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# Light Waves

What we call light is a small portion of the electromagnetic spectrum All the different colors are electromagnetic waves with different wave lengths Wave speed = frequency x wavelength EM radiation is alternating electric and magnetic fields. Light Waves. Remember our field induction rules

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## Light Waves

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1. What we call light is a small portion of the electromagnetic spectrum All the different colors are electromagnetic waves with different wave lengths Wave speed = frequency x wavelength EM radiation is alternating electric and magnetic fields Light Waves

2. Remember our field induction rules Changing magnetic/electric field induces electric/magnetic field Electromagnetic Waves

3. Remember, the lower the frequency the longer the wavelength and vice versa Wave speed of light is 300,000,000 m/s Electromagnetic Spectrum

4. Lowest frequencies of visible light are red Highest frequencies are violet The order is red, orange, yellow, green, blue, violet Colors

5. Need to vibrate charged particles We shake electrons in an antenna to make radio and TV waves We shake electrons in a magnetron to make microwaves We shake electrons in atoms to make light (this is a bit of a simplification which we will correct later this semester Producing EM Waves

6. Electromagnetic Waves carry energy When light strikes a material, it vibrates the lightweight electrons Light and Materials

7. Light and Materials

8. If the natural vibration frequency of the electrons is close to the frequency of the incoming light wave, the electron will be set into big vibrations (resonance) It will absorb some energy from the EM wave and will vibrate with larger amplitude It can smack into the electrons in neighboring atoms and vibrate them too Light and Materials

9. This cuts down on the intensity of the light If all the light is absorbed by shaking electrons we say the material is opaque If the light frequency doesn’t come close to the natural frequencies of the electrons, they shake a bit, but just re-emit the energy Materials that can’t hold the energy are transparent Light and Materials

10. There is a delay in transparent materials between the time the electrons start shaking and then re-emit the energy This means that the light travels more slowly in these materials!!! We will use this fact later to do interesting things with lenses Light and Materials

11. Light and Materials See why the light travels more slowly in a transparent material than in a vacuum?

12. Infrared EM waves shake entire atoms or molecules in materials Microwave EM waves effect the rotation of atoms about each other Light and Materials + Remember our discussion of water and how it has a positive end by the hydrogen atoms and a negative end by the oxygen -

13. Things that are opaque absorb the energy carried by the light wave This means that they heat up Metals are opaque and shiny The free electrons close to the surface shake and re-emit the energy back out of the material They are reflectors!!! Light and Materials

14. Our atmosphere is transparent to visible light, some ultraviolet light and some infrared light The ozone layer absorbs a lot of ultraviolet light The ozone molecule is O3 and breaks into a regular oxygen molecule O2 and an oxygen atom Light and Air

15. Colored objects are a result of selective reflection and absorption of light Sunlight is a mixture of colors We call this mixture white light When white light strikes a red object, the red frequencies are reflected and the other frequencies are absorbed Colors

16. Sunlight

17. White objects reflect all frequencies Black objects absorb all frequencies Objects can only reflect the colors of light that shine on them If you shine blue light on a red object it will look black Colors

18. We can also consider shining light through semi-transparent objects Filters used on the lights in a theater are materials that absorb some frequencies and let other frequencies pass through Red filters transmit red light through the material while other frequencies are absorbed Colors

19. Our eyes and brains are very complicated color processing sensors and computers See the discussion on page 289 in the text Artists have learned how to mix colors to make pleasing combinations and to create a huge array of different colors All based on what the sensors in our eyes respond to Mixing Colors

20. Visual Response

21. Visual Response Red + Blue = Magenta Red + Green = Yellow Blue + Green = Cyan The way your computer screen makes colors. And your TV set does the same thing.

22. Visual Response Red + Blue = Magenta Red + Green = Yellow Blue + Green = Cyan Magenta + Green = White Cyan + Red = White Yellow + Blue = White

23. This is why the sky is blue! The higher frequencies are scattered more by the molecules in the atmosphere Reds and oranges just pass through, so the sky appears blue Selective Scattering

24. Selective Scattering Sunsets are red!

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