# Why does the electric field dominate in light?

I read a book on the wave property of light where the author mentioned that the electric field, instead of magnetic field, dominates the light property. I don't understand why.

In Maxwell's theory, a light field has an electric and magnetic field at the same time and they are perpendicular. Also, in some books, where they consider the polarization, they only use the electric field as example. For example, if the vibration of the electric field is up and down, it cannot go through a polarizer which orients 90 degree to the vibration direction of the field, so no light goes through the polarizer. But what happened to the magnetic field? The magnetic field is perpendicular to the electric field, so in this case, the magnetic field should pass the polarizer, and we should have outgoing light -- but we don't. Why is this so?

Materials, and certainly materials transparent to light , have few magnetic properties. They are not composed out of atoms that have strong ferromagnetism. But all atoms have strong electric fields. This means that light, as it goes through a transparent medium has small probability to interact with its magnetic field component with the medium, which is mainly transparent to it.

Take the wire grid polariser as a more simple example

It consists of a regular array of fine parallel metallic wires, placed in a plane perpendicular to the incident beam. Electromagnetic waves which have a component of their electric fields aligned parallel to the wires induce the movement of electrons along the length of the wires. Since the electrons are free to move in this direction, the polarizer behaves in a similar manner to the surface of a metal when reflecting light; and the wave is reflected backwards along the incident beam (minus a small amount of energy lost to joule heating of the wire).

A wire-grid polarizer converts an unpolarized beam into one with a single linear polarization. Coloured arrows depict the electric field vector. The diagonally-polarized waves also contribute to the transmitted polarization. Their vertical components are transmitted, while the horizontal components are absorbed and reflected.

The magnetic component in this setup cannot interact to affect the absorption of the light the way the electric can with the free electrons in the metal of the wire.

• In your "wire grid polariser" example; How a 45° angle electric filed is able to passe through it, on other words How 50% of that light is able to induce the movement to the horizontally aligned electrons and then get absorbed while the other 50% passe through it? and what's the orientation of the passing light? – Hammar Nov 10 '17 at 15:24
• @Hammar both unpolarized light, and light polarized in an angle to the vertical to the grid pass vertically. all the other components are either absorbed (heating the wires) or reflected. Unpolarized light has all the angles, and just the vertical component is transmitted. – anna v Nov 10 '17 at 16:02
• let me take a step backward and lets assume this sentence"just the vertical component is transmitted" is a physical reality now if we introduce polarizing filter at 45° angle to the vertical component transmitted earlier and what we will gt is 50% reduction in intensity, notice it's the same question but from different point view, but at the smallest levels what's really happening how 50% of the vertically polarized able to base the 45° angle filter? – Hammar Nov 10 '17 at 16:53
• It is just a demonstration of how the electric field can affect directly matter. It is not a filter but a generator of polarized in one direction waves. I gave it as an example of interaction with the electric field. – anna v Nov 10 '17 at 17:03

The reason we tend to concentrate on the electric field is that it interacts strongly with charges, e.g. electrons, and there are a lot of electrons around. The magnetic field would interact strongly with magnetic charges, i.e. magnetic monopoles, but as far as we know magnetic monopoles don't exist. So generally speaking it's the electric field that dominates the interaction of light with matter.

we know that E(electic field magnitude) =B(magnetic field magnitude)*c(c is the speed of light in vacuum)... from that, B=E/c here c is very large ie, approx= 3*10^8 so magnetic field magnitude is one by 3*10^8 times the electric field intensity... so compared to electric field magnitude,magnetic field magnitude is very low, hence negligible

• Since your magnitudes have different units, this should not play a role as units are arbitrary (just redefine the metre to a light year and you'll have a completely different picture) – Martin Mar 17 '15 at 10:27
• @Martin: Perhaps anagha wanted to say that if you multiply magnetic field with some velocity that is not huge in SI, you'll find that Lorentz force is negligable. More general way to say that would have been (without assuming any specific units): if $v \ll c$, Lorentz force is $qvB \approx qEv/c \ll qE$, the electrostatic force. – kristjan Mar 17 '15 at 10:43

A different reason follows from “Planets and electromagnetic waves”. Light waves or rays interact with electric fields of electrons in a solar cell to produce a disturbance in electrons so that electricity is produced. In a tungsten bulb, electrons try to move with very close distance because of a voltage, and at the same time the electric fields of these electrons repel them. So, light energy is released. Light energy is associated only with electric fields. Visualize a light wave as electric component wave. No one practically observed combined form of magnetic field type and electric field type of waves with common wavelength. In Hertz’s experiment, it happens that both light waves and radio waves (magnetic type waves) are released but not with common wavelength. Some researchers do not consider microwaves as electromagnetic waves, because their velocity in vacuum is less than the velocity of light in vacuum