Chapter 26 Properties of Light Sources of light Luminous –Producing light –The Sun (luminous) versus the Moon (nonluminous) Incandescent –Glowing with.

Chapter 26Chapter 26Properties of Properties of

LightLight

Sources of light

• Luminous – Producing light– The Sun (luminous) versus the Moon (nonluminous)

• Incandescent – Glowing with visible light from high temperatures – Examples: flames, incandescent light bulbs – Produce light via electromagnetic waves

• Electrical charge is accelerated by external force• Acceleration produces wave consisting of electrical and

magnetic fields• Wave moves through space; the fields exchange energy

and continue on until the energy is absorbed by matter

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

1. Incandescence a. mercury vapor

light tubes

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

paints and plastics

3. Phosphorescence c. light bulbs

with filaments

1. 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

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 SpectrumThe Electromagnetic Spectrum

• 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 SpectrumThe Electromagnetic Spectrum

• 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.

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

Which characteristic of waves of visible light is most similar to waves of microwaves?

• A)speed

• B)wavelength

• C)frequency

• D)amplitude

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

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.

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.

The human eye

• Uses convex lens with muscularly controlled curvature to change focal distance

• Nearsightedness (myopia) - images form in front of retina• Farsightedness (hyperopia) - images form behind retina• Correction - lenses (glasses, contacts) used to move images onto retina