1 / 19

Diffraction

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. Diffraction of Light. If light passes through a very thin slit it forms a diffraction pattern

kmeyer
Download Presentation

Diffraction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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

  3. Two Source interference • Remember this pattern with water waves?

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

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

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

  7. n=1 n=0 n=1 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.

  8. Path difference 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. = nl

  9. Path difference 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)l

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

  11. Bright x Slits Central bright q L 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

  12. 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 sinq≈q for small angles)

  13. Diffraction Gratings • A diffraction grating is a series of many (eg. 6000 per cm) very fine parallel slits, closely spaced, on a piece of glass or plastic

  14. 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°)

  15. n=2 red Diffraction Grating n=1 violet White n=0 violet n=1 red n=2 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

  16. 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)

  17. 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:

  18. 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

  19. 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.

More Related