The Electromagnetic Spectrum

Historical Intro and Overview

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S&T club 27 Jun 2022

Freethinkers 22 Sep 2022

DMSP IR Aurora Imagery

Maxwell and the Electromagnetic Spectrum

• “Maxwell’s Equations are one of the great intellectual achievements of mankind.” Physics textbook

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• History of “discovery” of electromagnetic spectrum
• Guided tour of the E&M spectrum

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• MIT’s Maxwell’s Equations t-shirt day….

Maxwell’s Equations t-shirts�Electricity…Magnetism…Light (and math!)

19^th Century Science�The Royal Academy of Sciences

• Electricity: Ben Franklin understanding (i.e. not much…)
• Electricity as a strong natural phenomenon and weak lab curiosity
• DIY wires, resistors, capacitors, and batteries (box of leaky, smelly, toxic chemicals)
• Time traveling EE joke….
• Magnetism: very mysterious...“Magic” iron rocks attract iron…or not…
• Earth is a big magnet which we can use for compasses
• Otherwise, another weak lab curiosity (could not be “created”)
• Light: very useful!! But what is it???

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Wave: interference (Young)

Transverse: polarization

ROYGBIV: dispersion (frequencies)

Speed: Fast! 2.8 – 3.2 E8 m/s (M & M)

Why so fast? Physical determinants?

Energy type?

What oscillates?

What is the medium? Ether??

Maxwell and “his” Equations

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Gauss’s Law for Electricity

• Charges cause E fields
• Coulomb’s Law is equivalent

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Gauss’s Law for Magnetism

• No magnetic monopoles

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Ampere’s Law

• E Currents cause B fields
• Connects E and B!

Faraday’s Law

• Changing B causes E
• Wow! Civilization changing!!
• Electromagnetism is a thing…

These 4 equations explain 99.9% of observed E and B theory!

Just a few instances of ‘stray’ B fields unaccounted for……

net

So what did Maxwell do?

1) He ‘completes’ Ampere’s Law

• He’s a mathematician
• Argues from ‘symmetry’
• “what should be” to “what is”
• If changing B causes E, maybe changing E creates B…and it does!
• Pop Quiz: Why no equivalent term for I then in Faraday’s Law????

These 4 equations explain 100% of observed E and B theory!

2) He has the first complete set of electromagnetic equations

• Doesn’t seem fair or a big deal
• Pyramid joke

So what did Maxwell do?

3) He ‘figures out’ light!

• Tries to reduce four eqs to two – seems to be some symmetry
• “Sort of” can but lose info, but…

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See white board

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• “Obviously” those are sinusoidal traveling wave equations…really…
• And their speed is SQRT (1/ε0μ0)
• 3 x 10E8 m/s!!!!
• These “must” be light waves!!!!

These 4 equations explain 100% of observed E and B theory!

19^th Century Science�The Royal Academy of Sciences

• Light: very useful!! But what is it???

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Speed: Fast! 2.8 – 3.2 E8 m/s (M & M)… Maxwell “nails it”!!!

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Why so fast? Physical determinants? Inherent property of E and B fields!!

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Energy type? Electric and Magnetic energy!!!

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What oscillates? Electric and Magnetic fields!!!

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What is the medium? Ether?? E and B can travel in a vacuum!!! Duh!!! Apocryphal story

So what did Maxwell do?

4) He ‘discovers’ E&M waves! 1865

Logic: There “must” be E&M waves below (infra) red and above (ultra) violet

And there are!

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• Color vs B&W??
• Triangle vs symphony orchestra??

These 4 equations explain 100% of observed E and B theory!

Argh……..

Electromagnetic Spectrum (the basics)

• Low Frequency (f) Radio -------------------- gamma Hi frequency
• Long wavelength (λ) --------------------------------------- Short λ
• Low Energy (photonic) --------------------------------------- Hi Energy
• Low temperature source --------------------------------------- Hi temp
• Low resolution / info --------------------------------------- Hi res / info
• Wave-like ----------------------------------------- particle-like

(photon)

• SPEED: same for all (c is NOT just the speed of light – speed of ALL E&M waves)

c = f λ = 3E8 m/s = fast!!! Universe’s ultimate speed limit

• Waves interact with ‘things’ about their λ size

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The grand tour

• Who /How / When ‘discovered’?

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• Why named? (usually badly even wrongly named!)

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• Importance / relevance?

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• Something interesting / unique?

Visible

• Who /How / When ‘discovered’?

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• Why named? (usually badly even wrongly named!)
• Importance / relevance?
• Only a factor of 2x in the band…tiny unimportant part of E&M spectrum….
• Something interesting / unique?
• Color is NOT a thing…mental construct…Is your red the same as my red?
• ROYGBIV under a different star?
• Blu-Ray DVDs

Ultraviolet

• Who /How / When ‘discovered’?
• Maxwell theorized…..Silver chloride experiments
• Why named? (usually badly even wrongly named!)
• Importance / relevance?
• Can’t penetrate skin (UVA/B/C)…(outer electrons energy)
• Skin cancer! Melanoma
• Cataracts! We will ALL get cataracts. Cheap sunglasses should be banned!
• Sterilizing…as on Mars (lack of atmosphere) or in some hospitals now
• Something interesting / unique?
• Raptor eyes…better vision, why no cataracts???
• High tech silicon wafers…better than Blu-ray!

X-rays

• Who /How / When ‘discovered’?
• Roentgen tinkering with cathode ray tubes
• Showed his wife her skeletal hand (scary!)
• Why named? (usually badly even wrongly named!)
• Yeah, this is a bad name…”x as unknown, like in algebra” (Hi Freq UV?)
• Importance / relevance?
• Penetrates skin, but not bone (inner electrons energy)
• Something interesting / unique?
• Yes, x-rays are high E damaging rays

Gamma rays

• Who /How / When ‘discovered’?
• Tinkering with “magic” radium rocks … three things came out near magnets
• Why named? (usually badly even wrongly named!)
• Alpha, beta, gamma…might as well be A,B,C or 1,2,3 Greek sounds geekier?
• Electromagnetic, NOT nuclear radiation…but…
• Importance / relevance?
• Can penetrate bone (nucleus energy)
• Something interesting / unique?
• Does NOT turn you into the Hulk!
• Radiation treatment does NOT use ‘safe’ radiation….
• Generated by some of the most energetic events in the

universe and we didn’t know they were happening! “Picture” of a black hole

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Infrared

• Who /How / When ‘discovered’?
• Maxwell theorized…..prism light temperature experiments
• Why named? (usually badly even wrongly named!)
• Importance / relevance?
• Near IR (near what?) – basically “light” that we can’t see…GEN I NVGs
• Far IR (thermal…why? 100 degree Fahrenheit emission…humans/mammals)
• You are emitting FIR right now and it is detectable and “cooling”
• Something interesting / unique?
• GEN III NVGs use FIR (‘see’ thru smoke, invert INFIL planning!)

Microwave

• Who /How / When ‘discovered’?
• Radio engineer’s chocolate bar snacks melted
• Why named? (usually badly even wrongly named!)
• Micro means 10^-6 to scientists, not ‘really small’
• Silly English twits!
• Importance / relevance?
• Resonant frequency to water molecules energy…’cook’ organic matter
• Why do microwave ovens have metal mesh screen with little holes? Safe??
• 5G … 6G WiFi
• Something interesting / unique?
• Pulsar discovery (LGM-1, -2, -3, etc)
• Cosmic microwave background

Radio

• Who /How / When ‘discovered’?
• Heinrich Hertz first to deliberately create, send, and receive E&M waves
• Rotating spark generator…Hertz scholarship story?
• Why named? (usually badly even wrongly named!)
• Importance / relevance?
• RADAR (acronym)…+ Doppler now
• Something interesting / unique?
• Why only AM and FM? Why talk radio on AM and orchestra music on FM?

MM (millimeter) Wave

• Who /How / When ‘discovered’?
• Latest (last?) to exploit
• Why named? (usually badly even wrongly named!)
• Yay…..
• Importance / relevance?
• Airport scanners (less E so don’t penetrate hard plastic…pornographic???)
• Something interesting / unique?
• Pain ray