KaiserScience

Home » Physics » Electromagnetism » Light is an EM field

Light is an EM field

Advertisements

We’ve heard that light is an electromagnetic wave. What does this mean?

Abbreviations used in this section
B-field     magnetic field
E- field    electric field
EM            electromagnetic
I                 electrical current
Δ               (Greek letter delta) – “changing”

Consider the EM spectrum. This shows a range from the longest EM waves (far left) to the shortest EM waves (far right.)

Gamma rays Spectrum Properties NASA

What are EM waves made of?

Here we see e-fields radiating out of charged particles.

dipole animation electric

Here we see a B-field around a bar magnet, made visible with iron filings.

Fact: A changing E- field creates it’s own B-field

Fact: A changing B-field creates its own E- field

Hmm… so ΔE creates ΔB, which creates ΔE, which creates ΔB, and so on… forever. Yes, that’s right! Once we generate a E & B field like this, it travels through space forever, until it is absorbed by something. Let’s see what this looks like

em-wave-gif

The fields are at right-angles to each other

They travel through vacuum (empty space) at the speed of light

c  =  speed of light
c  =  3 x 108 m/s       =   186,282 miles/second

So all are parts of the EM spectrum – radio, light, Wi-Fi, X-rays,
are all made of exactly the same thing! The only thing different among them? wavelength and frequency.

How to make EM fields

Moving a magnet into a wire coil increases the B-field through the coil.
As B-field increases, I develops within the wire.

Moving magnet out of a wire coil decreases B-field through the coil.
As B-field decreases, I – in opposite direction – develops within the wire.

If magnet doesn’t move -> B-field stays constant -> I = 0

The following images are from Giancoli Physics, 6th edition, Pearson/Prentice Hall

Faraday law 1

______________________________________________

Given just this, how can we create radio waves, or any EM (electromagnetic) waves?

Consider two metal rods: they’ll be an antenna. We connect them to opposite terminals of a battery.

When the switch is closed, I develops, and E-fields are produced

E-fields move out in all directions

But changing E-fields generate (blue) B-fields.

dot in circle B field dot in a circle = B-field pointed out of the screen

X in circle B fieldX in a circle = B-field pointed into the screen

Directions of the B-field is always perpendicular to the E-fields.

B-fields are shaped in circles, like the E-fields:  since these circles are coming in/out of the screen, we can’t directly draw them.

The dots and X’s just show the parts of the circle coming at you, and away from you.

Generate EM field from wire

In the diagram below, we replace the DC battey with an AC generator. This changes the direction of the electrical current every second (or, for example, every thousandth of a second.)

When the current points up, we get E and B fields just like the previous diagram. But when the current points down, what happens?

* New E and B fields are made – this time pointing in the opposite direction.

* because the new fields have changed direction, the old lines fold back to connect up to some of the new lines – they form closed loops.

* The old fields don’t disappear – they continue on outards forever (or until they hit something that absorbs them.)

Generate EM field from wire 2

So once we set up a changing B-field, a changing E-field radiates away from it,
thus causing a new B-field to emerge, thus causing a new E-field to emerge,
and so on until infinity.
EM waves are thus self-propagating.

Here we see the EM fields far from the antenna.
They form loops and move outwards.

Generate EM field from wire 3

Below: An EM (electromagnetic) wave moving through space: We see the E and B-fields, perpendicular to each other.

Generate EM field from wire 4– – – – – – –

* Light is an E-field and B-field moving together

* The speed at which they move is the speed of light

= 186,282 miles/second = 3 x 10 8 meters/second

= 1,080 million kilometers/hour = 1.08 × 10 9 km/hr

_________________________________________________

Visible light is made of many colors

Dispersive Prism spectrum

Each color in a rainbow corresponds to a different wavelength of electromagnetic spectrum.

============================

Radio waves are an invisible form of electromagnetic radiation.

Their wavelength ranges from 0.04 inches (one millimeter) to over 62,000 miles (100,000 km) long.

What creates or uses radio waves?

* Wi-fi

* Bluetooth

* AM radio and FM radio

* over-the-air television (old fashioned TV)

* 2G, 3G and 4G cellphones

* Energy from the sun (yes, our Sun produces radio waves!)

 

How do we make AM radio waves?

We can use interference (a.k.a. superposition) to add two waves together to create a more complex wave. This lets us modulate the amplitude of the resulting wave. This is known as AM radio.
AM Radio waves Giancoli Physics

How do we make FM radio waves?

We may also add two waves together to modulate the frequency of the resulting EM wave. This is known as FM radio.
FM Radio waves Giancoli Physics

How do atoms emit light?

text

Milky Way Galaxy in Multiple Wavelengths

text

Milky Way Galaxy in Multiple Wavelengths Pearson Prentice

Above: Our galaxy – the Milky Way – viewed through:
(a.) radio wavelengths
(b.) infrared wavelengths
(c.) visible wavelengths
(d.) X-ray wavelengths
(e.) gamma-ray wavelengths

Below: The famous “Whirlpool Galaxy” (Messier 51a) viewed through different wavelengths of the EM spectrum:

The wavelength and energy of a photon relates to how fast electrons are accelerated.
Low energy radiation comes from cool regions of molecular gas.
High energy radiation comes hot spots where atoms are fully ionized.
Provides insight into the structure, temperature, and chemical composition of  .
http://ecuip.lib.uchicago.edu/multiwavelength-astronomy/astrophysics/05.html

Apps/Computer labs

Interactive Java Tutorials: Basic Electromagnetic Wave Properties

http://micro.magnet.fsu.edu/primer/java/electromagnetic/index.html

http://micro.magnet.fsu.edu/primer/java/wavebasics/
Broadcast radio waves from KPhET. Wiggle the transmitter electron manually or have it oscillate automatically. Display the field as a curve or vectors. The strip chart shows the electron positions at the transmitter and at the receiver.
PhET radio-waves-screenshot
http://phet.colorado.edu/en/simulation/radio-waves

What are electromagnetic waves? Physics 2000
http://www.colorado.edu/physics/2000/waves_particles/index.html
The electric and magnetic fields generated by an oscillating electric charge
http://www.cabrillo.edu/~jmccullough/Applets/Flash/Optics/EMWave.swf

Electromagnetic waves animation
http://www.walter-fendt.de/ph14e/emwave.htm

Propagation of Electromagnetic Wave
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35

Quotes

Poets say science takes away from the beauty of the stars— mere globs of gas atoms. Nothing is ‘mere’. I too can see the stars on a desert night, and feel them. But do I see less or more? The vastness of the heavens stretches my imagination— stuck on this carousel my little eye can catch one-million-year-old light. A vast pattern— of which I am a part…What is the pattern, or the meaning, or the why? It does not do harm to the mystery to know a little more about it. For far more marvelous is the truth than any artists of the past imagined it. Why do the poets of the present not speak of it? What men are poets who can speak of Jupiter if he were a man, but if he is an immense spinning sphere of methane and ammonia must be silent?

– Richard Feynman (1918 – 1988), Feynman Lectures on Physics, footnote
_______________________________________________________

Why are magnetic fields are called B-fields?

The origin of B was James Clerk Maxwell himself. See the article by Ralph Baierlein in the American Journal of Physics, v68, n8 (Aug 2000), p691.
In his text, “A Treatise on Electricity and Magnetism“, Maxwell presents a list of the vector quantities he will be dealing with. He then labels them in alphabetical order!

  • Electromagnetic momentum at a point: A (now called vector potential)
  • Magnetic induction: B (usually called magnetic field)
  • Total electric current: C
  • Electric displacement: D
  • Electromotive force: E
  • Mechanical force: F
  • Velocity at a point: G
  • Magnetic force: H (usually called magnetic intensity)

The use of A, B, D, F, and H has lived on, but C and G have been abandoned.

E for EMF has been replaced by the Greek letter epsilon ɛ , or ℰ (script capital E, Unicode U+2130).

So the letter E is now ‘electric field.’

https://www.harker.org/cfhttp.cfm?script=extensions/includes/resource/resourcecontent.cfm&rid=268&pageid=962

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

6.MS-PS4-1. Use diagrams of a simple wave to explain that (a) a wave has a repeating pattern with a specific amplitude, frequency, and wavelength, and (b) the amplitude of a wave is related to the energy of the wave.

HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling within various media. Recognize that electromagnetic waves can travel through empty space (without a medium) as compared to mechanical waves that require a medium.

HS-PS4-5. Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy. Clarification Statements:
• Emphasis is on qualitative information and descriptions.
• Examples of technological devices could include solar cells capturing light and
converting it to electricity, medical imaging, and communications technology.

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

6. Electromagnetic Radiation Central Concept: Oscillating electric or magnetic fields can generate electromagnetic waves over a wide spectrum. 6.1 Recognize that electromagnetic waves are transverse waves and travel at the speed of light through a vacuum. 6.2 Describe the electromagnetic spectrum in terms of frequency and wavelength, and identify the locations of radio waves, microwaves, infrared radiation, visible light (red, orange, yellow, green, blue, indigo, and violet), ultraviolet rays, x-rays, and gamma rays on the spectrum.

 

Advertisements

1 Comment

Comments are closed.

%d bloggers like this: