Major Concepts in Physics Lecture 5.

1. Major Concepts in Physics Lecture 5. Prof Simon Catterall Office 309 Physics, x 5978 smc@physics.syr.edu http://physics/courses/PHY102.08Spring PHY102

2. Topics for today • Iridescence – demo • Diffraction gratings (light) - demo • Diffraction – bending of waves around obstacles • Only important for waves with wavelength comparable to size of object PHY102

3. Oil on water • Create large, stable layer in bottle. • See multiple colors – gravity causes film to have varying thickness – constructive thin film interference picks out certain wavelengths (colors) as thickness varies (and angle of viewing) PHY102

4. A light ray can be reflected many times within a medium. PHY102

5. . PHY102

6. Recap - double slit interference The condition for constructive interference is sin q=q=y/Dfor q small The distance between two adjacent minima is: PHY102

7. More slits – diffraction gratings • Clearly we are not limited to 2 slits. We can make gratings with N parallel slits • The condition for constructive interference is same as 2 slits – but fringes are much brighter – obvious - more light gets thru • Less obviously – they are much narrower – because presence of many sources amplifies effects of destructive interference – width of bright fringe =1/N PHY102

8. Diffraction Gratings A grating has a large number of evenly spaced, parallel slits cut into it. PHY102

9. Demo • Grating pattern – bright isolated spots PHY102

10. Diffraction • In homogeneous media waves propagate in straight lines – the travel in fixed direction at constant speed. • If one places an obstacle in their path whose size is much bigger than wavelength – waves cannot reach the region behind the object – in case of light one talks about shadow • But if wavelength is comparable to size waves can bend and spread - diffraction PHY102

11. Water waves PHY102

12. Fig. 25.30 PHY102

13. Razor blade demo PHY102

14. Huygen’s principle • To understand diffraction – imagine that each point on wavefront acts as source of new wavelets. • To get resultant wave need to add up all these new wavelets (superposition) • Interference special case – where just 2 or a few wavelet sources • Depending on path differences may see constructive/destructive interference PHY102

15. . PHY102

16. Single slit diffraction • demo PHY102

17. The intensity pattern on the screen. PHY102

18. The minima occur when: where m = 1, 2,… PHY102

19. Fig. 25.32 PHY102

20. Example (text problem 25.49): Light from a red laser passes through a single slit to form a diffraction pattern on a distant screen. If the width of the slit is increased by a factor of two, what happens to the width of the central maximum on the screen? The central maximum occurs between =0 and  as determined by the location of the 1st minimum in the diffraction pattern: Let m =+1 and assume that  is small. From the previous picture,  only determines the half-width of the maximum. If a is doubled, the width of the maximum is halved. PHY102

21. Diffraction of light of wavelength430 nm from a slit of size 0.02 mm is viewed on screen. At what angle is first dark line seen ? • A: 0.2 radian • B: 0.02 radian • C: 0.04 radian • D: 0.4 radian PHY102

22. What is width of central bright line (assume screen is 1.2 m distant) • A: 2.6 cm • B 5.2 cm • C: 1.0 cm • D: Need more information PHY102

23. Other diffraction patterns • Circular aperture • Grid • etc PHY102

24. Other examples of diffraction. • X-ray diffraction – structure of crystals – wavelength = 1nm – spacing between atoms PHY102

25. Resolution of Optical Instruments The effect of diffraction is to spread light out. When viewing two distant objects, it is possible that their light is spread out to where the images of each object overlap. The objects become indistinguishable. Can resolve two sources at angular separation q if asin q=l where a is size of aperture and we use light of wavelength l PHY102