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Light is an EM field


Light is an electromagnetic wave

B-field     magnetic field
E- field    electric field
I                  electrical current

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.


What is the EM spectrum?

The spectrum is the full range, of all wavelengths, of EM waves.

How much of this can we see with our eyes? (very little)

For blue and purple light, the width of these vibrating EM waves are, according to this diagram, about how wide? Roughly, according to this pic, the width of a bacteria cell

For microwaves, the width of these vibrating EM waves are, according to this diagram, about how wide? Roughly, according to the pic, about the width of butterfly wings (within an order of magnitude)

Gamma rays Spectrum Properties NASA
NASA tour of the EM spectrum

From the webcomic XKCD (Randall Patrick Munroe)



Visible light

Wavelengths of EM radiation that your eyes react to. All the colors of the rainbow – and every combination of them.

Infrared light

Infrared means “below red,” as infrared light has less energy than red light. We typically describe light energy in terms of wavelength, and as the energy of light decreases, its wavelength gets longer. Infrared light, having less energy than visible light, has a correspondingly longer wavelength. The infrared portion of the spectrum ranges in wavelength from 1 to 15 microns, or about 2 to 30 times longer wavelength (and 2-30 times less energy) than visible light.

Infrared light is invisible to the unaided eye, but can be felt as heat on one’s skin. Warm objects emit infrared light, and the hotter the object, the shorter the wavelength of IR light emitted. This IR “glow” enables rescue workers equipped with longwave IR sensors to locate a lost person in a deep forest in total darkness, for example. Infrared light can penetrate smoke and fog better than visible light, revealing objects that are normally obscured. It can also be used to detect the presence of excess heat or cold in a piece of machinery or a chemical reaction.

“The image on the left shows an optical view of a star forming region. The same area is shown on the right in infrared radiation. Notice how the infrared observations penetrate the obscuring cloud to reveal many new details.”


You may be familiar with microwave images as they are used on TV weather news and you can even use microwaves to cook your food. Microwave ovens work by using microwave about 12 centimeters in length to force water and fat molecules in food to rotate. The interaction of these molecules undergoing forced rotation creates heat, and the food is cooked.

Microwaves are a portion or “band” found at the higher frequency end of the radio spectrum, but they are commonly distinguished from radio waves because of the technologies used to access them. Different wavelengths of microwaves (grouped into “sub-bands”) provide different information to scientists. Medium-length (C-band) microwaves penetrate through clouds, dust, smoke, snow, and rain to reveal the Earth’s surface. L-band microwaves, like those used by a Global Positioning System (GPS) receiver in your car, can also penetrate the canopy cover of forests to measure the soil moisture of rain forests. Most communication satellites use C-, X-, and Ku-bands to send signals to a ground station.

Microwaves that penetrate haze, light rain and snow, clouds, and smoke are beneficial for satellite communication and studying the Earth from space. The SeaWinds instrument onboard the Quick Scatterometer (QuikSCAT) satellite uses radar pulses in the Ku-band of the microwave spectrum. This scatterometer measures changes in the energy of the microwave pulses and can determine speed and direction of wind near the ocean surface. The ability of microwaves to pass through clouds enables scientists to monitor conditions underneath a hurricane.


Passive remote sensing refers to the sensing of electromagnetic waves that did not originate from the satellite or instrument itself. The sensor is merely a passive observer collecting electromagnetic radiation. Passive remote sensing instruments onboard satellites have revolutionized weather forecasting by providing a global view of weather patterns and surface temperatures. A microwave imager onboard NASA’s Tropical Rainfall Measuring Mission (TRMM) can capture data from underneath storm clouds to reveal the underlying rain structure.



“Microwave imaging has shown great potential to be used for structural health monitoring.
They Electromagnetic waves in low frequency (e.g., <10 GHz) can easily penetrate inside concrete and reach to object of interest which is usually rebar. If there is any rust on the rebar, since rust reflects less EM wave in comparison with the whole metallic rebar, the microwave imaging method can distinguish between rebars with and without rust (or corrosion).” – https://en.wikipedia.org/wiki/Microwave_imaging

Radio and TV waves

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?


Milky Way Galaxy in Multiple Wavelengths


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  .

Apps/Computer labs

Interactive Java Tutorials: Basic Electromagnetic Wave Properties


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

What are electromagnetic waves? Physics 2000
The electric and magnetic fields generated by an oscillating electric charge

Electromagnetic waves animation

Propagation of Electromagnetic Wave


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.’


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.



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