What Is Radiation?
Radiation simply means energy that travels through space or matter in the form of waves or particles. Despite the word sometimes sounding alarming, most radiation is completely harmless—and essential to life!
There are two main types of radiation we encounter:
- Electromagnetic radiation (energy waves like light, radio, X-rays)
- Nuclear (or particle) radiation (tiny particles emitted from unstable atoms)
In this module, we’ll focus mostly on electromagnetic radiation—the kind that powers everything from sunlight to Wi-Fi—and briefly touch on everyday sources of low-level nuclear radiation.
⚡ Electromagnetic Radiation: Energy on the Move
Electromagnetic (EM) radiation is energy that travels as oscillating electric and magnetic fields moving together through space at the speed of light (about 300,000 km/s!). It doesn’t need air or water to travel—which is why sunlight reaches us across the vacuum of space.
This energy comes in tiny “packets” called photons. Photons have no mass, but they carry energy—and the more energy a photon has, the shorter its wavelength.
The Electromagnetic Spectrum: One Family, Many Forms
All electromagnetic radiation—from radio waves to gamma rays—is fundamentally the same phenomenon, just with different wavelengths and energies. Scientists organize these into the electromagnetic spectrum:
Text description of the figure 1.1.
The image illustrates the electromagnetic spectrum, comparing different types of radiation from radio waves to gamma rays. It is organized into several sections, each providing different details about the characteristics of these waves. At the top, a wavy red line represents the increasing frequency from left (radio waves) to right (gamma rays). Immediately below, a row of rectangles indicates whether each radiation type penetrates Earth's atmosphere, with "Y" for yes and "N" for no.
The wave types are listed with their corresponding wavelengths in meters: Radio (10³), Microwave (10⁻²), Infrared (10⁻⁵), Visible (0.5×10⁻⁶), Ultraviolet (10⁻⁸), X-ray (10⁻¹⁰), Gamma ray (10⁻¹²). Each type is accompanied by an image for approximate scale: Buildings for radio, humans for microwave, butterflies for infrared, needle points for visible, protozoans for ultraviolet, molecules for X-ray, atoms for gamma ray, and atomic nuclei for the smallest scales.
Below, the frequency range in Hertz (Hz) is shown, mapping the spectrum from 10⁴ for radio waves to 10²⁰ for gamma rays. A colored horizontal bar displays the visible spectrum from red at the left to violet at the right.
The lowest section describes the temperature of objects emitting the most intense wavelength, depicted with a gradient bar ranging from 1 Kelvin (-272°C) to 10,000,000 Kelvin (~10,000,000°C).
Real-World Examples of Electromagnetic Radiation
- Sunlight: A mix of visible light, UV, and infrared—powers photosynthesis, warms Earth, and gives you a tan (or sunburn!).
- Wi-Fi & Bluetooth: Use microwaves to send data between your phone and router.
- Remote controls: Send infrared signals to your TV.
- Night-vision goggles: Detect infrared radiation (heat) emitted by people and animals.
- Medical X-rays: Pass through soft tissue but are absorbed by bones—creating diagnostic images.
- Solar panels: Capture photons from sunlight and convert them directly into electricity.
Please watch the following 5:04 video about the electromagnetic spectrum:
Tour of the Electro Magnetic Spectrum (5:03)
Transcript: Tour of the Electro Magnetic Spectrum (5:03)
Something surrounds you. Bombards you some of which you can't see, touch, or even feel. Everyday. Everywhere you go. It is odorless and tasteless. Yet you use it and depend on it every hour of every day. Without it, the world you know could not exist. What is it? Electromagnetic radiation.
These waves spread across a spectrum from very short gamma rays, to x-rays, ultraviolet rays, visible light waves, even longer infrared waves, microwaves, to radio waves which can measure longer than a mountain range. This spectrum is the foundation of the information age and of our modern world.
Your radio, remote control, text message, television, microwave oven, even a doctor's x-ray, all depend on waves within the electromagnetic spectrum.
Electromagnetic waves (or EM waves) are similar to ocean waves in that both are energy waves - they transmit energy. EM waves are produced by the vibration of charged particles and have electrical and magnetic properties. But unlike ocean waves that require water, EM waves travel through the vacuum of space at the constant speed of light.
EM waves have crests and troughs like ocean waves. The distance between crests is the wavelength. While some EM wavelengths are very long and are measured in meters, many are tiny and are measured in billionths of a meter...nanometers. The number of these crests that pass a given point within one second is described as the frequency of the wave. One wave - or cycle - per second, is called a Hertz.
Long EM waves, such as radio waves, have the lowest frequency and carry less energy. Adding energy increases the frequency of the wave and makes the wavelength shorter. Gamma rays are the shortest, highest energy waves in the spectrum.
So, as you sit watching TV, not only are there visible light waves from the TV striking your eyes...But also radio waves transmitting from a nearby station; and microwaves carrying cell phone calls and text messages; and waves from your neighbor's WiFi; and GPS units in the cars driving by. There is a chaos of waves from all across the spectrum passing through your room right now!
With all these waves around you, how can you possibly watch your TV show? Similar to tuning a radio to a specific radio station, our eyes are tuned to a specific region of the EM spectrum and can detect energy with wavelengths from 400 to 700 nanometers, the visible light region of the spectrum.
Objects appear to have color because EM waves interact with their molecules. Some wavelengths in the visible spectrum are reflected and other wavelengths are absorbed. This leaf looks green because EM waves interact with the chlorophyll molecules. Waves between 492 and 577 nanometers in length are reflected and our eye interprets this as the leaf being green.
Our eyes see the leaf as green but cannot tell us anything about how the leaf reflects ultraviolet, microwave, or infrared waves.
To learn more about the world around us, scientists and engineers have devised ways to enable us to 'see' beyond that sliver of the EM spectrum called visible light. Data from multiple wavelengths help scientists study all kinds of amazing phenomena on Earth, from seasonal change to specific habitats.
Everything around us emits, reflects and absorbs EM radiation differently based on its composition. A graph showing these interactions across a region of the EM spectrum is called a spectral signature. Characteristic patterns, like fingerprints within the spectra allow astronomers to identify an object's chemical composition and to determine such physical properties as temperature and density.
NASA's Spitzer space telescope observed the presence of water and organic molecules in a galaxy 3.2 billion light years away.
Viewing our Sun in multiple wavelengths with the SOHO satellite allows scientists to study and understand sunspots that are associated with solar flares and eruptions harmful to satellites, astronauts and communications here on Earth.
We are constantly learning more about our world and Universe by taking advantage of the unique information contained in the different waves across the EM spectrum.