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Chapter 26 Properties of Light

Part VI: Light. Chapter 26 Properties of Light. Electromagnetic Waves. Moving charges (currents) create magnetic fields. Oscillating magnetic fields create electric fields. These effects create electromagnetic waves . Demo : Light & Sound.

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Chapter 26 Properties of Light

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  1. Part VI: Light Chapter 26Properties of Light Physics 1 (Garcia) SJSU

  2. Electromagnetic Waves Moving charges (currents) create magnetic fields. Oscillating magnetic fields create electric fields. These effects create electromagnetic waves. Physics 1 (Garcia) SJSU

  3. Demo: Light & Sound Sound waves can only travel through a material, such as air, but light waves can travel through vacuum. Radio Wave Can see cell phone ringing inside vacuum chamber but don’t hear the sound. Sound waves are not electromagnetic waves Physics 1 (Garcia) SJSU

  4. Speed of Light Speed of light is 300,000,000 m/s Since (Wavelength) = then radio station at 100 Megahertz has (Wavelength) = = 3 meters (Wave Speed) (Frequency) 300,000,000 m/s 100,000,000 Hz Physics 1 (Garcia) SJSU

  5. Electromagnetic Spectrum Cell phone Very broad spectrum of electromagnetic waves Physics 1 (Garcia) SJSU

  6. Transparent & Opaque Materials A material is transparent or opaque to different wavelengths depending on its atomic properties. Some opaque materials absorb certain waves while other materials reflect those electro-magnetic waves. For visible light, glass is transparent, while rubber and metal are opaque (rubber absorbs, metal reflects) Physics 1 (Garcia) SJSU

  7. Check Yourself The forced oscillations of ultraviolet light happen to match the natural frequency of electrons in glass. So what happens? So is glass opaque to ultraviolet light? Physics 1 (Garcia) SJSU

  8. Demo: Ultraviolet Light Fluorescent (Day-Glo) paint converts invisible ultraviolet light into visible light. Test opacity of: • Glass • Water • Sunscreen Ultraviolet Lamp Wavelength of ultraviolet light is about the size of bacteria (shorter than visible light). Physics 1 (Garcia) SJSU

  9. Demo: Microwaves Test transparency, opacity, & reflectivity of: • Metal (aluminum foil) • Wood (book) • Glass • Water Microwave Receiver Microwave Transmitter Wavelength of microwaves is fraction of a centimeter (longer than visible light). Physics 1 (Garcia) SJSU

  10. Perception of Distance Visually, we experience distance by • Occultation (objects hide what’s behind them) • Geometric Perspective (objects look smaller as they get further away) • Atmospheric Perspective (distant objects are hazy and bluish) • Lighting and shadows • Stereopsis (different view in each eye) • Relative motion (as you move, nearby objects shift more than distant objects)

  11. Occlusion The simplest way that we perceive distance is by the fact that closer objects occlude (hide) the objects behind them. Even in this surreal painting we immediately see the boy as being closer to us than the woman because he partially blocks our view of her. Detail from The Madonna of Port Lligat, Salvador Dali, 1950

  12. Distorted Occlusion The image is disturbing but the reason isn’t immediately apparent. When occlusion is incorrect, we are very cognizant of the distortion. This channel of water needs to be behind the lower part of the right tower. Detail from Waterfall, M.C. Escher, 1961

  13. Pre-15th Century Paintings Occlusion but no sense of distance Mongol Ruler and consort enthroned, 14th century Road to Calvary, Martini, 1315

  14. Renaissance Paintings Scenes in these paintings look realistic Marriage of the Virgin, Raphael, 1504 The Annunciation, Botticelli, 1489

  15. Perspective The difference is the introduction of visual perspective by Filippo Brunelleschi of Florence. Objects in the distance look smaller as determined by geometric rules. Florence, Italy

  16. Perspective Example The gazelles in this photo appear to be roughly the same physical size. Thanks to John Clapp for these slides

  17. Perspective Example Move from here…to here Let’s move one using cut-and-paste. How big will it be?

  18. Perspective Example Surprised? Objects appear much smaller with distance! Your brain adjusts and “sees” the animals as equal size.

  19. Perspective Example Move down Even a short distance into the background makes a surprising difference.

  20. Perspective Example Move from here…to here Now let’s go the other way and move from foreground to background. Again, try to visualize how large the gazelle will be when cut-and-pasted. Try to visualize it’s size…

  21. Perspective Example Surprised? Instead of a gazelle it’s now Godzilla. Try this at home with your own photos.

  22. Drawing with Perspective From that example we see that it’s not easy to predict how large or small objects will be at different distances. How do artists create images with realistic perspective? By using geometry!

  23. Perspective Demonstration Now add the rest of the figures and draw this.

  24. Perspective Demonstration Horizon We can check that the persons in the foreground and background are the same height by drawing lines back to the horizon.

  25. Distorted Perspective Modern painters sometimes distort the perspective for dramatic effect. The two buildings converge to two different horizons. This feels weird and unnatural, which is what the artist intended (note the title of the painting). Mystery and Melancholy of a Street, de Chirico, 1914

  26. Shadows Size and sharpness of a shadow depends on size and distance of light source and of object casting the shadow. Physics 1 (Garcia) SJSU

  27. Penumbra UMBRA Penumbra Ray Tracing for Shadows Trace rays from light source to wall to map out location of deep shadow (umbra) and fuzzy shadow (penumbra). Light source Object Wall Physics 1 (Garcia) SJSU

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