Lecture 14
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Electromagnetic waves consist of vibrating electric and magnetic fields that propagate at the speed of light. The electromagnetic spectrum includes visible light as well as other types of electromagnetic radiation such as radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays. Electromagnetic waves can be characterized by their wavelength or frequency, with radio waves having the longest wavelengths and lowest frequencies, and gamma rays having the shortest wavelengths and highest frequencies. Materials can absorb, transmit, or reflect electromagnetic waves depending on the wavelength and properties of the material.
Lecture 14
- 1. Light
- 2. Electromagnetic Radiation • We know that a changing magnetic field can cause the creation of an electric field. – Electromagnetic Induction • A changing electric field may also cause the creation of a magnetic field. • The result? The form of radiation we think of as both light and radio.
- 3. Electromagnetic Waves • Electromagnetic waves consist of vibrating electric and magnetic fields. • All EM waves move at the speed of light (in a vacuum). – velocity = frequency * wavelength still holds, but the velocity is now the speed of light.
- 5. Example Problems • Is it correct to say that a radio wave is a low- frequency light wave? Is a radio wave also a sound wave? • What is the principal difference between a radio wave and light? Between light and an x- ray?
- 6. Matter and EM Waves • A vibrating electric charge creates an EM wave. • An EM wave can cause a charge to vibrate. • This is similar to how a vibrating tuning fork can cause another tuning fork to vibrate. • In changing the way it vibrates, a material may absorb or emit EM waves.
- 7. Transparency • When light is absorbed and quickly re-emitted an object may appear transparent. – Reemission occurs before the energy can be changed to heat through collisions.
- 8. Color • White light is made up of many colors (together they appear white). • Most objects do not emit light – they reflect it. • An object will appear to be the complement of the colors it absorbs. • Absorbing all colors = black. Absorbing none (reflecting all) = white. • The color something appears depends on the lighting.
- 9. Human Eye • Rods (color insensitive) and cones (color sensitive). • Diagram in lower right shows the sensitivity of the 3 cone types. • Sometimes one of these cone types is missing or mutated.
- 10. Colorblindness
- 11. Color Mixing • By mixing three colors of light (red, green and blue) we may produce most any color we can see. • Red + Green + Blue = white. • Red + Blue = Magenta • Red + Green = Yellow • Blue + Green = Cyan
- 13. Pigments • If you mix red, green and blue pigments you get an ugly dark brown mess. • Mixing of paints and dyes is different from mixing of colored light. • Pigments work by color subtraction: white – cyan = red white – yellow = blue • We tend to use Cyan, Magenta, Yellow + Black for printing.
- 14. Why is the sky blue? • Scattered light. – Absorbed light is re- emitted in all directions. – High frequency light (blue) is more strongly scattered than low frequency (red).
- 15. Why sunsets are red • Blue light is scattered away (subtracted) from the white light. • When the path length is very long (sunrise, sunset) this subtraction becomes visible.
- 16. Why clouds are white • Different sized particles tend to scatter different color light: – small particles scatter blue – medium particles scatter green – larger particles scatter red • Together we get white.
- 17. Polarization • Light is a transverse wave. • Transverse waves can have an orientation. – In the picture to the left, the top wave is oriented up/down. – The bottom wave is oriented left/right.
- 18. • A polarizer has the ability to block waves with their electric fields oriented in particular directions. • When two polarizers are aligned, they pass light. • When they are crossed they block light. – LCD monitors work using this principle. – Some 3d movies take advantage of the effect. – Also used in some sunglasses.