Electromagnetism and Light

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1 Electromagnetism and Light Monday Properties of waves (sound and light) interference, diffraction [Hewitt 12] Tuesday Light waves, diffraction, refraction, Snell's Law. [Hewitt 13, 14] Wednesday Lenses, polarization [Hewitt 14] Thurday/Friday Magnetic fields forces and induction [Hewitt 11]

2 TUESDAY This morning how electromagnetic radiation is made, interference, diffraction, spectra, doppler effect, interference Afternoon Refraction, snell's law.

3 This lecture will help you understand: Electromagnetic Spectrum Spectrum of Gasses Doppler Effect Bow Waves/Shock Waves and the Sonic Boom Why the Sky is Blue, Sunsets are Red, and Clouds are White Diffraction Interference of Light

4 Light Waves The light of stars that were extinguished ages ago still reaches us. So it is with great men who died centuries ago, but still reach us with the radiations of their personalities. Kahlil Gibran

5 Wave Speeds Speed of light (in vacuum/air) 8 c=3 10 m / s Speed of sound depends on wind conditions, temperature, humidity speed in dry air at 0 C is about 330 m/s in water vapor slightly faster in warm air faster than cold air speed in water about 4 times speed in air speed in steel about 15 times its speed in air

6 A situation to ponder Consider a person attending a concert that is being broadcast over the radio. The broadcast microphone is sitting right on the stage. The person sits about 45 m from the stage and listens to the radio broadcast with one ear while listening to the concert in person with the other ear. Further suppose that the radio signal is sent around the Earth before reaching the listeners radio. HINT: Don't guess on this one. You can do two calculations which will tell you the answer.

7 A situation to ponder CLICKER QUESTION Which signal will be heard first? A. B. C. D. radio signal the live sound signal both at the same time none of the above

8 A situation to ponder CHECK YOUR ANSWER Which signal will be heard first? A. radio signal B. the live sound signal A. both at the same time D. none of the above Explanation: A radio signal travels at the speed of light m/s. Time to travel 45 m at 340 m/s 0.13 s. Time to travel m (Earth s circumference) at m/s 0.13 s. Therefore, if you sit farther back at the concert, the radio signal would reach you first!

9 Light Waves Light is the only thing we can see originates from the accelerated motion of electrons electromagnetic phenomenon

10 Electromagnetic Spectrum Electromagnetic wave made up of vibrating electric and magnetic fields

11 k phet

12 Electromagnetic Spectrum CHECK YOUR NEIGHBOR If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a A. period of 1000 seconds. B. speed of 1000 m/s. C. wavelength of 1000 m. D. none of the above

13 Electromagnetic Spectrum CHECK YOUR ANSWER If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a A. period of 1000 seconds. B. speed of 1000 m/s. C. wavelength of 1000 m. D. none of the above Explanation: The vibrating electron would emit a wave with a frequency of 1000 Hz, which is not in the list above.

14 Electromagnetic Spectrum Electromagnetic spectrum classification of electromagnetic waves according to frequency lowest frequency of light we can see appears red highest frequency of light we can see appears violet higher frequency of light is ultraviolet more energetic and causes sunburns beyond are X ray and gamma ray

15 Electromagnetic Spectrum Electromagnetic spectrum Below Radio? more radio Above Gamma? more gamma Some gamma rays have as much energy as a fastball no one knows how.

16 Transparent and Opaque Materials Light is transmitted similar to sound light incident on matter forces some electrons in matter to vibrate

17 Transparent and Opaque Materials How light penetrates transparent material such as glass

18 Transparent and Opaque Materials How light penetrates transparent material such as glass (continued) electrons or molecules in the glass are forced into vibration energy is momentarily absorbed and vibrates the electrons in the glass this vibrating electron either emits a photon or transfers the energy as heat Time delay between absorption and reemission of energy of vibrating electrons results in a lower average speed of light through a transparent material

19 Transparent and Opaque Materials Average speed of light through different materials vacuum c (300,000,000 m/s) atmosphere slightly less than c (but rounded off to c) water 0.75 c glass 0.67 c, depending on material diamond 0.41 c Refraction (the bending of light as it enters a material) is CAUSED BY this speed change

20 CLICKER QUESTION Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted A. is slightly less. B. is the same. C. is slightly higher. D. depends on the type of plastic.

21 CHECK YOUR ANSWER Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted A. is slightly less. B. is the same. C. is slightly higher. D. depends on the type of plastic Explanation: Speed of light in plastic may vary, but the frequency transmitted doesn t.

22 CLICKER QUESTION The average speed of light is less in A. air before entering glass. B. glass. C. air after emerging from glass. D. none of the above

23 CHECK YOUR ANSWER The average speed of light is less in A. B. C. D. air before entering glass. glass. air after emerging from glass. none of the above

24 Interference Two patterns of interference constructive interference increased amplitude when the crest of one wave overlaps the crest of another wave destructive interference reduced amplitude when the crest of one wave overlaps the trough of another wave

25 Double slit constructive interference (diffraction) dsin =m y d =m L L ymax =m d m is an integer, 0, 1, 2, 3 etc.

26 Single slit diffraction Light from different Parts of a single slit interferes with itself Destructive Condition: asin θ=mλ

27 Single slit diffraction (Wide bright area in middle)

$Diffraction Diffraction bending of waves by means other than reflection and$

28 Diffraction Diffraction bending of waves by means other than reflection and refraction property of all kinds of waves seen around edges of many shadows

$Diffraction Waves diffract after passing through a narrow opening.$

29 Diffraction Waves diffract after passing through a narrow opening. Plane waves passing through openings of various sizes. The smaller the opening, the greater the bending of the waves at the edges.

$Diffraction Amount of diffraction depends on wavelength of the$

30 Diffraction Amount of diffraction depends on wavelength of the wave compared to the size of the obstruction that casts the shadow.

31 Animation Huyghen's principle. Huyghen's Principle Every piece of a slit makes circular waves Their interference makes what we see.

$Doubles slit diffraction is superposition of single and$

32 Doubles slit diffraction is superposition of single and double slit dsin =m Dot spacing = Wavelength * Distance to wall / d

33 Diffraction grating Same formula as two slits but sharper dots.

34 Summary: Interference & Diffraction dsin =m Condition for constructive interference between slits separated by d. asin =m Condition for destructive interference for single slit of width a. min=1.22 D Rayleigh Condition Minimum resolvable angular separation for lens of diameter D.

$Single slit interference leads to Diffraction limit Destructive Condition slit: asin θ=mθ θmin$

35 Single slit interference leads to Diffraction limit Destructive Condition slit: asin θ=mθ θmin λ/a For circular aperture Theta is full width θmin 1.22 λ/d To see a 100 kly galaxy 2 M Ly away

36 Very Large Array min=1.22 D m/s = Hz 0.1 m 4 min=1.22 =10 rad 1220m

37 Air-borne laser, laser missile defense =2.8 m min=1.22 D

38 Death Star Large Mirror Good for destroying planets! min=1.22 D

39 Use Diffraction gratings to identify gasses.

40 Identifying atomic energy levels E=h f c E=h 34 h= Joule sec

41 Interference at thin films

$Reflection and Refraction of Sound Refraction bending of waves caused$

42 Reflection and Refraction of Sound Refraction bending of waves caused by changes in speed affected by wind variations temperature variations

43 Roswell! The Roswell incident has been explained. It WAS a top secret government program. It wasn't just a weather balloon that crashed. Sonnenfeld will tell all.

Doppler Effect change of frequency due to the motion of the wave source most evident in change of pitch frequency of waves increases as the siren

44 Doppler Effect change of frequency due to the motion of the wave source most evident in change of pitch frequency of waves increases as the siren approaches (hear higher pitch) frequency of waves decreases as siren moves away (hear lower pitch) (Nice animations)

45 CLICKER QUESTION When a fire engine approaches you, the A. speed of its sound increases. B. frequency of sound increases. C. wavelength of its sound increases. D. all of the above increase

46 CHECK YOUR ANSWER When a fire engine approaches you, the A. speed of its sound increases. B. frequency of sound increases. C. wavelength of its sound increases. D. all of the above increase Comment: Be sure you distinguish between sound speed, and sound frequency.

47 Doppler Effect Doppler effect also applies to light increase in light frequency (blue shift) when light source approaches you decrease in light frequency (red shift) when light source moves away from you The expansion and the age of the Universe is entirely measured by the Doppler Effect. v=h*d v= speed of galaxy relative to Earth H= Hubble constant D=distance to galaxy

48 CLICKER QUESTION Doppler Effect The panel on the left are spectral lines from the sun. The panel at right are spectral lines of a distant star. The star is [A] A different composition of gasses as our sun [B] Moving away from Earth [C] Moving toward Earth [D] Not enough information.

49 CHECK YOUR ANSWER Doppler Effect The star is [A] A different composition of gasses as our sun [B] Moving away from Earth [C] Moving toward Earth [D] Not enough information. Explanation: The lines are in correct relative orientation, so are same gasses, but they move into the red. Thus star is moving away.

50 Wave Barriers and Bow Waves Wave barrier waves superimpose directly on top of one another producing a wall example: bug swimming as fast as the wave it makes

51 Wave Barriers and Bow Waves Supersonic aircraft flying faster than the speed of sound Bow wave V shape form of overlapping waves when object travels faster than wave speed an increase in speed will produce a narrower V shape of overlapping waves.

52 Shock Waves and the Sonic Boom Shock wave pattern of overlapping spheres that form a cone from objects traveling faster than the speed of sound

53 Sonic Boom Shock wave consists of two cones a high pressure cone generated at the bow of the supersonic aircraft a low pressure cone that follows toward (or at) the tail of the aircraft it is not required that a moving source be noisy Why does the air condense?

54 Cerenkov Radiation and Tachyons Particles traveling faster than light make a shock wave too! Fast charged particles in the water bath of a nuclear reactor make a blue glow. Tachyons would glow in a vacuum

55 Hewitt Problem 5 An electric field does 12 J of work on a charge of C as it moves from point A to point B. What is the voltage change between point A and point B? How much work does this same field do on a charge of Coulombs?

Electromagnetism and Light

Electromagnetism and Light Monday Properties of waves (sound and light) interference, diffraction [Hewitt 12] Tuesday Light waves, diffraction, refraction, Snell's Law. [Hewitt 13, 14] Wednesday Lenses,