chapter 27 properties of light light photons electromagneticradiation

Chapter 27Chapter 27Properties of LightProperties of Light

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 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.

Michelson’s 1878 Rotating Mirror Experiment

Picture credit

• German American physicist A.A. Michelson • Using this method, Michelson was able to calculate c = 299,792 km/s

The Electromagnetic The Electromagnetic SpectrumSpectrum

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

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 Visible SpectrumThe Visible Spectrum

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

Questions• Is it correct to say that radio

wave is a low-frequency light wave?

• Is a radio wave also a sound wave?

• See question on page 455.

*

Transparent Transparent MaterialsMaterials

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

Opaque Opaque MaterialsMaterials

• 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.

Polarization

Electric field only goingup and down – say it islinearly polarized.

Light can have other types of polarizations such as circularly polarized or elliptically polarized. We will only look at linearly polarized light.

Net electric field is zero – Unpolarized light!

How do we make Polarized Light?

I. Polarizers-

Polarizers are made of longchained molecules which absorblight with electric fields perpendicular to the axis.

• Light is totally polarized when an object with a vertical Polarization axis and another object with a horizontal polarization axis are placed next to each other.

• This causes no light to be able to get through the lens.

Questions• Which type of eclipse is

dangerous to view with the unprotected eye?

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

• See question on page 460.

*

Chapter 27 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

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

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.

b. cones.

End of Chapter 25