Chapter 26

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# Chapter 26 - PowerPoint PPT Presentation

Chapter 26. Properties of Light. Electromagnetic Waves. Traveling, oscillating, electric and magnetic fields which are emitted by vibrating charges. The electric and magnetic fields carry energy.

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## Chapter 26

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Presentation Transcript

### Chapter 26

Properties of Light

Electromagnetic 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)
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 thespeed 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

-ray

Micro

I.R.

Visible

U.V.

X-ray

104

106

1010

1013

1023

Hz

Visible Part of EM Spectrum

red =700nm

fred =4.29 x 1014Hz

violet=400nm

f violet= 7.50 x 1014Hz

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:

OR

• the vibrational frequencies of the atoms and molecules as a whole.
• then a resonant condition is established, and the light is absorbed.
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 is transmitted

Visible light

UV & IR get absorbed

Diffraction

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

Light Source

Wide Window

Light Source

Interference Pattern

Narrow Slit

Two or more slits – Diffraction Grating

Light Source

Interference Pattern

Two or more slits

Air

OIL

Water

Polarization

The alignment of the transverse waves.

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.

Light rays from the sun are essentially parallel

Sharp

Large far away light source

Sharp

Small nearby light source

Large nearby light source

Penumbra

Umbra

Eclipses

Sun

Full Moon

Not To Scale

Lunar Eclipse

Earth

Moon

Sun

Not To Scale

Solar Eclipse

Earth

Moon

Sun

SUN

Drawing Not To Scale

SOLAR

ECLIPSE