The simplest definition of an electromagnetic wave is that it is the way in which certain kinds of energy (light, radio, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays) travel through space. These are all examples of electromagnetic radiation.
In other words, when you see light, what is happening is that something (the sun, a star, a flame, etc.) is giving off electromagnetic waves. These waves travel through space until they hit your eye, which reacts to the waves, and you perceive this as light.
If the waves travel through other objects, then they will travel at a lower speed. Some electromagnetic waves (such as visible light) are blocked by most solid objects. This is why you can't see through solid walls. But some electromagnetic waves, such as x-rays, gamma rays, and radio waves, are able to travel through some solid objects.
An electromagnetic wave gets its name from the fact that it is a combination of an electrical field and a magnetic field.
All waves have two properties called the frequency and wavelength. The intensity of the wave (the amplitude) keeps reversing on a regular basis as it moves through space. If you were able to measure the intensity at a given point, it would go up and down at a certain rate. This rate is called the frequency, and it's measured in cycles per second, or Hertz (Hz).
The other important property is wavelength. Since the wave is changing intensity as it moves, it is "high" at some points and "low" at others. To measure the wavelength, you find two points where the intensity is exactly the same. Wavelength is usually measured in meters.
The frequency times the wavelength always equals the speed of light. For example, a radio wave changing a million times per second (1 MHz) has a wavelength of 300 meters.
The different kinds of electromagnetic waves are different because they have different frequencies. You can't see x-rays or radio waves because your eyes are only sensitive to a narrow band of frequencies. This full range of frequencies is called the electromagnetic spectrum.
Visible light is just a small part of the spectrum. Red light has the longest wavelength, and violet has the shortest. You can remember the order of colors in a rainbow using the name Roy G. Biv: Red, Orange, Yellow, Green, Blue, Indigo, Violet.
For hundreds of years, scientists thought light was only a wave. But in the 20th century, physicists like Einstein realized that light is also made of tiny particles called photons. Because light behaves like both, we call this "Wave-Particle Duality."
Is light a "thing" or a "wave"?
It is both! It travels through space as a wave, but it hits objects like a tiny particle (a photon).
Why can't I see Wi-Fi or Radio waves?
Your eyes are like a radio tuned to only one station. They can only "see" the frequencies of visible light. Wi-Fi and Radio waves are vibrating too slowly for your eyes to detect.
How does light travel if there is no air in space?
Unlike sound waves (which need air to vibrate), electromagnetic waves are made of electric and magnetic fields. These fields can exist and "ripple" even in the empty vacuum of space.
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