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Properties of Light

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Electromagnetic Waves:An electromagnetic wave is an oscillating combination of a

magnetic and an electric field.It can be visualized as two perpendicular waves

(electric and magnetic).

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Speed of Light:

In a vacuum, all electromagnetic waves have a velocity of

300,000,000 m/s! ( 3x108m/s)

That’s 186,000 miles per second!

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Electromagnetic Spectrum:

An electromagnetic spectrum is a “map” of the total range of waves.

Usually, high frequency (low λ) is on the right.Low frequency (high λ) is on the left.

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Notice the wide range of waves. We only perceive visible light, a tiny fraction of the

whole spectrum...

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Sometimes an object may have a very different appearance depending on the wavelength of light you are using to observe it. Ex: visible light and

infrared picture of the constellation Orion

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Historical attempts to explain lightHistorical attempts to explain light• Ancient ideaAncient idea

– light emanates from eye to illuminate objectlight emanates from eye to illuminate object• Newton (18Newton (18thth century) century)

– light is particulate and moves faster in light is particulate and moves faster in transparent materialstransparent materials

• Young (1801)Young (1801)– interference experiments demonstrates wave interference experiments demonstrates wave

nature of lightnature of light• Einstein (early 20Einstein (early 20thth century) century)

– explains photoelectric effect by assuming a explains photoelectric effect by assuming a particulate nature of light (lumps of waves with particulate nature of light (lumps of waves with E = hf) E = hf)

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Known Properties of LightKnown Properties of Light• Explained classically by Maxwell’s Equations Explained classically by Maxwell’s Equations

(in 1860)(in 1860)

• Transverse WaveTransverse Wave

– polarizationpolarization

• Wave speedWave speed

– speed of light in a vacuumspeed of light in a vacuum

• 3.00 3.00 10 1088 m/s m/s

– speed of light slower in mediaspeed of light slower in media

• Particle (?)Particle (?)

– the photon (E = hf)the photon (E = hf)

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Dual Nature of Light

      Light exhibits wave phenomena as a light wave is propagated by interchange of energy between varying electric and magnetic fields (Maxwell)

      Light acts like particles composed of kinetic energy and momentum when light interacts with matter

•Both wave and particle!

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Wave Nature of Light •Wave Nature

      Light exhibits wave characteristics like:

–Diffraction – bending around an obstacle

–Interference – “adding” & “subtracting” energy

–Doppler Effect – change in frequency due to motion

–Polarization - filtering

•Particles are not able to do any of those characteristics

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PolarizationPolarization

UnpolarizedUnpolarized

PlanePlanepolarizedpolarized

(Source of light is(Source of light isexcited atoms)excited atoms)

)sin( ˆ)sin(, tkxEjtkxEtxE mm

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Methods of chararacterizing the Methods of chararacterizing the light wavelight wave

• ray constructionray construction

– lines perpendicular to lines perpendicular to wavefronts showing wavefronts showing direction of motion of direction of motion of wavewave

• the plane wavethe plane wave

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Polarizer-AnalyzerPolarizer-Analyzer

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Particle Nature of Light •Particle Nature

        The Photoelectric Effect

–Light having a frequency above some minimum value and incident on metals causes electrons to be emitted from the metal

Albert Einstein explained the phenomenon using quantum theory developed by Max Planck

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Transparent Materials:

Transparent materials allow light to pass through in a straight line.

Examples:

glass, water

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The explanation for why certain materials are transparent and others aren’t, depends on the

atoms of the material and their vibrations.

It’s fairly accurate to imagine atoms in continual motion. Imagine they are linked

together with springs!

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When light hits a transparent material, it causes the atoms to vibrate only a little. The

wave has very little energy lost to those vibrating atoms. The wave is free to pass

through the transparent material.

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Opaque Materials:

An opaque material doesn’t allow light to pass through it.

In this case, the light causes the atoms of the material to vibrate a great deal. This dissipates

the energy of the wave, which can’t pass through the material...

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When light doesn’t pass through an opaque object, it is absorbed.

The energy of the wave is transferred into heat.

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If not absorbed, the light can be reflected from the object.

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Materials can be transparent to certain wavelengths, but not others..

For example, glass is transparent to visible light, but not to ultraviolet or infrared: