1. Electromagnetic Radiation

2. Electromagnetic Spectrum

3. Light 1600’s – sunlight was considered purest form of light 1665 – Isaac Newton – passed a beam of sunlight through a prism. Beam spread out. Got a band of colors. Rejoined the colors using a 2 nd prism & got white light again.

4. Notice that red light is bent the least, violet the most. (Fill in page 185, wkbk.)

6. Newton: Particle Guy Light: made up of tiny particles, but with no mass. Explains why shadows have sharp edges. Couldn’t explain … –how particles of different colors were different & why they were refracted differently by a prism. –why 2 beams of light didn’t affect each other when they crossed. Would think particles of light would collide off each other.

7. Christian Huygens: Wave Guy 1678: suggested that light was composed of waves. Explained … –why 2 beams of light could cross each other without being disturbed. –refraction in prism. Different colors could have different wavelengths. Could NOT explain … –Waves couldn’t explain shadows with sharp edges. People were used to waves in water. Water waves move around an obstruction.

8. Wave Vocabulary Transverse and Longitudinal Wavelength Frequency Amplitude Velocity (Speed)

9. Direction of displacement = direction of travel

10. Direction of displacement is at right angle to direction of travel. Transverse Wave

11. Wavelength vs. Frequency (pg 187) WAVELENGTH = Distance light travels to complete 1 cycle. Symbol = (lambda) FREQUENCY = Number of cycles completed in 1 second. Symbol =  (nu)

12. Amplitude: Distance from origin to crest or trough.

13. Frequency Units: Cycles per second, sec -1, or Hz

16. Relationship between frequency and wavelength Which wave has a short wavelength? Which has a long wavelength? What happens to the frequency as the wavelength gets smaller?

17. Wave Velocity The speed of the wave in the direction it is traveling in. Velocity = wavelength X frequency Velocity = = 3.0 X 10 8 m/s for all electromagnetic radiation. So wavelength and frequency are inversely related.

18. Comparing Long wavelength Slow frequency Low energy Short wavelength Rapid frequency High energy All electromagnetic radiation has the same velocity: 3.00 x 10 8 m/sec. 3.00 X 10 8 m/sec =

19. Light: Particles vs. Waves Huygens might have had better arguments, but Newton was more famous, so people went with his theory: Light was particles! Speed of light 1 st determined (approximately) in 1676 by a Danish astronomer. Speed of light = 3.0 X 10 8 m/sec

20. Maxwell 1864-1873 worked out equations describing electricity & magnetism. Electric & magnetic fields cannot exist independently. The electromagnetic radiation in Maxwell’s equation moved at the same speed as light! notCould not be a coincidence!

21. Electromagnetic Radiation

22. Light that can’t be seen? Light was electromagnetic radiation! Maxwell’s equations unified electricity, magnetism, & light. But visible light only accounted for a fraction of the solutions to Maxwell’s equations. What about other frequencies and wavelengths? Java Tutorial on Electromagnetic Radiation

23. Discovered infrared radiation with a thermometer. Discovered ultraviolet radiation using a strip of paper soaked in silver nitrate.

24. Radio Waves 1888, Hertz used an oscillating electric current to emit electromagnetic radiation. Had a detector that he could move around to map the electromagnetic wave & determine its length. Found radio waves far beyond infrared radiation. Could have wavelengths from cm to km.

25. X-Rays 1895: Roentgen discovered that his cathode ray tube was emitting radiation – X-rays.

29. Units of Wavelength Nanometers –Visible light has wavelengths ranging from 400 nm to about 700 nm 0.4 μm to 0.7 μm Angstroms –4000 Å to 7000 Å

31. Electromagnetic Spectrum Water Diffraction Applet Light Diffraction Applet Single Slit Diffraction - Light Physics 2000 – Double Slit Double Slit Diffraction - Light Double Slit Diffraction

32. Energy of Radiation: Einstein E photon = h = hc/ The higher the frequency, the higher the energy. The shorter the wavelength, the higher the energy. h = Planck’s constant = 6.626 X 10 -34 J  s Remember, Joules (energy) are derived units. 1 J = 1 kgm 2 /s 2

33. The next step… Einstein – all radiation is quantized!  E = h  Each frequency or wavelength or color of light has its own unique energy. Radiation from an object consists of a series of packets of energy, but it’s also a wave, since it has its own frequency = photons!

34. Wave-particle Duality Light can’t be forced into categories like everyday objects. In some situations, light exhibits interference phenomena, like water waves. In some situations, light shows energy transfers like particles in a collision.

35. E = mc 2 E photon = h = hc/ = mc 2 So m = hc/  c 2 m = h/ c for a photon.

36. Electrons as Waves 1924: Louis de Broglie Suggested that since light can act like a particle, maybe a particle, the electron, can act like a wave. m = h/ c for a photon. Electrons are not electromagnetic waves. They are “matter waves.” m = h/ v where v = velocity of the particle.

37. deBroglie’s Equation m = h/ v where v = velocity of the particle Rearrange: = h/mv Verification: Diffraction of electron beams by a crystal lattice.

38. An integral number of wavelengths must fit around the “orbit.”