Properties of Properties of LightLight

LightLightPhotonsPhotons

ElectromagneticElectromagneticRadiationRadiation

These three are the same…

• Light * pure energy

• Electromagnetic Waves* energy-carrying waves emitted by vibrating electrons

• Photons* particles of light

Electromagnetic Electromagnetic WavesWaves

• Demo– Tesla Coil, Radio, Fluorescent light

Electromagnetic Electromagnetic Wave VelocityWave Velocity

• The speed of light is the same for all seven forms of light.

• It is 300,000,000 meters per second or 186,000 miles per second.

The Electromagnetic The Electromagnetic SpectrumSpectrum

• Radio Waves - communication

• Microwaves - used to cook

• Infrared - “heat waves”

• Visible Light - detected by your eyes

• Ultraviolet - causes sunburns

• X-rays - penetrates tissue

• Gamma Rays - most energetic

THE ELECTROMAGNETIC SPECTRUM

EM Waves Sources

Radio Waves

Microwaves

Infrared

Visible

Ultraviolet

X-rays

Gamma rays

Vibrating charges

Molecular vibrations

Molecular vibrations

Atomic vibrations

Atomic vibrations

Atomic vibrations

Nuclear vibrations

The Electromagnetic The Electromagnetic SpectrumSpectrum

• A range of light waves extending in wavelength from radio waves to gamma rays

The Visible SpectrumThe Visible Spectrum

•A range of light waves extending in wavelength from about 400 to 700 namometers.

Transparent Transparent MaterialsMaterials

• Transparent - the term applied to materials through which light can pass in straight lines

Opaque MaterialsOpaque Materials

• Opaque - the term applied to materials that absorb light

Example QuestionsExample Questions• Are clouds transparent or opaque to

visible light?– Answer: opaque

• Are clouds transparent or opaque to ultraviolet light?

– Answer: transparent

ShadowsShadows• Umbra - the darker part of a shadow

where all the light is blocked

• Penumbra - a partial shadow

• These terms also apply to Solar Eclipses and Lunar Eclipses.

Penumbra

Umbra

Partial Shadow

Full Shadow

Solar Eclipse

Sun

Moon

Earth

• A solar eclipse occurs when the Moon passes in front of the Sun.

Lunar Eclipse

Sun

EarthMoon

• A lunar eclipse occurs when the Moon passes into the Earth's shadow.

Questions• Which type of eclipse is dangerous to

view with the unprotected eye?

• Why are lunar eclipses more commonly seen than solar eclipses?

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 (red eye demo)

does remainder of focusing

• Retina -location of light sensors, has rods and cones

• Blind spot -

• Fovea - center of vision, predominantly conesoptic nerve exit, no light sensors

Color Deficiency

Myopia (Near-Sightedness)

People with near-sightedness cannot see clearly at distance.

Hyperopia (Farsightedness)

People with far-sightedness cannot see clearly up close.

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

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

In the dark at late evening, no color is seen because of lack of stimulation of

a. rods.

b. cones.

c. cornea.

d. crystalline lens.

Light Emission

• The following slides will illustrate facts about Light and electrons as they relate to atomic structure and release of energy

Structure of the Atom• Proton

• Neutron

• Electron

• Energy Levels

ExcitationExcitation• When an electron is raised to a higher

energy level, the atom is said to be excited.

EmissionEmission• When the electron returns to a lower

energy level, energy is released in the form of light.

• Different transitions from high levels to low levels result in different colors of light.

Robert BunsenGustav Kirchhoff

“The Taste Test”versus

“The Flame Test”

The Kirchhoff-Bunsen ExperimentThe Kirchhoff-Bunsen Experiment• These two scientists found that burning chemicals over an open flame resulted in a spectrum with bright lines.

• They found that each chemical element produced its own characteristic pattern of bright spectral lines.

Emission Spectra of Hydrogen

Prism

Photographic Film

Film

Slit

Low DensityGlowing

Hydrogen Gas

Discrete Emission Spectrum

• Hot gas produces a bright line emission spectrum.

• Demo - hot hydrogen gas and diffraction gratings

Emission Spectrum

Emission SpectraEmission Spectra

Helium

Hydrogen

Oxygen

Carbon

Every element can be “fingerprinted” by it spectra.

Emission Spectra

Continuous Emission Spectrum

Prism

Photographic Film

Slit

White Light Source

IncandescenceIncandescence

• Hot, dense solids produce a continuous spectrum.

• Demo - an incandescent light bulb and diffraction gratings

Continuous Spectrum

• The brightness and color of light emitted by a hot object changes with its temperature.

• Glowing object colors:• Reddish coolest glowing object

• Orange-ish

• Yellowish

• White

• Bluish hottest glowing object

Discrete Absorption Spectrum

Absorption Spectraof Hydrogen

Prism

Photographic Film

Film

Slit

White Light Source

Discrete Emission Spectrum

Hydrogen Gas

Absorption SpectraAbsorption Spectra• Cool gas in front of a continuous source of

light produces an absorption line spectrum.

• Fraunhofer lines in our Sun's spectrum showed that cool helium gas surrounds the Sun.

Absorption Spectrum

Matching QuestionsMatching Questions

Type of Spectrum Appearance

1. Emission Spectra a. All Colors

2. Continuous Spectra b. Dark Lines

3. Absorption Spectra c. Bright Lines

Matching QuestionsMatching Questions

1. Emission Spectra a. Hot Solids

2. Continuous Spectra b. Hot Stars

3. Absorption Spectra c. Hot Gases

Fluorescence Fluorescence • Some materials that are excited by

ultraviolet light can emit visible light upon de-excitation. This is fluorescence.

• Demo - Black light and chalk

• See Figures 30.10 & 30.11

Fluorescent LampsFluorescent Lamps• Primary excitation - electron collisions with

low pressure mercury vapor, and ultraviolet light is given off

• Secondary excitation - ultraviolet light is absorbed by phosphors and these emit visible light

Phosphorescence Phosphorescence • Phosphorescence - a type of light emission

that is the same as fluorescence except for a delay between excitation and de-excitation.

• Electrons get "stuck" in an excited state and de-excite gradually.

• Demos - glow-in-the-dark objects

Lasers Lasers • Laser

– Light amplification by stimulated emission of radiation

• Lasers produce coherent light.• Coherent light means that all the light waves

have the frequency, phase and direction.• Demo - Laser and chalk dust• Demo - Laser and prism or diffraction grating

Matching QuestionsMatching Questions1. Incandescence a. mercury vapor

light tubes

2. Fluorescence b. glow-in-the-dark

paints and plastics

3. Phosphorescence c. light bulbs

with filaments

Matching QuestionsMatching Questions1.Fluorescent Lamp a. color changes

with temperature

2. Incandescent Bulb b. only one color

of light

3. Laser c. converts ultraviolet

light to visible light

End of Chapter 30

• Brightness versus color curve for different temperatures

T f

(measured in Kelvins)

0.0

0.1

0.1

0.2

0.2

0 500 1000

Wavelength (nm)

Re

lativ

e E

nerg

y

Emission Spectra

• Therefore, each element emits is own characteristic pattern of light frequencies.