# What is light?

Gold Member
Hello everyone,
Although there are lots of things that I know about light, I have trouble really understanding it. I can't visualize a wave that doesn't need a medium to move. Photons are electrically neutral, so how is the thing with vertical and horizontal oscillating electric and magnetic fields happen? Everything that I've learned in the school seems to be just repetitions of subconscious memorisations.

Thank you
Note: I know this is a frequenly asked question, but other threads couldn't help me understand.

Nugatory
Mentor
Hello everyone,
Although there are lots of things that I know about light, I have trouble really understanding it. I can't visualize a wave that doesn't need a medium to move. Photons are electrically neutral, so how is the thing with vertical and horizontal oscillating electric and magnetic fields happen? Everything that I've learned in the school seems to be just repetitions of subconscious memorisations.

First step is to try to forget that you ever heard the word "photon" - the popular notion of photons as particles of light is seriously misleading and just getting in the way here. You won't need photons until you get to quantum mechanics; until then, every time you find yourself thinking "photon" you should try mentally substituting "flash of light" or "radio signal".

Second step is to consider that electrical and magnetic fields exist in a vacuum just fine: The electric field of a charged particle in outer space will interact with other nearby charged particles even though there's just vacuum between the particles.

Now it's easy at a non-mathematical level: the electromagnetic waves are rippling fluctuations in the strength of the electrical and magnetic fields, the same way that water waves are rippling fluctuations in the height of the water's surface. The fields are everywhere so the electromagnetic waves can go everywhere.

At a more mathematical and less hand-waving level, you can read through the sections of Purcell's E&M textbook (I believe that @bcrowell may have a pointer to a licensed non-pirated online copy) where he shows how these "ripples" form and propagate; and if you're really going to understand, sooner or later you're going to have to take on Maxwell's equations (discovered in 1861 and the basis for all of classical electrodynamics).

sophiecentaur
Gold Member
First step is to try to forget that you ever heard the word "photon" - the popular notion of photons as particles of light is seriously misleading and just getting in the way here. You won't need photons until you get to quantum mechanics; until then, every time you find yourself thinking "photon" you should try mentally substituting "flash of light" or "radio signal".

Second step is to consider that electrical and magnetic fields exist in a vacuum just fine: The electric field of a charged particle in outer space will interact with other nearby charged particles even though there's just vacuum between the particles.

Now it's easy at a non-mathematical level: the electromagnetic waves are rippling fluctuations in the strength of the electrical and magnetic fields, the same way that water waves are rippling fluctuations in the height of the water's surface. The fields are everywhere so the electromagnetic waves can go everywhere.

At a more mathematical and less hand-waving level, you can read through the sections of Purcell's E&M textbook where he shows how these "ripples" form and propagate; and if you're really going to understand, sooner or later you're going to have to take on Maxwell's equations (discovered in 1861 and the basis for all of classical electrodynamics).

What is the reason that they aren't carry electrical or magnetic charge even though their strenght fluctates?
What is the reason that the two field fluctations are right angled? Is it so that they somehow negate each other?

...and I have already stirred my mind enough with quantum physics, aren't the mediating particle of em force virtual photons instead of photons?

Nugatory
Mentor
What is the reason that they aren't carry electrical or magnetic charge even though their strenght fluctates?
Because the ripples in the field carry energy and momentum but they don't move the charge itself. It's easy to see this if you consider two like-charged particles near each other - they're pushed apart, gaining kinetic energy and momentum in the process, but their charges are unchanged.
What is the reason that the two field fluctuations are right angled? Is it so that they somehow negate each other?
The right-angled field fluctuations jump out at you if look at Maxwell's equations, but I know of no easy non-mathematical explanation for why that happens. Your choices here are basically to learn the math (typically in the first year of a college physics program) or take the word of someone who has learned it.
and I have already stirred my mind enough with quantum physics, aren't the mediating particle of em force virtual photons instead of photons?
They are, but that's quantum field theory which combines relativity and quantum mechanics; you won't come to it until several years after you first learn quantum mechanics. However, none of this matters for understanding electromagnetic waves at the level we're discussing in this post, and none of it changes my recommendation that you try to forget that you ever heard the word "photon" for now.

Electromagnetic waves are a classical phenomenon, discovered and well understand decades before either relativity or quantum mechanics, and you understand them by applying classical electrodynamics.

Gold Member
Because the ripples in the field carry energy and momentum but they don't move the charge itself. It's easy to see this if you consider two like-charged particles near each other - they're pushed apart, gaining kinetic energy and momentum in the process, but their charges are unchanged.

The right-angled field fluctuations jump out at you if look at Maxwell's equations, but I know of no easy non-mathematical explanation for why that happens. Your choices here are basically to learn the math (typically in the first year of a college physics program) or take the word of someone who has learned it.

They are, but that's quantum field theory which combines relativity and quantum mechanics; you won't come to it until several years after you first learn quantum mechanics. However, none of this matters for understanding electromagnetic waves at the level we're discussing in this post, and none of it changes my recommendation that you try to forget that you ever heard the word "photon" for now.

Electromagnetic waves are a classical phenomenon, discovered and well understand decades before either relativity or quantum mechanics, and you understand them by applying classical electrodynamics.

Okay, thank you very much!