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

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 1 st determined about 1676 by a Danish astronomer –Speed of light = 3.0 X 10 8 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! notCould 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. 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 =

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:Planck: –Energy consists of tiny particles that can’t be divided into anything smaller