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Lecture 20

Lecture 20. Electromagnetic Waves Nature of Light Reflection and Refraction. Fig. 22-CO, p.726. Electromagnetic Waves are Transverse Waves. The and fields are perpendicular to each other Both fields are perpendicular to the direction of motion Therefore, em waves are transverse waves.

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Lecture 20

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  1. Lecture 20 • Electromagnetic Waves • Nature of Light • Reflection and Refraction

  2. Fig. 22-CO, p.726

  3. Electromagnetic Waves are Transverse Waves • The and fields are perpendicular to each other • Both fields are perpendicular to the direction of motion • Therefore, em waves are transverse waves

  4. Properties of EM Waves • Electromagnetic waves are transverse waves • Electromagnetic waves travel at the speed of light • Because em waves travel at a speed that is precisely the speed of light, light is an electromagnetic wave

  5. Properties of EM Waves, 2 • The ratio of the electric field to the magnetic field is equal to the speed of light • Electromagnetic waves carry energy as they travel through space, and this energy can be transferred to objects placed in their path

  6. Properties of EM Waves, 3 • Energy carried by em waves is shared equally by the electric and magnetic fields

  7. Properties of EM Waves, final • Electromagnetic waves transport linear momentum as well as energy • For complete absorption of energy U, p=U/c • For complete reflection of energy U, p=(2U)/c • Radiation pressures can be determined experimentally

  8. Determining Radiation Pressure • This is an apparatus for measuring radiation pressure • In practice, the system is contained in a vacuum • The pressure is determined by the angle at which equilibrium occurs

  9. The Spectrum of EM Waves • Forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths • c = ƒλ • Wavelengths for visible light range from 400 nm to 700 nm • There is no sharp division between one kind of em wave and the next

  10. The EMSpectrum • Note the overlap between types of waves • Visible light is a small portion of the spectrum • Types are distinguished by frequency or wavelength

  11. Notes on The EM Spectrum • Radio Waves • Used in radio and television communication systems • Microwaves • Wavelengths from about 1 mm to 30 cm • Well suited for radar systems • Microwave ovens are an application

  12. Notes on the EM Spectrum, 2 • Infrared waves • Incorrectly called “heat waves” • Produced by hot objects and molecules • Readily absorbed by most materials • Visible light • Part of the spectrum detected by the human eye • Most sensitive at about 560 nm (yellow-green)

  13. Notes on the EM Spectrum, 3 • Ultraviolet light • Covers about 400 nm to 0.6 nm • Sun is an important source of uv light • Most uv light from the sun is absorbed in the stratosphere by ozone • X-rays • Most common source is acceleration of high-energy electrons striking a metal target • Used as a diagnostic tool in medicine

  14. Notes on the EM Spectrum, final • Gamma rays • Emitted by radioactive nuclei • Highly penetrating and cause serious damage when absorbed by living tissue • Looking at objects in different portions of the spectrum can produce different information

  15. Fig. 21-CO, p.693

  16. Radio Fig. 21-23d, p.717

  17. X-ray Fig. 21-23a, p.717

  18. optical Fig. 21-23b, p.717

  19. Infra-red Fig. 21-23c, p.717

  20. A Brief History of Light • 1000 AD • It was proposed that light consisted of tiny particles • Newton • Used this particle model to explain reflection and refraction • Huygens • 1678 • Explained many properties of light by proposing light was wave-like

  21. A Brief History of Light, cont • Young • 1801 • Strong support for wave theory by showing interference • Maxwell • 1865 • Electromagnetic waves travel at the speed of light

  22. A Brief History of Light, final • Planck • EM radiation is quantized • Implies particles • Explained light spectrum emitted by hot objects • Einstein • Particle nature of light • Explained the photoelectric effect

  23. The Particle Nature of Light • “Particles” of light are called photons • Each photon has a particular energy • E = h ƒ • h is Planck’s constant • h = 6.63 x 10-34 J s • Encompasses both natures of light • Interacts like a particle • Has a given frequency like a wave

  24. Dual Nature of Light • Experiments can be devised that will display either the wave nature or the particle nature of light • In some experiments light acts as a wave and in others it acts as a particle • Nature prevents testing both qualities at the same time

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