Understanding Light Diffraction: A Comprehensive Guide

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.