mrs. "Chapter 4: Young’s Double Slit Experiment." Singapore A Level Physics Syllabus. N.p., n.d. Web. 6 Feb. 2011.<http://h2physics.org/>.

• Constructive interference produces a bright Constructive interference produces a bright central band and other bands on either side. central band and other bands on either side. Dark areas are where destructive Dark areas are where destructive interference occurs between bright bands. interference occurs between bright bands.

• White light in double slit diffraction causes White light in double slit diffraction causes colors instead of light/dark bands. Positions colors instead of light/dark bands. Positions of constructive/destructive bands depends of constructive/destructive bands depends on light's wavelength.on light's wavelength.

Diffraction of white lightDiffraction of white light

Constructive: positive growth.

Destructive: negative growth.

Constructive Interference. Tipler, Paul A. Physics for Scientists and Engineers. 4th Ed. New York: W.H. Freeman and Company, 1999.Destructive Interference. Tipler, Paul A. Physics for Scientists and Engineers. 4th Ed. New York: W.H. Freeman and Company, 1999. 

Red LightBlue Light White Light

White Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.Red Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.

Karen Cooper

Karen Cooper

Equation: Wavelength Using Equation: Wavelength Using Double Slit InterferenceDouble Slit Interference

• Distance between P0 and P1: Distance between P0 and P1: xx

• Distance between screen and Distance between screen and slits: slits: LL

• Separation of two slits: Separation of two slits: dd

• Equation: Equation: xx//LL = λ/ = λ/dd

• Wavelength Using Double-Slit Wavelength Using Double-Slit Interference: λ = Interference: λ = xdxd//LL

Single-Slit Single-Slit DiffractionDiffraction

• Light passes through a single slit's small Light passes through a single slit's small opening. This creates a central band with opening. This creates a central band with dimmer bands on either side.dimmer bands on either side.

Red Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.

From one to many slitsFrom one to many slits

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Measuring a Measuring a wavelength of lightwavelength of light

• The slit is illuminated, so a central The slit is illuminated, so a central bright band appears at Po on bright band appears at Po on screen.screen.

• The dark band from the central The dark band from the central light band create a situation similar light band create a situation similar to the double-slit interference.to the double-slit interference.

• Paths differ by λ/2 and separation Paths differ by λ/2 and separation between slits is between slits is ww/2 instead./2 instead.

White Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.

Red Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.

Karen Cooper

Equation: Distance Between Equation: Distance Between Central and 1Central and 1stst Dark Bands Dark Bands

• xx = = λLλL//ww

• Distance between slits and Distance between slits and screen: screen: LL

• Distance between Po and P1: Distance between Po and P1: xx• Wavelength: Wavelength: λλ• Width of a single slit: Width of a single slit: ww

Diffraction GratingsDiffraction Gratings

• Diffraction gratings are used to measure Diffraction gratings are used to measure the wavelength of light. Diffraction the wavelength of light. Diffraction gratings are made by scratching very gratings are made by scratching very fine lines on glass using the point of a fine lines on glass using the point of a diamond. The spaces between the diamond. The spaces between the scratches are like slits.scratches are like slits.

• EX: jewelry creates spectrum like that on EX: jewelry creates spectrum like that on the surface of a CD.the surface of a CD.

CD. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb. 2011. <http://www.itp.uniannover.de/~zawischa/I TP/multibeam.html>.

Diamond. N.d. Colour Museum. N.p., 2009. Web. 7 Feb. 2011. <http://www.coloure.experience.org/index.htm>.

• The interference pattern produced by The interference pattern produced by diffraction grating has bright bands cause diffraction grating has bright bands cause by a double-slit. Individual colors can be by a double-slit. Individual colors can be distinguished more easily with this and distinguished more easily with this and wavelengths can be measured more wavelengths can be measured more precisely in diffraction grating then precisely in diffraction grating then double-slits.double-slits.

Equation: Wavelength Using a Equation: Wavelength Using a Diffraction GratingDiffraction Grating

• λ = λ = xd/L = d sin xd/L = d sin θθ

• Distance between P0 and P1: Distance between P0 and P1: xx

• Distance between screen and slits: Distance between screen and slits: LL

• Separation of two slits: Separation of two slits: dd

• Wavelength: Wavelength: λλ

• Phase separation: Phase separation: θθ

Ng., H. K., Dr. Diffraction Grating Spectrometer. N.d. N.p., 30 Dec. 1996. Web. 8 Feb. 2011.<http://www.physics.fsu.edu/users/ng/courses/phy2054c/ labs/expt09/expt-09.htm>.