How is light considered “electromagnetic” radiation, What does it have to do with electromagnetism?



How is light considered “electromagnetic” radiation, What does it have to do with electromagnetism?

In: Physics

Before we knew what light was, James Clerk Maxwell was studying electricity and magnetism. He developed a series of equations describing how the two forces related, and realized that these equations could describe an oscillation in space that could travel on its own without a nearby source. It only works if you quickly change the electric field, though, and a constant field won’t do this.

He did the math, and discovered that all such waves would travel at a specific speed. Around this time, the speed of light was measured by someone else, and they turned out to be the same the speed.

Further experimentation revealed that these waves behaves differently at different frequencies of oscillation. Higher frequencies became more like visible light, and higher still like x-rays and gamma rays.

light is considered EM radiation because it just simply is.

EM Radiation describes any radiation with electric and magnetic fields that vary simultaneously. (heres a gif of that [](, its a bit easier to wrap your head around seeing it that way). Basically light waves have two components, an electric component E, and a magnetic component B.

All light, both visible and invisible (like microwaves, radiowaves, UV rays, Xrays, and Gamma Rays) are made up with those same EM fields/waves, just with different Wavelengths/Frequencies.

It just is. We use the word “electromagnetism” to describe the phenomena of which light is a subset. Visible light, gamma radiation, radio waves…is all radiative energy transfer that causes simultaneous electric and magnetic field changes in a repeating manner.

Light is made out of wiggles in the electric and magnetic fields. These are the same fields that electrically and magnetically charged things interact through.

An obvious example is through one of the ways we commonly create a certain kind of light: radio waves. If you have a metal antenna and you send electrons back and forth through it, the electrons are making an electric field that wiggles back and forth. Since they are moving, they also create a magnetic field that wiggles back and forth. This wiggling creates radio waves (a form of light), which are wiggles in the electric and magnetic fields.

There is an electric field and a magnetic field filling every point in the cosmos, in the same sense that water in the ocean has a height at every point on the ocean’s surface.

The electric field is responsible for attracting and repelling charged particles, while the magnetic field causes the movement of charged particles to curve. The electric field arises because charges exist, the magnetic field arises because charges move. The two are deeply related and, more properly, can be thought of as two manifestations of a single electromagnetic field.

Light, at its core, is a small “bump” in these fields where they have different values than they do in the surrounding area. In the same way that a wave in the ocean is a small area where the height of the water is different, light is a small area where the “height” (or more abstractly, value) of the electromagnetic field is different. It turns out that both waves are described by the same kind of equations, which is why we talk about light as a “wave”. Just as a water wave can travel through the ocean, electromagnetic waves travel through the electromagnetic field, and just as water waves can break on a shoreline, electromagnetic waves can hit (and interact with) objects.

Electromagnetic radiation interacts with various materials based on [its frequency]( (the energy of the photons is based on their frequency). Radio frequencies interact with metals (antennas), microwave frequencies interact with water and other materials (microwave oven), visible frequencies interact with the receptors in your eyes, gamma ray frequencies interact with your cells / damage your DNA causing radiation sickness, and so on.

Visible light is just a frequency of electromagnetic radiation.

Electricity and magnetism are caused by “charges”, and “charge” is a fundamental property of matter, just like “mass”, “spin”, and other such things. Atoms have electrons and protons (these are charged particles), and electromagnetism interacts with atoms / is caused by these charges in the atoms.

Stars are made of atoms, and their atoms create electromagnetism (a star emits all sorts of frequencies, not just visible light), and your eyes have atoms in them that interact with (receive) electromagnetism. Radio antennas are made of atoms, electrical wires are made of atoms, magnets are made of atoms, and so on.

Electricity and magnetism are two things that humans think of as being separate, only because we’re very big and not very smart.

If you work out all the mathematical laws, and zoom down very small, these concepts are really two sides of the same coin. How exactly they’re connected is expressed in mathematics, so I won’t try to use words here. Just know that they make up electromagnetism.

Packets of electromagnetic energy can travel through space. (again, look to mathematics if you want to know what the packet is “like”) The packet is called a photon. Photons exist at all different “frequencies”, and have different “wavelengths”.

These waves are literally in the air, and can be measured in size. It just so happens that there are two 1/8ths inch holes in your head that let waves of only a certain size in. Your eyes then interprets the slight differences between the waves smaller than 1/8ths inch and your brain sees that as color.

Your brain is measuring electromagnetic waves, using your eyes.