# Light in changing magnetic field

1. Aug 30, 2008

### spidey

what will happen if light passes through changing magnetic field?

2. Aug 30, 2008

### granpa

same thing that happens when a beam of light crosses another beam of light.

3. Aug 31, 2008

### granpa

Last edited: Aug 31, 2008
4. Aug 31, 2008

### Astronuc

Staff Emeritus
Assuming it's in a vacuum, i.e. a charge-free space, it keeps on going straight with no change in frequency.

5. Aug 31, 2008

### spidey

Hi Astronuc..why there is no change at all in light in changing magnetic field..are there any reasons behind this?

6. Aug 31, 2008

### granpa

yes. the wave has no charge. only charged particles are affected by electric fields.

you may as well ask why water waves pass right through one another without interacting.

7. Aug 31, 2008

### Defennder

You do get a change in colour when you mix different coloured lights together.

8. Sep 1, 2008

### spidey

what i meant,will there be any change in the frequency or energy of light after passing through changing magnetic field

9. Sep 2, 2008

### atyy

A changing magnetic field will produce an electromagnetic wave. So your question is basically asking what happens when two electromagentic waves meet each other.

1) Maxwell's (vacuum) wave equation is linear, which means that two waves which are themselves solutions (ie. physically possible) can be added together to produce another wave which is also a solution.

2) A sinusoidal wave of any frequency is a solution of the wave equation.

3) This means that any electromagnetic wave, no matter how complex, can be considered to be due to the addition of many sinusoidal waves of different frequencies. In this sense, sinusoidal waves "preserve their identity" and hence "preserve their frequency" even when they sum into a very complex wave.

This is not true for electromagnetic waves (at high intensities) in materials, nor is it true for water waves - the wave equations there are nonlinear.

So why is the sum of two waves a different visual colour? Well, each wave itself stimulates a particular colour receptor in your retina. Different combinations of waves stimulate different combinations of colour receptors, and what colour you see is really a biological question (and there are interesting effects where light of the same wavelength appears a different colour depending on the colour of its surroundings):http://www.designmatrix.com/pl/cyberpl/cic.html.

10. Sep 4, 2008

### spidey

11. Sep 5, 2008

### Defennder

That's interesting. I always thought that the two different waves mix with each other to form one with a different frequency.

12. Sep 5, 2008

### atyy

We don't know how. It's is experimental observation which is encoded in two of the Maxwell equations:

1) A changing magnetic field produces a changing electric field (Faraday's law).
2) A changing current or a changing electric field produces a changing magnetic field (Ampere-Maxwell law).

So those two laws together say: a changing magnetic field produces a changing electric field produces a changing magnetic field produces a changing electric field produces ... [ad infinitum].

The infinitely propagating change in electric and magnetic fields is what we call an electromagnetic wave.

Every time you turn a current on, the current creates a magnetic field (don't ask why, it's just another experimental fact which we know how to describe mathematically). The creation of a magnetic field is, of course, a change of the magnetic field, and therefore creates an electromagentic waves. This is the fundamental idea behind radio broadcasting.

13. Sep 5, 2008

### Staff: Mentor

Maxwell's Equations. Loosely speaking, equation III says that a time-varying magnetic field produces an electric field, and equation IV says that a time-varying electric field produces a magnetic field.

Maxwell put forth these equations in the 1860s as a unifying theory of observations of electric and magnetic phenomena that were available at that time. He showed mathematically that they imply the existence of self-propagating collections of electric and magnetic fields: the changing electric field produces a changing magnetic field which in turn produces a changing electric field which in turn produces...

14. Sep 5, 2008

### atyy

Perhaps you're thinking of amplitude modulation. The Fourier frequencies of the wave don't change, but the frequency of the wave envelope changes.

There's a similar thing in sound. The envelope of two nearby frequencies will beat. If your "ear" is not able to resolve the frequencies, but only hears their envelope, then you will hear the beating (both frequencies within a critical band). If your "ear" is able to resolve the frequencies, it will not hear any beat (frequencies separated by a critical band).
http://www.sfu.ca/sonic-studio/handbook/Critical_Band.html