1. Diffraction and Coherence 16-2 and 16-3

2. CAN WAVES BEND AROUND CORNERS? ·Can you hear me when I stand around the corner and yell? ·What about light? Think about the double slit experiment.

3. What is Diffraction? Spreading of light into a region behind an obstruction

4. When does diffraction happen? ·Small Openings ·Around Obstacles ·By sharp edges ·http://www.lon-capa.org/~mmp/kap13/cd372.htm

5. Interference vs. Diffraction ·Interference assumes slits behave as point sources ·Diffraction considers the width of a single slit (Huygens’ principle) ·Each portion of the slit acts as a source of waves so one portion interferes with another portion

6. Destructive Interference in Single-Slit Diffraction b) Each wavlet from upper half cancels a wavelet that travels one-half wavelength farther than lower half (dark fringe)

7. ·In a diffraction pattern, the central maximum is twice as wide as the secondary maxima. ·Secondary maxima aren’t as bright – some of light waves remain uncanceled

8. Light Diffracted by an Obstacle also produces a pattern.

9. Diffraction Pattern of a Penny ·The Arago bright spot is explained by the wave theory of light ·Waves that diffract on the edges of the penny all travel the same distance to the center ·The center is a point of constructive interference and therefore a bright spot ·Geometric optics does not predict the presence of the bright spot · The penny should screen the center of the pattern

10. A rainbow on a CD? -Alternating pits and smooth surfaces form closely spaced rows -Rows of data don’t reflect as much light as thin part separating rows -Reflected light undergoes constructive interference (Depends on direction, orientation of disc, light’s wavelength) -Each wavelength seen at different angle

11. Diffraction Gratings ·A diffraction grating uses diffraction and interference to disperse light into its component colors. ·The position of a maximum depends on the separation of the slits in the grating, d, the order of the maximum m,, and the wavelength of the light, l. d sin q = ±mlm = 0, 1, 2, 3, …

12. Notes about Diffraction Gratings ·Sharpness of principal maxima and broad range of dark areas depends on # of lines in grating ·# of lines per unit length is inverse of line separation d ·Example: 5000 lines/cm, then d = 1/5000 cm ·More lines = less separation between the lines and individual wavelengths are spread apart farther

13. Spectrometers ·Uses a diffraction grating to separate light into its monochromatic components ·Can determine chemical composition of light source ·Used to study temperatures of stars, interstellar gas clouds, and galaxies

14. Diffraction Gratings Monochromatic light from a helium-neon laser (l = 632.8 nm) shines at a right angle to the surface of a diffraction grating that contains 150 500 lines/m. Find the angles at which one would observe the first-order and second-order maxima.

15. Diffraction and Instrument Resolution ·The ability of an optical system to distinguish between closely spaced objects is limited by the wave nature of light. ·Resolving power is the ability of an optical instrument to form separate images of two objects that are close together. ·Resolution depends on wavelength and aperture width. For a circular aperture of diameter D:

16. Resolution of Two Light Sources

17. Resolution Example ·Pluto and its moon, Charon ·Left – Earth based telescope is blurred ·Right – Hubble Space Telescope clearly resolves the two objects

18. Resolving Power ·To get high resolution, the angle between resolved objects should be as small as possible ·Since What must l and the size of the opening be to get high resolution? ·Wavelength must be as short as possible ·Wider opening (or aperture)

19. Lasers and Coherence ·A laser is a device that produces coherent light at a single wavelength. ·The word laser is an acronym of “light amplification by stimulated emission of radiation.” ·Lasers transform other forms of energy into coherent light.

20. Applications of Lasers ·Lasers are used to measure distances with great precision. ·Compact disc and DVD players use lasers to read digital data on these discs. ·Lasers have many applications in medicine. ·Eye surgery ·Tumor removal

21. Attachments how lasers work.swf