# High level defintion of electromagnetic radiation?

Logical1
Can someone give me a high level defintion of electromagnetic radiation with some intuition on what the wave form represents (specifically the peaks and valleys)?

I am comfortable with the electric and magnetic fields. I would like an explanation similar to the my intuition on the physical meaning of a sound wave (assuming it is correct):

A sound wave is the mathematical representation of air density disturbance.
(Magnitude vs Time) Plotting a sinusoidal sound wave as the density of the disturbed air vs time, the line y=0 would represent the normal density of the air, where peaks represent a higher density, and a valley represents a low density of air for a given value t.

I believe this gives definition provides good intuitions on what exactly a "sound wave" means. However I'm trying to grasp the concept of an EM wave in this manner. Specifically, what the actual wave form means.

Here is my attempt:
The wave is a graphical or mathematical representation of the changing electric and magnetic field at a point. Therefore, the peak of a wave should represent a higher magnitude of the fields at that point, and a valley represents a lower magnitude of the fields at that point.

While I don't know if this is even correct, I'm having a hard time figuring out what the base line y=0 of the graph means. Maybe the magnitude of the E and B fields before the EM wave passes through? That is, what is a negative points in the wave represent?

Also, how is this 'light'?

Can someone clarify things for me?

## Answers and Replies

Staff Emeritus
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The squiggly graph is a representation of the EM wave over time. The Y=0 is simply the mark where the fields go from positive to negative. Above this mark the wave will provide a force identical to applying a positive electric field to a particle, while below the mark it will be negative. The direction of this force depends on the vector of the EM wave at the time of interaction.

Also, how is this 'light'?

I'm not sure what you are asking here. Light, and all other forms of EM radiation, are all EM waves. The frequency of the wave determines how it interacts with matter. Visible light has a wavelength of about 380 nanometers - 740 nanometers. Shorter wavelengths have more energy while longer wavelengths have less energy.

Perhaps the key thing to understand is that the EM wave interacts with stuff only in small packets. IE when an EM wave passes over something, say an antenna, the electrons are "bumped" by many small nudges, one at a time and with the same amount of energy. We call these packets "Photons", the particle of light.

Mentor
Here is my attempt:
The wave is a graphical or mathematical representation of the changing electric and magnetic field at a point. Therefore, the peak of a wave should represent a higher magnitude of the fields at that point, and a valley represents a lower magnitude of the fields at that point.

You're close! What you're missing is that the electric and magnetic fields are vector quantities, that is, they have both magnitude and direction.

I'm having a hard time figuring out what the base line y=0 of the graph means. Maybe the magnitude of the E and B fields before the EM wave passes through? That is, what is a negative points in the wave represent?

Above the baseline, the E and B fields each point in a certain direction (the directions for the two fields being perpendicular to each other and to the direction of propagation of the wave). Below the baseline, the fields point in the opposites of those directions.

For example, points above the baseline might correspond to E being upward and B being horizontal towards the east. In this case points below the baseline would correspond to E being downward and B being horizontal towards the west. (In this case the wave would be propagating towards the north.)

Staff Emeritus
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JT, was my post accurate, or have I completely misunderstood EM waves? *Breaks out his Optics for Dummies book*

Mentor
Your description is OK as long as you associate "positive" and "negative" with (in my example) "upward" and "downward" for the E field, and with "eastward" and "westward" for the B field.

Vectors are not intrinsically "positive" or "negative", so you have to specify this correspondence.

Staff Emeritus
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Hmm. I have misunderstood something then. I thought the fields oscillated from + to -. Can you elaborate on what the vector means for an EM wave?

Staff Emeritus
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Homework Helper
Hmm. I have misunderstood something then. I thought the fields oscillated from + to -. Can you elaborate on what the vector means for an EM wave?
I think JT's point is that technically you have to define what the + and - directions are if you are trying to fully specify an EM wave in a specific situation-- though in some cases it may be understood from the context.

Otherwise it is just understood that "+" refers to some unspecified direction if we are talking in a very general sense.

In still other cases we might really care about the power $\propto E^2$, so that nobody cares much which direction is defined as "+". Electric solar panels or laser beams are good examples of such a situation.

Hope that helps.

Staff Emeritus
Science Advisor
2022 Award
Oh yeah, I definitely misunderstood something Redbelly. I'll have to look more into it later.