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Diffraction The bending/spreading of waves as they go through gaps or around edges The effect is greatest when gap width is equal to or smaller than the wavelength

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Diffraction of Light If light passes through a very thin slit it forms a diffraction pattern It is seen as a bright central fringe with dark and bright fringes on either side As the gap width increases the pattern width decreases

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Two Source interference Remember this pattern with water waves?

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Two Source Interference Remember this pattern with water waves? Anti-nodal lines are lines of constructive interference. Nodal lines are lines of destructive interference.

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Double Slit Interference When monochromatic (single colour) light passes through 2 closely spaced, thin slits, the waves overlap and form an interference pattern. Bright Dark Screen

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Double Slit Interference The pattern is seen on a screen as evenly spaced bright and dark fringes

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Double Slit Interference The fringes are numbered by their order The central fringe is n=0 The first fringe either side of centre is n=1 The second n=2 etc. n=0 n=1

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Double Slit Interference For constructive interference (bright) the waves must arrive at the screen in phase so the path difference between the two interfering waves must be a whole multiple of the wavelength Path diff. = n Path difference

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Double Slit Interference For destructive interference (dark) the waves must arrive at the screen out of phase so the path difference between the two interfering waves must be a half multiple of the wavelength Path diff. = (n+ 1/2) Path difference

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Double Slit Interference The angle of the fringe can be calculated from the formula below d=distance between slits n=order number =wavelength d S1 S2 pd To screen

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Double Slit Interference The angle of the fringe can also be worked out using the following formula x=distance between centre and fringe L=distance from slits to screen Central bright x Bright Slits L

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Double Slit Interference So the spacing of the fringes depends on: The distance between the slits The wavelength of the light used How far away the screen is from the slits (NB sin ≈ for small angles)

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Diffraction Gratings A diffraction grating is a series of many (eg per cm) very fine parallel slits, closely spaced, on a piece of glass or plastic

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Diffraction Gratings The interference pattern produced is similar to the double slit pattern The differences are: There are lots of slits so fringes are brighter Slits are closer together so fringes are widely spread Slits are narrow so the light is diffracted through a wider angle (almost 180°)

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Diffraction Gratings If we shine white light onto a grating, we produce a series of spectra each side of the central fringe This is because white light is made of many frequencies (colours) which all diffract at slightly different angles White n=0 violet red n=1 n=2 Diffraction Grating

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Diffraction Gratings The spectrum produced by a grating is more widely spread that that produced by a prism It is also the other way around (ie red is diffracted the most)

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Diffraction Gratings The formula for working out angles with a diffraction grating is the same for two slit patterns However, often N, the number of slits per m (or slits per cm) is given. Slit spacing d is related to N by:

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Diffraction Gratings CD surfaces can act like diffraction gratings because it has many finely spaced lines on it’s surface. Light is reflected off the disc, but produces spectra in the same way

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Diffraction Gratings Diffraction gratings are a useful tool for determining the chemical composition of substances. This is down by analysing the light produced when the atoms are excited by heat or electricity. This is how astronomers can tell what stars are made of.

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