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Electromagnetic spectrum

What is the EM spectrum?

The spectrum is the full range of electromagnetic waves.

We only can see a very small portion of this with our eyes.

How wide are the EM waves that we can see? Look at the diagram below:

Blue/violet light – width of EM waves are about the size a bacteria.

What about microwaves? About the width of butterfly wings – within an order of magnitude.

Radio waves are shown to be hundreds to thousands of meters long

X-rays are EM waves about the width of a single atom.


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” – infrared light has less energy than red light.

As the energy of light decreases, its wavelength gets longer.

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.

from What is infra-red light? FLIR components

“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


Gravity is the force that pulls matter together over great distances (tens of millions of light years). The electromagnetic force is also very influential, but it works at very small distances (down to submillimeter scales) causing positively charged atomic nuclei to attract negatively charged electrons allowing atoms and molecules to form.

The electromagnetic force is responsible for generating visible light as well as radiation in other wavebands not detectable by the human eye. As electrons and protons fly around bumping into each other in a light source, the electromagnetic force produces photons of all wavelengths across the electromagnetic spectrum.

Slow, randomly moving charged particles create radio, infrared, optical and ultraviolet photons with wavelengths, respectively, from meters to microns to thousands of nanometers to hundreds of nanometers.

Fast moving particles may create X-rays.

These forms of radiation are refered to as thermal radiation because the energy of the photons depends on the temperature of the gas.

Other processes such as directed motion of charged particles in magnetic fields or decays of particles into photons create additional radiation that is refered to as non-thermal radiation because the main cause of the radiation is something other than the temperature of the gas. It is often difficult to say how a particular photon is created because they have similar wavelengths.

The highest energy radiation, known as gamma rays, is usually non-thermal radiation.

Multiwavelength Whirlpool Galaxy: Each image shows a narrow band of wavelengths of light and invisible radiation across the electromagnetic 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, and high energy radiation comes from hot spots where atoms are fully ionized. The combined images provide insight into the structure, temperature, and chemical composition of the Whirlpool Galaxy. The stars in the infrared image represent most of the mass of the galaxy, excluding dark matter, which can’t be seen. The optical image represents a slightly smaller amount of mass and the other three images represent only traces of mass in molecules (radio image) massive hot stars, (ultraviolet image) and hot plasma (x-ray image).

the above text is from http://ecuip.lib.uchicago.edu/multiwavelength-astronomy/astrophysics/05.html

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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