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Sunlight: Observations and Phenomena

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Presentation Transcript

  1. Sunlight

  2. Question: When you look up at the sky during the day, is the light from distant stars reaching your eyes?

  3. Observations About Sunlight • Appears whiter than most light • Casts shadows • Makes the sky appear blue • Becomes redder at sunrise and sunset • Reflects from many surfaces, not all metallic • Bends and separates into colors in materials

  4. Light • Medium-wavelength electromagnetic waves • The range of wavelengths we can see

  5. Spectrum of Sunlight • Thermal agitation make charges accelerate • Highly agitated charges emit light • The sun is a black-body at 5800° C

  6. Rayleigh Scattering • Passing sunlight polarizes particles in air • Fluctuating polarization  light emission • Air particles scatter light (absorb & reemit) • Air particles too small to be good antennas • Long-wavelengths (reds) scatter poorly • Shorter-wavelengths (violets) scatter better • Scattered light is bluish in appearance

  7. Question: When you look up at the sky during the day, is the light from distant stars reaching your eyes?

  8. Refraction • Polarization of matter delays light’s passage • Light slows as it passes through matter • As sunlight slows, it bends – refraction • On slowing, bend is toward normal line • As sunlight speeds up, it also refracts • On speeding up, bend is away from normal line • Index of refraction • factor by which light’s speed is reduced

  9. Reflection • Light polarizes different materials differently • In different materials, light has different • speeds of travel • relationships between electric & magnetic fields • These changes lead to reflections • As sunlight slows, some of it reflects • As sunlight speeds up, some of it reflects

  10. Dispersion • Light’s speed in a material depends on color • Violet light usually moves slower than red • Refraction (bending) depends speed change • Violet light usually bends more than red

  11. Rainbows • Refraction, reflection, and dispersion

  12. Interference • Light from different paths can interfere • Constructive – fields are in same direction • Destructive – fields are in opposite directions • The two reflections froma film interfere • Different colors mayinterfere differently

  13. Reflection of Polarized Light • Angled reflection varies for polarized light • Fluctuating electric field parallel to surface • large fluctuating surface polarization • big reflection • Electric field perpendicular to surface • small fluctuating surface polarization • small reflection

  14. Polarized Sunlight • Most glare is horizontally polarized light • Polarizing sunglasses • block horizontally polarized light • block glare from horizontal surfaces • Much of the blue sky is polarized light, too

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