15-May-15 Physics 1 (Garcia) SJSU Chapter 26 Properties of Light Part VI: Light

15-May-15 Physics 1 (Garcia) SJSU Electromagnetic Waves Moving charges (currents) create magnetic fields. Oscillating magnetic fields create electric fields. These effects create electromagnetic waves.

15-May-15 Physics 1 (Garcia) SJSU Demo: Light & Sound Sound waves can only travel through a material, such as air, but light waves can travel through vacuum. Can see cell phone ringing inside vacuum chamber but don’t hear the sound. Sound waves are not electromagnetic waves Radio Wave

15-May-15 Physics 1 (Garcia) SJSU 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

15-May-15 Physics 1 (Garcia) SJSU Electromagnetic Spectrum Very broad spectrum of electromagnetic waves Cell phone

15-May-15 Physics 1 (Garcia) SJSU 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)

15-May-15 Physics 1 (Garcia) SJSU 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?

15-May-15 Physics 1 (Garcia) SJSU 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).

15-May-15 Physics 1 (Garcia) SJSU Demo: Microwaves Test transparency, opacity, & reflectivity of: Metal (aluminum foil) Wood (book) Glass Water Microwave Transmitter Microwave Receiver Wavelength of microwaves is fraction of a centimeter (longer than visible light).

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)

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

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

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

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

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

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

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

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

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

Move from here…to here Try to visualize it’s size… Perspective Example 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.

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

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!

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

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

Distorted Perspective Modern painters sometimes distort the perspective for dramatic effect. Mystery and Melancholy of a Street, de Chirico, 1914 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).

15-May-15 Physics 1 (Garcia) SJSU Shadows Size and sharpness of a shadow depends on size and distance of light source and of object casting the shadow.

15-May-15 Physics 1 (Garcia) SJSU Ray Tracing for Shadows Trace rays from light source to wall to map out location of deep shadow (umbra) and fuzzy shadow (penumbra). UMBRA Penumbra Light source Object Wall