Chapter 26Chapter 26

Properties of Light

Origin and Nature of LightOrigin and Nature of Light

Light originates with accelerated motion of

electrons.

It is an electromagnetic wave phenomenon.

1. ELECTROMAGNETICWAVES

James Clerk Maxwell

Electromagnetic Wave VelocityElectromagnetic Wave Velocity

The speed of light (E&M waves) is called c.

c = 300,000 km/s = 186,000 mi/s

Galileo’s Experiment

AB

c = Round Trip Distance/Total Time

The Speed of LightThe Speed of Light

Earth

Jupiter

Ole Roemer (1675)Ole Roemer (1675)

THE ELECTROMAGNETIC SPECTRUM

Visible Light

Infrared

Visible Light

Ultraviolet

X-rays

Gamma Rays

Radio Light

Microwaves

Previous Slide

The Visible SpectrumThe Visible Spectrum

400nm 500nm 600nm 700nm

Wavelength means

COLOR

THE ELECTROMAGNETIC SPECTRUM

EM Waves SourcesRadio Waves Microwaves

InfraredVisible

UltravioletX-rays

Gamma rays

Vibrating charges Molecular rotations Molecular vibrations

Atomic vibrations Atomic vibrations Nuclear vibrations

Atomic vibrations

Gamma RaysGamma Rays

Compton Gamma Ray Observatory

X-raysX-rays

Chandra X-Ray Observatory

UltravioletUltravioletUltraviolet Imaging Telescope

Messier 74

InfraredInfrared

RadioRadio

Very Large Array

Cygnus A

Speed, Wavelength, Speed, Wavelength, FrequencyFrequency

c = f

Properties of LightProperties of Light Speed (c)

Nothing travels faster Absolute constant

Wavelength () Gamma rays, X-rays, UV, Visible, IR, Radio In visible wavelength means color

Frequency (f) Number of complete waves per second

Energy of a Photon (E) E = hf

2. TRANSPARENT MATERIALS 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.

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

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

All visible gets through clear glass.

Light Speed in a MediumLight Speed in a Medium

There is an apparent slow down in the speed of light.

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.

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 sunburns.

Demo - Radiometer and FiltersDemo - Radiometer and Filters

4. SHADOWS When light is blocked, shadows are created. The kind of shadow depends on the source

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

For instance, a total shadow (umbra) occurs when an object is illuminated by a point light source.

Extended light sources cause shadows of objects to be partial shadows (penumbras).

The shadows may or may not have umbras. A penumbra will be small if the light source

is far from the object and its shadow.

Penumbra

Umbra

Partial Shadow

Full Shadow

Solar EclipseSolar Eclipse

Sun

Moon

Earth

Annular EclipseAnnular Eclipse

Lunar EclipseLunar EclipseSun

EarthMoon

Earth’s Shadow

The Eye

5. SEEING LIGHT - THE EYE

Cornea - does most of the focusing

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

Color DeficiencyColor Deficiency

Myopia (Near-Sightedness)Myopia (Near-Sightedness)

People with near-sightedness, also known as myopia, cannot see clearly at distance. Myopia occurs when the eyeball is too long or when the cornea (the clear part on the front of the eye) is too steep.

Hyperopia (Farsightedness)Hyperopia (Farsightedness)or or presbyopia

Demo - Blind Spot (in text)Demo - Blind Spot (in text) 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– Colors second

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

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.

Lateral InhibitionLateral Inhibition

We 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.

Measured BrightnessMeasured Brightnessof Last Slideof Last Slide

Optical IllusionsOptical Illusions

Show Pinwheel

Chapter 26 Review QuestionsChapter 26 Review Questions

You will observe a total eclipse of the sun when

(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

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

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

(d) a point source of light

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

(d) is very fast, but not infinite

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

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