Chapter 26Chapter 26

Properties of LightProperties of Light

Electromagnetic WavesElectromagnetic Waves• Traveling, oscillating, electric and

magnetic fields which are emitted by vibrating charges.

• The electric and magnetic fields carry energy.

• All electromagnetic waves travel at the speed of light in vacuum. (i.e.the wave speed = the speed of light)

Typical Electromagnetic Wave

Michael Faraday & James Clerk Maxwell

• c 1831: Michael Faraday discovers electromagnetic Induction. Faraday also introduces the field-line concept for electricity and magnetism.

•c 1865: James Clerk Maxwell develops mathematical relationships between electric and magnetic fields. Shows that all of electricity and magnetism are wave phenomena.

• When Maxwell calculated the speed of these “Electro-magnetic” waves, he came up with the speed of light.

The speed of light = constant in vacuum

c 3.00 x 108 m/s = 300,000,000m/s

• electromagnetic wave in vacuum c = f

• Note that everything you have learned about mechanical waves, with the exception of the need for a medium, also applies here to electromagnetic waves

Electromagnetic Spectrum

Radio Micro I.R. Visible U.V. X-ray -ray

104 106 1010 1013 1023Hz

Visible Part of EM Spectrum

red =700nm

fred =4.29 x 1014Hz

violet=400nm

f violet= 7.50 x 1014Hz

Electromagnetic Spectrum

Transparent Materials

• When light is incident on a particular material, the electrons and atoms of the material begin to vibrate while the light propagates through the material.

• If the frequency of the light matches the:

–natural frequency of the electrons of the atoms in the material.

then a resonant condition is established, and the light is absorbed.

the vibrational frequencies of the atoms and molecules as a whole.

OR

• For example:

Most glass is made up of atoms (Si and other elements).

They possess electrons that have natural frequencies of vibration that closely match the frequency of ultra-violet (UV)light.

(1015- 1017) Hz

The larger atoms and molecules will vibrate at lower IR frequencies. So if IR is present, it will also be absorbed with the energy being dissipated as heat.

• The visible radiation is sequentially absorbed, and then re-emitted from atom to atom. Eventually, the visible light passes through the entire piece of glass.

• A state of resonance occurs for UV light.

•We say that the glass is transparent to visible light, but not to UV or IR.

Glass

Visible light

UV radiationUV & IR get

absorbed

Visible light is transmitted

IR radiation

Diffraction

Any bending of light by means other than reflection and refraction is called diffraction.

Light Source

Wide Window

Light Source

Narrow Slit

Interference Pattern

Two or more slits – Diffraction Grating

Light Source

Interference PatternTwo or

more slits

Thin Film Phenomena – Constructive and destructive Interference

Water

OIL

Air

Polarization

The alignment of the transverse waves.

Time delay between absorption and re-emission of visible light

Medium Average speed

Vacuum c = 3 x 108 m/s

Water v = 0.75c

Glass v = 0.67c

Diamond v = 0.41c

Opaque Materials

• In Opaque materials, light is absorbed without re-emission. The energy of the light including visible, goes into the internal energy of the material – heating.

• Metals are opaque but reflective.

•These free electrons are responsible for why metals make such good electrical and thermal conductors.

•Also, the light does not propagate from atom to atom as it does in a transparent material, but goes into reflection.

ShadowsShadowsLight rays from

the sun are

essentially parallel

Sharp

shadowLarge far away light source

Small nearby light sourceSharp

shadow

Large nearby light source

Penumbra(Partial shadow)

Umbra(Total shadow)

EclipsesEclipses

Sun

Full Full MoonMoon

Not To Scale

Lunar EclipseLunar Eclipse

Sun

Earth

Moon

Not To Scale

Solar Eclipse

Sun

MoonEarth

Drawing Not To Scale

SUN

Lunar Eclipse

Solar Eclipse

SOLAR

ECLIPSE

The End of Chapter 25