Diffraction Topic 13.5

Outcomes You will describe, qualitatively, diffraction, interference and polarization You will describe, qualitatively, how the results of Young’s double-slit experiment support the wave model of light You will solve double-slit and diffraction grating problems using, You will compare and contrast the visible spectra produced by diffraction gratings and triangular prisms.

What is Diffraction? Diffraction is the change in shape and direction of a wave front as it encounters a small opening or corner of a barrier Poisson’s Bright Spot: Provided evidence for light as a wave As light bent around the spherical object, the waves had destructive and constructive interference. The bright spot was caused by constructive interference,

Huygens Principle (From Physics 20) Every point in a wave front is the source of a new wave When the opening is small compared to the wavelength, the new wave front is acting more like a point source A point source produces waves that spread out. From Physics 20 - Remember: Constructive Interference vs. Destructive Interference

Wave Interference Patterns Constructive interference occurs when a crest and crest or a trough and trough meet (in phase) Antinodal lines pass through points of constructive interference

Wave Interference Patterns Destructive interference occurs when a crest and trough meet (out of phase) Nodal lines pass through points of destructive interference.

Thomas Young & Interference Patterns In 1801, Thomas Young discovered that when monochromatic light passed through two narrow closely spaced openings, an interference pattern of bright and dark fringes was produced on a screen The bright fringe was the end of an antinodal line (constructive interference) and the dark fringe was the end of a nodal line (destructive interference)

Thomas Young & Interference Patterns *If the path difference is a whole number, constructive interference results (bright spot) *If the path difference is not a whole number, destructive interference occurs (dark spot)

Homework Concept Q’s: p. 697 # 1-4

DAY #2

Polarization Demonstrated the light travelled as a transverse wave. Since depolarized light vibrates in all planes at right angles to the direction of motion. When depolarized light is passed through a polarizing filter, only the waves vibrating parallel to the filters slits pass through. The light that emerges is said to be polarized. If light travelled in longitudinal waves, then all the light should pass through the polarizing filter

Comparing Diffraction Gratings and Prisms REFRACTION DUE TO PRISMS Longer  = more diffraction (see equation relationship between x and) Spectrum appears violet to red Multiple spectra (bright spots) are created Shorter  = greater refraction Spectrum appears red to violet Only one spectrum (red to violet) will be created

Calculations with Diffraction Gratings The wavelength of visible light can be calculated from an interference pattern using the following equations:

2nd order of maxima n=2 2nd nodal line n=1.5 1st order of maxima n=1 1st nodal line n=0.5 Central order of maxima n=0 1st nodal line n=0.5 1st order of maxima n=1 2nd nodal line n=1.5 2nd order of maxima n=2

Sample Problem Calculate the angle of deviation of the 2nd order maximum produced by directing monochromatic light of wavelength 4.30 x 10-7 m through a diffraction grating ruled with 2.00 x 105 lines/m.

Sample Problem Light falls on a pair of openings 1.28 x 10-5 m apart. The maxima are 4.11 x 10-2 m apart and the screen is 1.00 m from the slits. What is the wavelength of the light?

Sample Problem A student performing a diffraction experiment using light with a wavelength of 6.00 x 102 nm found that the distance between the 1st and 4th dark bands was 0.120 m on a screen placed 1.40 m away. What is the diffraction grating ruling?

Homework Calculation Q’s: p. 119 #1-16 Microwave lab – Calculate the speed of EMR