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# 9diff 090515034704-phpapp02

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### 9diff 090515034704-phpapp02

1. 1. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS Interference pattern (“fringes”) MONOCHROMATIC LIGHT SOURCE Single slit acts as a single monochromatic source for the double slit Double Slits Beams interfere in region of overlap
2. 2. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS <ul><ul><li>The monochromatic source is a source of a single wavelength. A sodium lamp is usually used. </li></ul></ul><ul><ul><li>The single slit diffracts and so illuminates both of the double slits. </li></ul></ul><ul><ul><li>It and the first slit can be replaced by a laser as it is a monochromatic, coherent source of light. </li></ul></ul><ul><ul><li>The interference pattern formed will only occur in the area of overlap. </li></ul></ul>
3. 3. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS w = the distance between adjacent fringes (fringe width) s = the distance between slits (slit width) D = distance from slits to screen Optical path distance b s This is too w D
4. 4. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS DOUBLE SLIT INTERFERENCE YOUNG’S SLITS and for small angles For a bright fringe b=  (or some multiple of it) so w/D=s/  ie Optical path distance b s This is too w D
5. 5. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS <ul><li>Note that so </li></ul><ul><ul><ul><li>since  at the red end of the spectrum is larger </li></ul></ul></ul><ul><ul><ul><li>for larger  then w will be larger </li></ul></ul></ul><ul><ul><ul><li>i.e. blue fringes will be closer together than red fringes </li></ul></ul></ul><ul><ul><ul><li>with white light: </li></ul></ul></ul><ul><ul><ul><ul><li>the central fringe will be white with red edges </li></ul></ul></ul></ul><ul><ul><ul><ul><li>the other fringes will be spectra with the blue end towards the middle of the overlap area </li></ul></ul></ul></ul>What would the fringes look like if we used a white light source?
6. 6. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS You must write out your calculations!
7. 7. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS
8. 8. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS
9. 9. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS
10. 10. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS Note that you can increase W by: <ul><ul><ul><li>increasing D the distance from the slits. This makes measurement easier and more accurate but the fringe intensity is decreased. </li></ul></ul></ul><ul><ul><ul><li>decreasing s the slit width but there is a practical limit to this ( the slides readily available now for light have a 1mm slit separation ). </li></ul></ul></ul><ul><ul><ul><li>Increasing  the wavelength. </li></ul></ul></ul>
11. 11. DOUBLE SLIT INTERFERENCE YOUNG’S SLITS The End
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