Chapter 26Chapter 26

Properties of Light

Visible light originates with accelerated

motion of electrons.

It is an electromagnetic wave phenomenon.

1. ELECTROMAGNETICWAVES

Accelerating charges give off EM waves.

Demo - Tesla Coil, Radio, and Fluorescent Demo - Tesla Coil, Radio, and Fluorescent

LampLamp

EM Wave – Next SlideEM Wave – Next Slide

Electromagnetic Wave Electromagnetic Wave VelocityVelocity

A certain speed is required to satisfy

conservation of energy requirements.

That speed is c.

c = 300 million m/s

Large Number of Dots – Next SlideLarge Number of Dots – Next Slide

= 186,000 mi/s

50, 000 Dots

Maxwell discovered the nature of light.

Jump to Light SpeedJump to Light Speed

Hollywood’s View of Light SpeedHollywood’s View of Light Speed

Star Trek IVStar Trek IV

Hollywood’s View of Light SpeedHollywood’s View of Light Speed

THE ELECTROMAGNETIC SPECTRUM

Different kinds of sources produce

the different regions of the EM

spectrum.

THE ELECTROMAGNETIC SPECTRUM

THE ELECTROMAGNETIC SPECTRUM

EM Waves Sources

Radio Waves

Microwaves

Infrared

Visible

Ultraviolet

X-rays

Gamma rays

Vibrating charges

Molecular rotations

Molecular vibrations

Atomic vibrations

Atomic vibrations

Atomic vibrations

Nuclear vibrations

We live in an “ocean” of waves.

For more information visit URL1.

Video - Microwave Video - Microwave

URL - X-ray of Hand - X-ray of Hand

URL - X-ray of Lungs - X-ray of Lungs

Video - The Atomic BombVideo - The Atomic Bomb

2. TRANSPARENT MATERIALS

Atom Spring Model – Next SlideAtom Spring Model – Next Slide

For glass the natural frequency is

in the UV.

Energy of EM wave in UV stays

with atoms longer causing

vibrations with neighboring atoms

with loss of energy.

Thus UV is absorbed.

The atom holds the energy for less time, with less chance of collision with neighboring atoms, and less energy transformed to heat.

All visible frequencies get through clear glass. (Not all their energies do.)

For visible light the electrons in the glass are forced into vibration, but at less amplitude.

There is an apparent slow down in the

speed of light.

Light in Transparent Medium – Next Light in Transparent Medium – Next

SlideSlide

Besides the UV, heat waves (IR)

don't get through either.

TRANSLUCENT MATERIALS

Frosted glasses are good examples of

translucent materials.

They scatter light on transmission.

You can’t read or identify objects from

light passing through translucent

materials.

Demo – Wax PaperDemo – Wax Paper

3. OPAQUE MATERIALS

Most things around us are opaque. Metals are shiny and reflect.

Because of free electrons - they don’t pass on energy to atoms.

Our atmosphere is transparent in visible and part of IR.

It's good that UV is absorbed in our atmosphere. UV causes sunUV causes sunburnsburns.

Demo - Radiometer and Filters Demo - Radiometer and Filters

Umbra

Penumbra

SHADOWSSHADOWS When light is blocked, shadows are created. The kind of shadow (umbra or penumbra)

depends on the source of light and the relative positions of the light source, object, and shadow of the object.

Extended light sources cause shadows of objects to be partial shadows (penumbras) that may or may not have umbras.

A point source of light will create a shadow that only has an umbra.

A penumbra will be small if the light source is far from the object and its shadow.

Demo – Shadows using Projector then PenlightDemo – Shadows using Projector then Penlight

Partial ShadowPenumbra

Umbra

Solar EclipseSolar Eclipse

Sun

Moon

EarthFull Shadow

Looking at the sun while the moon passes in front of it.

Annular EclipseAnnular Eclipse

Again looking at the sun while the moon passes in front of it.

Lunar EclipseLunar Eclipse

Looking at the moon while it passes through the earth’s shadow.

Earth’s shadow

New Moon PhaseFull Moon Phase

Cornea

Iris

Pupil

- has the eye color and controls

light intensity

Lens

- the hole in the eye

- does remainder of focusing

Retina- location of light sensors, has rods and cones

Blind spot

Fovea- center of vision, predominantly cones

- optic nerve exit, no light sensors

4. SEEING LIGHT - THE EYE

- does most of the focusing

Primates and a species of ground squirrel are the only mammals that experience full color vision.

At the periphery of our vision we see

Motions first as objects come from behind

Colors second

Demo - Blind SpotDemo - Blind Spot

Examples: stars and twilight Demo - Turn off Room LightsDemo - Turn off Room Lights Rods see blue better. Blue will

appear much brighter than red in dim light, though the red might be much brighter than the blue in bright light.

Rods are more sensitive to low light levels than cones are.

Red Eye Photography

Lateral InhibitionWe don't perceive the actual differences in brightness in fields of light and dark.We even out our visual field.This allows us to discern detail in very bright areas and in dark areas at the same time.We accentuate differences rather than similarities.

Lateral InhibitionLateral Inhibition – Next Slide– Next Slide

Chapter 26 Review QuestionsChapter 26 Review Questions

The speed of light

(a) has never been measured

(b) is about the same as that of sound

(c) is infinitely fast

(d) is very fast, but not infinite

(a) you stand in the penumbra of the moon’s shadow

(b) you stand in the umbra of the moon’s shadow

(c) sunlight diffracts around the moon(d) sunlight reflects from the moon to the

earth

You will observe a total eclipse of the sun when

Which of the following will cast a shadow that has an umbra but no penumbra?

(a) the sun(b) an incandescent lamp(c) a fluorescent lamp(d) a point source of light

What is the purpose of the pupil in an eye?

(a) to adjust the focal length of the lens

(b) to adjust the amount of light passing through the lens

(c) to focus the image(d) to change the width of the field

of view