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Light Introduction

Light Introduction. General Information. Light is a Wave. Exactly as ocean waves, sound waves, slinky waves. . . Amplitude Wavelength Frequency Wave speed v = f . The speed of light is constant (in a given material). 3 x 10 8 m/s (186,000 miles per second) Nothing travels faster

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Light Introduction

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  1. Light Introduction General Information

  2. Light is a Wave • Exactly as ocean waves, sound waves, slinky waves. . . • Amplitude • Wavelength • Frequency • Wave speed v = f 

  3. The speed of light is constant (in a given material) • 3 x 108m/s (186,000 miles per second) • Nothing travels faster • Although light does slow as it enters a more dense medium • Therefore, 3x108m/s = f 

  4. So if v is constant . . . • v = f  • As f goes up  goes down • As f goes down  goes up • Also E is related to amplitude • Big wave high energy (tsunami)

  5. Light is different from most waves • Because it does not need a medium to be transmitted • A water wave needs water • A sound wave needs air • A light wave needs nothing!

  6. Light needs no medium because • It is an electro-magnetic wave • An electron moves • A changing electric field is produced • The electric field produces a magnetic field • The changing magnetic field produces an electric field • A changing electric field…

  7. Moving at website http://www.phy.ntnu.edu.tw/java/emWave/emWave.html

  8. Light is Electromagnetic Waves Only a small part are visible, most are not. The spectrum is organized by frequency and wavelength

  9. All light is a wave • Can only see visible light • Can’t see the wave form • We can see white light which is all colors together • Gamma has high frequency small wavelength • Radio has low frequency large wavelength

  10. Energy is related to frequency • As you increase frequency you increase energy • As you decrease frequency you decrease energy • Gamma high f, high E • Radio low f, low E

  11. High Energy Low Energy

  12. Wait! • In the wave unit we learned energy was proportional to amplitude • In a classic wave there is no connection between f and E • Again, light is different from a traditional wave

  13. Why is energy related to frequency for light not for other waves? • We describe light as having a dual nature • We try to explain light in terms of human experience, but we can’t really describe it • Sometimes we notice light acts like a particle • Sometimes we notice light acting like a wave • Actually, light is both and neither.

  14. Energy • High frequency = high Energy • Short wavelength = high Energy E = hf

  15. What is more dangerous: • Visible light or microwaves? • How does a microwave work then? • Why doesn’t it hurt us? • Cell phones use microwaves

  16. Intensity • Brightness • The number of light waves per area • Nothing about the wave changes

  17. We think of light as just brightness • Really light is a SPECTRUM • All wavelengths of light together creates white light • When a room is dark you see - black! Black is the absence of light • A prism separates the light into colors • But light is more than just the colors

  18. Remember • Light is electromagnetic radiation • From gamma to radio waves • Energy is proportional to frequency • Light can be described as a wave and particle • Otherwise, light behaves as any other wave

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