Major Concepts in Physics Lecture 6.

1. Major Concepts in Physics Lecture 6. Prof Simon Catterall Office 309 Physics, x 5978 smc@physics.syr.edu http://physics/courses/PHY102.08Spring PHY102

2. Topics • Recap/finish diffraction • Light as electromagnetic wave PHY102

3. Announcements • Exam 1 – Monday 11 – example exam available from web page soon … • Also hw1 solutions • Review session – this Wednesday in class • Also review in workshop. • Homework 1 due this week. Homework 2 goes out in recitation (due in 2 weeks) PHY102

4. Diffraction - recap • When waves encounter obstacle of size comparable to wavelength – bend around it. • Understand using Huygen’s principle • Build wavefront at time t by adding together wavelets originating along wavefront at earlier time • Why we can hear `around corners’ • Limits resolution of optical instruments • Simplest example – single slit diffraction. PHY102

5. The intensity pattern on the screen. PHY102

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

7. Diffraction of light of wavelength400 nm from a slit of size 0.02 mm is viewed on screen. What angle is first dark line seen ? • A: sin-1(0.02) • B: sin-1(0.04) • C: sin-1(0.2) • D: sin-1(0.4) PHY102

8. What is width of central bright line (assume screen is 1.0 m distant) • A: 2.0 cm • B 4.0 cm • C: 1.0 cm • D: 2.0 mm PHY102

9. Other diffraction patterns • Circular aperture – circular rings • Human hair • etc PHY102

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

11. Resolution of Optical Instruments The effect of diffraction is to spread light out. When viewing two distant objects eg stars, 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

12. Electromagnetic waves – what is light? • Last semester you learnt about forces of electricity and magnetism • Forces arise from interaction of charges with electric and magnetic fields • Static charges generate electric fields (Coulomb) • Charges moving at constant velocity generate magnetic fields (Ampere) • Magnetic fields changing in time yield currents and electric fields (Faraday) PHY102

13. Maxwell • Maxwell, Scottish physicist, noticed a lack of symmetry in these equations • In absence of charges and currents: • Time varying magnetic fields generate electric fields BUT not the opposite! PHY102

14. New proposal • This led to other problems (violation of charge conservation …) • Proposed adding a new term to equations Time varying electric fields generate magnetic fields Maxwell equations PHY102

15. Consequences • Charges that accelerate will generate waves consisting of oscillating electric and magnetic fields! • These waves can exist in vacuum ! • Speed of these waves c=1/sqrt(e0m0) This number 3.0x108 m/s was exactly the measured speed of light ! PHY102

16. Fig. 22.1 PHY102

17. Fig. 22.2 PHY102

18. Unification • Thus Maxwell had unified not only the forces of electricity with magnetism • But also with the science of optics! • Furthermore, these electromagnetic waves could have a wide range of wavelengths • Visible light was just a small part • One speaks of the electromagnetic spectrum PHY102

19. . PHY102

20. Visible light • White light is composed of a range of wavelengths l=400-700 nm (colors) 2000-4000 times size of atom f=3.0x108/500x10-9=6x1014 Hz • See with diffraction grating – demo • Human eye sensitive to these wavelengths which corresponds to where Sun puts out most of its energy. • Other wavelengths invisible to eye PHY102

21. . PHY102

22. Infrared – l = 800 nm – 1 mm PHY102

23. Ultraviolet l=350 nm – 10 nm PHY102

24. Demos • IR camera, UV light PHY102

25. Microwaves l=10 cm PHY102

26. X-rays l=1x10-12 m PHY102