Light

1. Light what is it?

2. Light • what is it? moving energy • wave or particle?

3. Light • what is it?moving energy • wave or particle? • how do we decide?

4. Light • what is it? moving energy • wave or particle? • how do we decide? • if a wave, what is waving? (waving even in a vacuum?)

5. Light • what is it? moving energy • wave or particle? • how do we decide? • if a wave, what is waving? (waving even in a vacuum?) Electric & Magnetic Fields

6. Properties of Light • speed of light • colors • reflection • shadows • refraction (bending) • energy theory • absorption of light • emission of light

7. Property 1: Speed of Light • particle (photon) ? no prediction • wave (E&M) ? in vacuum, v = c; in material, v < c From experiment, we find that the wave prediction works! (Here c stands for the speed of light in vacuum, which is 300,000,000 meters/second, or about 670 million miles per hour.)

8. Property 2: Color • experiment ? • particle (photon) ? • wave (E&M) ?

9. Property 2: Color experiment ? visible order: • red • orange • yellow • green • blue • violet

10. Property 2: Color experiment ? invisible as well as visible total spectrum order: • radio • microwave • IR • visible • UV • x-ray and gamma ray

11. Property 2: Color particle (photon) ? amount of energy per photon determines “color”

12. Property 2: Color particle (photon) ?amount of energy among different types: x-ray - most energy;radio - least in visible portion: violet - most energy; red - least

13. Property 2: Color • particle (photon) ? amount of energy • wave (E&M) ?

14. Property 2: Color • particle (photon) ? amount of energy • wave (E&M) ? frequency among different types of “light”: low frequency is radio(AM is 500-1500 KHz) high frequency is x-ray & gamma ray in visible spectrum: red is lowest frequency (just above IR) violet is highest frequency (just below UV)

15. Wavelength and Frequency “Nice” sine waves have a simple relation for wavelength and frequency: λ*f = v where λ is the wavelength (distance from one crest to the next one), where f is the frequency (how many times one location goes up and down a second), and where v is the speed of the wave (how fast the crest of the wave moves). λ v

16. Light For light in vacuum, the speed of the light wave is 300,000,000 meters/sec, or about 670 million miles/hour. We use the symbol “c” to denote this value. Therefore for light in vacuum, we have: λ*f = c . Example: for a radio wave of frequency 100 MHz, the wavelength is: λ * (100 * 1,000,000 Hz) = 300,000,000 m/s, or λ = 300,000,000 m/s / 100,000,000 Hz = 3 meters.

17. Nanometers The wavelength of visible light is in the range of 0.000000400 meters to .000000700 meters. This is an awkward way to write these numbers. In Scientific Notation, this becomes 4 x 10-7 m to 7 x 10-7 m. This is still somewhat awkward, so we often use the unit of nanometers (nm) which is 10-9 m; this gives the range for the wavelengths of visible light to be 400 nm to 700 nm.

18. Colors: frequencies & wavelengths (in vacuum) AM radio  1 MHz100’s of m FM radio  100 MHz m’s microwave  10 GHzcm - mm Infrared (IR)1012 - 4x1014Hzmm - 700 nm visible 4x1014 - 7.5x1014700nm -400nm Ultraviolet (UV) 7.5x1014 - 1017400 nm - 1 nm x-ray &  ray > 1017 Hz< 1 nm

19. Property 3: Reflection • particle (photon) ? • wave (E&M) ?

20. Property 3: Reflection • particle (photon) ?bounces “nicely” • wave (E&M) ? bounces “nicely” bounces nicely means: angle incident = angle reflected

21. Property 4:Light and Shadows Consider what we would expect from particletheory: sharp shadows dark dark light

22. Light and Shadows Consider what we would expect from wave theory: shadows NOT sharp crest crest crest dark dark dim light dim

23. Light and Shadows What DOES happen? Look at a very bright laser beam going through a vertical slit. (A laser has one frequency unlike white light.)

24. Diffraction: single slit How can we explain the pattern from light going through a single slit? screen x w L

25. Diffraction: single slit In fact, we can break the beam up into 2n pieces since pieces will cancel in pairs. This leads to: (w/2n) sin(n) = /2 , or w sin(n) = nfor MINIMUM. screen x w L

26. Diffraction: circular opening If instead of a single SLIT, we have a CIRCULAR opening, the change in geometry makes: the single slit pattern into a series of rings; and the formula to be: 1.22 n = D sin(n)

27. Diffraction: circular opening Since the light seems to act like a wave and spreads out behind a circular opening, and since the eye (and a camera and a telescope and a microscope, etc.) has a circular opening, the light from two closely spaced objects will tend to overlap. This will hamper our ability to resolve the light (that is, it will hamper our ability to see clearly).

28. Rayleigh Criterion: a picture The lens will focus the light to a fuzzy DOT rather than a true point. lens D

29. Rayleigh Criterion: a picture If a second point of light makes an angle of limit with the first point, then it can just be resolved. lens D x x’ s’ s

30. Limits on Resolution: • Imperfections in the eye (correctable with glasses) • Rayleigh Criterion due to wavelength of visible light • Graininess of retinal cells

31. Limits on Resolution: further examples • hawk eyes and owl eyes • cameras: • lenses (focal lengths, diameters) • films (speed and graininess) • shutter speeds and f-stops • Amt of light  D2 t • f-stop = f/D • f-stops & resolution: resolution depends on D

32. Limits on Resolution: further examples • other types of light • x-ray diffraction (use atoms as slits) • IR • radio & microwave • surface must be smooth on order of 