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Optics: Young’s Double Slit Experiment

Optics: Young’s Double Slit Experiment. 24-3. Thomas Young. In 1801, Thomas Young conducted an experiment demonstrating the wave nature of light. (Englishman) For 100 years Newton’s “Corpuscular” theory of light held sway. He was 28 years old.

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Optics: Young’s Double Slit Experiment

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  1. Optics: Young’s Double Slit Experiment 24-3

  2. Thomas Young • In 1801, Thomas Young conducted an experiment demonstrating the wave nature of light. (Englishman) • For 100 years Newton’s “Corpuscular” theory of light held sway. • He was 28 years old. • Learned to read at 2, knew Latin, Greek, French and Italian by 13 • Attempted to translate the Rosetta Stone (left the efforts worse off than when he started)

  3. Double Slit Experiment • Light from a single source (Young used the Sun) is shown on two slits spaced closely together. • IF light consists of tiny particles, then you’d expect to see two bright lines placed behind the slits as in (b) of the diagram to the left. • BUT Young observed a series of bright lines as in (c) of the diagram. Young EXPLAINED this result as a wave-interference phenomenon

  4. Double Slit Experiment • Imagine plane waves of light of a single λ, (monochromatic) falling on the two slits as shown here. • Because of diffraction, the waves leaving the 2 small slits spread out [sort of like 2 rocks thrown into a pond]

  5. Double Slit Experiment • How is the interference pattern made on the screen? What is a bright line? What causes the dark lines? • Waves of wavelength λare shown here entering the slits S1 and S2 which are a distance, d, apart

  6. Double Slit Experiment • Constructive interference occurs whenever the paths of 2 rays differ by any whole number of wavelengths • The waves reaching the center of the screen [θ= 0] have traveled the same distance so are IN PHASE and interfere constructively resulting in a bright area

  7. Double Slit Experiment • The waves spread out in all directions, we are focusing on only three different angles • IF one ray travels at a ½ λ[ or 3/2 λor 5/2 λand so on] therefore destructive interference occurs and the screen is dark

  8. Double Slit Experiment • To Determine WHERE the bright lines fall • the distance d between the slits is small compared to L the distancetothe screen • This makes the rays from each slit essentially parallel and θis the angle they make with the horizontal • From the shaded right triangle in (b), we can see that the extra distance traveled by the lower ray is • d sin θ= mλ, m = 0,1,2...

  9. Double Slit Experiment • The value of m is the order of the interference fringe • Destructive interference occurs when the extra distance • d sin θ= (m + ½)λ, m = 0,1,2... • The INTENSITY of the bright fringes is greatest for the central fringe (m = 0) and decreases for higher orders

  10. Double Slit Experiment • We can see that except for the zeroth-order fringe at the center, the position of the fringes depends on the • Consequently when white light falls on the 2 slits, as Young found in his experiments, the central fringe is white, BUT the first-and higher-fringes contain a spectrum of colors like the rainbow. • Young could then determine the wavelengths of visible light.

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