Understanding Electromagnetic Radiation: Discoveries, Theories, and the Electromagnetic Spectrum

1. Electromagnetic Radiation

2. Electromagnetic Spectrum

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

4. Notice: red light is bent the least, violet the most

6. Newton • Thought light made up of tiny particles with no mass • explains why shadows have sharp edges • Couldn’t explain how particles of different colors were different or why were refracted differently by prism • Couldn’t explain why 2 beams of light didn’t affect each other when they crossed • particles of light should collide off each other

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

8. Wave Vocabulary • Transverse and Longitudinal • Wavelength • Frequency • Amplitude • Velocity

9. Direction of displacement = direction of travel

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

11. Wavelength vs. Frequency • WAVELENGTH = Distance light travels to complete 1 cycle • FREQUENCY = Number of cycles completed in 1 second

13. Wave Velocity • The speed of wave in direction it is traveling • Velocity = wavelength x frequency • Velocity = 

14. Light: Particles vs. Waves • Huygens had the better argument, but Newton was more famous • people went with Newton's theory: Light was particles! • Speed of light 1st determined about 1676 by a Danish astronomer • Speed of light = 3.0 X 108 m/sec

15. Calculating Wavelengths of Light • Visible light waves have lengths of ~ 1/20,000 cm • Red was a little longer, violet a little shorter • short wavelength explains why light cast sharp shadows despite being waves • Waves can only bend around obstacles that are about the same length • cannot bend around anything substantially longer than itself

16. The Ether • Water waves move in water • Sound waves move in air • Light waves move through a vacuum • Gravity, Electricity, & Magnetism also felt across a vacuum • Couldn't accept this: • Postulated a subtle form of matter called ether • not easily detectable • Didn’t give up “Ether” idea until 1900

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

18. Light that can’t be seen? • Light was electromagnetic radiation! • Maxwell’s equations unified electricity, magnetism, & light • But visible light only accounts for a fraction of Maxwell’s equations • What about other frequencies and wavelengths?

19. Heat & Light: Separate or the same? • Herschel – 1800 studied the spectrum with a thermometer & found the highest temperature at the red end and coolest at violet end • Placed the thermometer beyond the red & the temp was higher there than anywhere in the visible spectrum • Discovered infrared rays, which we cannot see

21. Infrared Radiation • By 1850, infrared rays were demonstrated to have all the properties of light • except that they could not be seen by eye

22. Silver Nitrate as detector • 1614: knew that silver nitrate (white cmpd) darkens on exposure to sunlight • 1770: Scheele soaked strips of white paper in silver nitrate solutions and placed them in different parts of the spectrum • Darkened least quickly in red and fastest in violet • ? the first photographs

23. Discovery of ultraviolet! • 1801: After Herschel's used thermometers to discover infrared • Ritter repeated Scheele’s experiment with the paper soaked in silver nitrate & put the strips beyond the violet • Strips darkened even quicker than in violet light • ultraviolet light was discovered!

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

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

27. Electromagnetic Spectrum

28. Short wavelength Rapid frequency High energy Comparing • Long wavelength • Slow frequency • Low energy All electromagnetic radiation has the same velocity: 3.00 x 108 m/sec 3.00 X 108 m/sec = 

29. Heat Flow • Objects hotter than their surroundings lose heat as electromagnetic radiation • the higher the temp, the more intense the radiation • Hot objects glow! • they glow different colors at different temperatures!

30. Ultraviolet Catastrophe • Classical physics - Assume that every wavelength has an equal chance of being radiated • Classical wave model could not explain why different colors were emitted at different temperatures

31. Planck • The shorter the wavelength, the less chance it has to be emitted! • Matter can gain or lose energy only in very small increments = quanta

32. What physical explanation goes with Planck’s assumption? • Pre-Planck: • Energy was considered to be continuous • could be broken into smaller & smaller bits indefinitely • Planck: • Energy consists of tiny particles that can’t be divided into anything smaller