What is light? Maxwell's model 1870

1870 Maxwell's model that light is a wave in the electromagnetic field.

Electromagnetic waves and charge are closely related.

Most objects have close to zero net electric charge since they are made of neutral atoms. The atoms themselves are neutral but made from equal numbers of positive and negative charges. However, if you rub two objects together one of them will acquire electric charge from the other. A rubber comb rubbed through hair becomes negatively charged leaving your hair positively charged. If you then hold the comb near some strands of your hair your hair will be attracted to the comb( as long as you haven't moosed (sic), sprayed, conditioned or otherwise treated your hair). Positive electric charges attract negative charges. This attraction occurs through the air which separates the charges, and even occurs if there is a vacuum between the charges. So electrical attraction can go through a vacuum.

At first, scientists said that charges attracted and repelled via "action at a distance" one thing just pulled or pushed on the other. Newton could easily explain refraction by saying that the glass pulled on the light speeding it up in the direction toward the glass and bending the light beam into the glass. The speed of light in glass could not be measured in Newton's day although Roemer had already measured the speed of light in a vacuum by observing the changing times of the eclipses of the moons of Jupiter as the earth circled the sun in its orbit.

But, now we know that this simple model isn't correct. If one charge is moved, the force on the other charge doesn't change for a while. It is as if something is carrying the force from one charge to the other. Maxwell building on the work of Faraday in the mid 1800's worked out a field theory which says that each charge creates an electric field that spreads out through all of space. When another charge is placed in this field it feels a force. So the electric field contains the potential for producing a force. Wiggle one charge and the other one responds to the wiggles after a time delay. The further apart the charges are, the longer the time it takes for the wiggles to go from one charge to the other. The wiggles in the field move at the speed of light because they are light (used in its broad sense to mean electromagnetic wave.)

(By the way, wiggles in the gravitational field, gravity waves, also travel at the speed of light.)

Whenever a charge, either plus or minus, is wiggled &emdash; that is accelerated &emdash; then waves move along the lines of the electric field that go out from that charge. The wiggles in this model are electromagnetic radiation one form of which is light.
Whenever light- a wave in the electric field - hits a charge, then the charge is wiggled.
Light is made by accelerating charges.
Charges are accelerated by light.

Which means that when light hits matter, which is made of atoms which are made of charged particles, the charges in the matter wiggle and send out light. The light sent out by matter adds to the incoming light, the resulting sum travels slower than the original light, and is the source of the slowing of the speed of light in matter that produces the index of refraction.

Light is a form of electromagnetic radiation.
Electromagnetic radiation includes:
radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays

These are all waves in the electric field, they have different frequencies and wavelengths. Wiggle your charged comb up and down slowly and you have made a radio wave, move it up and down more often(about 1015 times per second) and you have visible light, faster still and you have x-rays.

You are free to choose whatever frequency you want to wiggle your comb up and down,

but, once the light is created its frequency will not change.

( Well actually it can change a little as it climbs out of or drops into a gravity well or if it goes through a nonlinear optical material but we will neglect these cases, it might seem to change if you look at it while you change your speed, this is called the Doppler shift.)

The frequency of light and its wavelength are related by the speed of light.

c = fL

where c is the speed of light, f is the frequency and L is the wavelength.

In the wave model:

and the purity of the color, saturation, is the mix of frequencies.

return to day 4 Light Is.

Scientific Explorations
© 1999

5 Apr 99