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- 1. Interference
- 2. Superposition in water <ul><li>Create your own wave superposition in your tub. </li></ul><ul><li>Take a photograph and print off a picture </li></ul><ul><li>Show on the picture </li></ul><ul><li>where the waves have a supercrest, </li></ul><ul><li>where they have a supertrough </li></ul><ul><li>Where a crest meets a trough and the resultant displacement is zero </li></ul>
- 3. Hearing superposition 1m Same frequency from Signal generator Walk along a path that is one metre away What do you hear?
- 4. Today you will... <ul><li>Find out about interference patterns </li></ul><ul><li>and answer these questions </li></ul><ul><li>What are coherent sources? </li></ul><ul><li>What is the general condition for the formation of a bright fringe? </li></ul><ul><li>Are Young’s fringes equally spaced? </li></ul><ul><li>What factors could be (i) increased or (ii) decreased, to increase the fringe spacing </li></ul><ul><li>Why are slits used, rather than two separate light sources, in Young’s double slits experiment? </li></ul><ul><li>What are the roles of diffraction, and interference, when producing Young’s fringes? </li></ul><ul><li>What do we mean by diffraction? </li></ul><ul><li> What feature of two waves must combine in order to produce reinforcement? </li></ul><ul><li>What is the phase difference between two waves if they produce maximum cancellation? </li></ul><ul><li>Why is total cancellation rarely achieved in practice? </li></ul>
- 5. Thomas Young <ul><li>Newton’s view that light was made from particles was questioned by young in his ‘Young’s double slit’ experiment. </li></ul><ul><li>The experiment showed that light had wave properties because they formed interference patterns. </li></ul><ul><li>Interference patterns happen for light, water and sound. </li></ul>http:// www.youtube.com/watch?v =DfPeprQ7oGc
- 6. The Interference Pattern
- 7. The laser interference <ul><li>Laser light is highly monochromatic </li></ul><ul><li>(only gives out 1 frequency of light) </li></ul><ul><li>The emitted light is also coherent </li></ul><ul><li>(of the same phase) </li></ul>This gives a very sharp image
- 8. What are the factors that affect fringe separation? Fringe separation = distance between bright lines
- 9. Fringe separation w = separation of the fringes (bright to bright or dark to dark) λ = wavelength of the light s = separation of the two slits D = distance between slits and screen w D s W = λD S
- 10. Young’s Questions <ul><li>Finding the wavelength of sodium light </li></ul><ul><li>In a two-slit apparatus the slits are 0.3 mm apart. Fringes in sodium are observed at a distance of 1.2 m from the slits. The separation of the fringes is 2.4 mm. </li></ul><ul><li>1. What is the wavelength of sodium light? </li></ul><ul><li> 2. The same light gives a fringe separation of 3.6 mm with a different pair of slits. What is the slit separation if the distance between the slits and the fringes is the same? </li></ul><ul><li> </li></ul><ul><li> Red light of wavelength 7.0 x 10 –7 m is shone at right angles through two slits of separation 0.3 mm. Fringes are formed at a distance of 1.3 m from the slits. </li></ul><ul><li>3. What is the fringe spacing? </li></ul><ul><li> 4. The same light gives a fringe spacing of 2 mm when passed through a different pair of slits. What is the slit separation if the distance between the slits and the fringes is the same? </li></ul><ul><li> In a two-slit apparatus the slits are 0.3 mm apart. White light passes through the slits and fringes are observed at a distance of 2 m from the slits. Red light has a wavelength of 700 nm and blue light has a wavelength of 400 nm. </li></ul><ul><li>5. Calculate the fringe spacing for each colour. </li></ul><ul><li> 6. Use your answers to explain the coloured fringes seen on the screen. </li></ul>W = λD S
- 11. Young’s answers
- 12. Ripple Tank
- 13. The geometric analysis of Young’s experiment s
- 14. What is the use of knowing θ ? For it to be in phase (first fringe) θ = sin -1 d/ λ For it to be out of phase θ = sin -1 2d/ λ
- 15. At the first dark fringe The waves must be in anti-phase λ2 difference at the point on the wall λ2 = s sin θ θ = sin -1 2d/ λ θ s θ What is this length? O = s sin θ Opposite Sin θ Hypotenuse s s
- 16. At the first Bright fringe The waves must be in phase λ difference at the point on the wall λ = s sin θ θ = sin -1 s/ λ s s s
- 17. What is the use of knowing θ ? For it to be in phase (first fringe) θ = sin -1 d/ λ For it to be out of phase θ = sin -1 2d/ λ

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