MadSci Network: Physics

Re: Is there any experimantal proof that electromagnetic wave exert pressure?

Date: Thu Oct 21 12:26:39 2004
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Physics
ID: 1098223906.Ph

You seem to have asked two different questions, one in the subject line of your post, the other in the body of the post. I'm not sure if I understand exactly what you are trying to say, so let me address each part separately; perhaps that will answer your real question.

So, the first thing you write is:

Is there any experimantal proof that electromagnetic wave exert pressure?

Yes, there is. Now, it's a little hard to see this in our everyday, ordinary life, because the pressure exerted by light waves (or radio waves, or X-rays) is very small compared to other common pressures: due to wind, or currents of water, or gravity pulling a box down onto a floor, etc. However, if we could eliminate all those other effects, we would be able to see macroscopic objects pushed around by the pressure of light.

A good way to eliminate other sources of pressure is to into space. With no air, and no water, and very little matter at all, objects are so isolated that the small pressure of sunlight can take over. Scientists have designed "solar sails" to take advantage of this isolation. Solar sails are basically large, very thin pieces of reflective foil. If they are placed into space so that one large, flat side faces the Sun, then electromagnetic waves from the Sun will strike the foil and bounce back, towards the Sun. As Newton's Third Law states, if the foil exerts a force on the photons (bouncing them backwards towards the Sun), then the photons exert a force of equal size but opposite direction on the foil (pushing it away from the Sun). These forces are small, so they don't produce large accelerations -- they are very much smaller than the accelerations of ordinary rocket engines. But they do require no fuel, just sunlight, and that's free. So for some purposes, these solar sails could be used to ferry material around the solar system. You can find a good reference page describing sails at

Okay, let's move on to your second question, which appears in the body of your text. You ask:

I want to understand how the changing eletric and magnetic fields in an electromagnetic wave will exert a force on an eletrical charge.

Hmmm. Okay, this sounds a bit different. In this case, you specify that the object of interest is electrically charged -- like an electron or proton. That means that we have to consider different physical processes than in the case of a large, neutral solar sail.

Physicists have known for over one hundred years that electric and magnetic fields both exert forces on charged particles. If you look in any college textbook on physics, you'll find several chapters describing the equations that describe these forces and how to use them. I don't think that I can write several chapters worth of material here, in this reply to you, so I'm going to recommend that you visit a library or bookstore and do that reading for yourself.

However, I can give you an example from ordinary, everyday life which involves electromagnetic waves interacting with charged particles. Consider a radio -- how does it work?

Well, first, a radio station has to create the radio waves. It does this with an antenna, which is basically a very long, straight metal wire. The metal is full of electrons, some of which are free to move throughout the material (that's what makes a substance a conductor). This antenna is attached to an electric circuit. The circuit, in turn, is connected to a microphone. As a person speaks into the microphone, it sends an oscillating current through the circuit, which in turn sends an oscillating electric field through the wire. The electric field exerts a force on the electrons, causing them to flow through the antenna first in one direction, and then in the other ... and then again in one direction, and then back again ... and over and over and over. The oscillating electrons emit electromagnetic waves (radio waves) which shoot outward from the wire at the speed of light.

Now, if these waves should happen to pass through a radio in your home, the same sort of thing happens in reverse. The electromagnetic waves contain within them a small electric field (and a small magnetic field, too). Inside your radio is an antenna, much smaller than the one at the radio station, but the same sort of device: a wire of conducting material. As the waves pass by the wire, they exert electric forces on the electrons in that wire, which cause the electrons to flow through the wire in one direction, and then back in the other, then forward, then back ... over and over and over. If the antenna is connected to a circuit inside the radio, then the circuit shares the oscillating flow of electrons: a current. The circuitry translates this oscillating current into a sound we can hear with our ears.

I hope that perhaps I have managed to answer some of the questions you had.

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2003. All rights reserved.