Apr 18, 2023 Physics 1 (Garcia) SJSU

Chapter 26Properties of Light

Part VI: Light

Apr 18, 2023 Physics 1 (Garcia) SJSU

Electromagnetic Waves

Moving charges (currents) create magnetic fields.

Oscillating magnetic fields create electric fields.

These effects create electromagnetic waves.

Apr 18, 2023 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

Wav

e

Apr 18, 2023 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/s100,000,000 Hz

Apr 18, 2023 Physics 1 (Garcia) SJSU

Electromagnetic Spectrum

Very broad spectrum ofelectromagnetic waves

Cell phone

Apr 18, 2023 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)

Apr 18, 2023 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?

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Ultraviolet Light

Fluorescent (Day-Glo) paint converts invisible ultraviolet light into visible light.

Test opacity of:

• Glass

• Water

• SunscreenUltraviolet

LampWavelength of ultraviolet light is about the size of bacteria (shorter than visible light).

Apr 18, 2023 Physics 1 (Garcia) SJSU

Demo: Microwaves

Test transparency, opacity, & reflectivity of:

• Metal (aluminum foil)

• Wood (book)

• Glass

• Water

MicrowaveTransmitter

MicrowaveReceiver

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-15th Century Paintings

Mongol Ruler and consort enthroned, 14th century Road to Calvary, Martini, 1315

Occlusion but no sense of distance

Renaissance PaintingsScenes 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 backgroundare 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).

Apr 18, 2023 Physics 1 (Garcia) SJSU

ShadowsSize and sharpness of a shadow depends on

size and distance of light source and of object casting the shadow.

Apr 18, 2023 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

PenumbraLight source

Object

Wal

l