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diffraction

diffraction. Diffraction is the apparent ``bending'' of light waves around obstacles in its path http://theory.uwinnipeg.ca/physics/light/node7.html. This bending is due to Huygen's principle This priciple states that all points along a wave front act as if they were point sources.

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diffraction

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  1. diffraction Diffraction is the apparent ``bending'' of light waves around obstacles in its pathhttp://theory.uwinnipeg.ca/physics/light/node7.html. This bending is due to Huygen's principleThis priciple states that all points along a wave front act as if they were point sources. Thus, when a wave comes against a barrier with a small opening (a slit), all but one of the effective point sources are blocked, and the light coming through the opening behaves as a single point source, so that: the light emerges in all directions, instead of just passing straight through the slit.or sizeable diffraction effects to occur the width of the opening must be of the same order or less than the wavelength of the light used.

  2. Young's Double Slit Experiment This is a classic example of interference effects in light waves. Two light rays pass through two slits, separated by a distance d and strike a screen a distance, L , from the slits if d < < L then the difference in path length r1 - r2 travelled by the two rays is approximately: r1 - r2 ± d sin q

  3. If the rays were in phase when they passed through the slits, then the condition for constructive interference at the screen is: dsinq = ml , where m = ±1, ±2,... whereas the condition for destructive interference at the screen is: dsinq = (m +.0.5 )q ,m = ±1, ±2,... The points of constructive interference will appear as bright bands on the screen and the points of destructive interference will appear as dark bands. These dark and bright spots are called interference fringes. Note:

  4. In the case that y , the distance from the interference fringe to the point of the screenopposite the center of the slits is much less than L ( y < < L ), one can use the approximate formula: sinq ±= y/Lso that the formulas specifying the y - coordinates of the bright and dark spots, respectively are: yBm = mlL/d brightspots yBm = (m + 0.5) lL/d darkspots The spacing between the dark spots is Dy = lL/d

  5. • If d < < L then the spacing between the interference can be large even when the wavelength of the light is very small (as in the case of visible light). This give a method for (indirectly) measuring the wavelength of light. (See Ex.6 at the end of this Chapter.) • The above formulas assume that the slit width is very small compared to the wavelength of light, so that the slits behave essentially like point sources of light. http://theory.uwinnipeg.ca/physics/light/node9.html

  6. in phase Two waves (of the same wavelength) are said to be in phase if the crests (and troughs) of one wave coincide with the crests (and troughs) of the other

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