Chapter 26 Properties of Light

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# Chapter 26 Properties of Light - PowerPoint PPT Presentation

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 &amp; Sound.

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Part VI: Light

### Chapter 26Properties of Light

Physics 1 (Garcia) SJSU

Electromagnetic Waves

Moving charges (currents) create magnetic fields.

Oscillating magnetic fields create electric fields.

These effects create electromagnetic waves.

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

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

Physics 1 (Garcia) SJSU

Electromagnetic Spectrum

Cell phone

electromagnetic waves

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)

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?

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).

Physics 1 (Garcia) SJSU

Demo: Microwaves

Test transparency, opacity, & reflectivity of:

• Metal (aluminum foil)
• Wood (book)
• Glass
• Water

Microwave

Microwave

Transmitter

Wavelength of microwaves is fraction of a centimeter (longer than visible light).

Physics 1 (Garcia) SJSU

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)
• 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.

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

Pre-15th Century Paintings

Occlusion but no sense of distance

Mongol Ruler and consort enthroned, 14th century

Renaissance Paintings

Scenes in these paintings look realistic

Marriage of the Virgin, Raphael, 1504

The Annunciation, Botticelli, 1489

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

Perspective Example

Move from here…to here

Let’s move one using cut-and-paste. How big will it be?

Perspective Example

Surprised? Objects appear much smaller with distance!

Perspective Example

Move down

Even a short distance into the background makes a surprising difference.

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…

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

Horizon

We can check that the persons in the foreground and background

are the same height by drawing lines back to the horizon.

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

Size and sharpness of a shadow depends on size and distance of light source and of object casting the shadow.

Physics 1 (Garcia) SJSU

Penumbra

UMBRA

Penumbra