Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Probability Amps
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993

How do physicists determine the direction of probability amplitude for all types of light? I know that red light has a rotation of about 36,000 times per inch traveled. Does this have any relation to the frequency of this light?

Yes, your number 36,630 is the number of wavelengths of red light in an inch. The wavelength times the frequency is equal to the speed of light, so there is an indirect relationship to the frequency. For my number I assumed that the wavelength of red light is 7000 angstroms. The frequency would be 4.2 times 10 to the 14th power. Probability amplitudes are something different.

Sam Bowen

The relation between the wavelength of light and its frequency is given by: wavelength * frequency = speed of light (=300,000 km/second). So, for visible light, which covers wavelengths from 400 to 700 nanometers (4 to 7 X 10^-7 meters, or 1.5 to 2.7 X 10^-5 inches) the frequencies go from 7.5 to 4.2 x 10^14 Hertz (1 Hertz = 1 cycle per second). More on the human scale are radio stations - for example, an FM station at 100 Megahertz (a frequency which can be generated by ordinary electronics has a wavelength of: 3 x 10^8 meters/second/1 x 10^8 Hz = 3 meters. The ideal antenna for picking up radio signals is about one half of a wavelength, or about 1.5 meters (= 5 feet) for this frequency which, if you think about it (think of a car antenna), is about the size people actually use! For our radio station, the radio waves, like light, are known as "electro-magnetic" waves, and have electric and magnetic fields associated with them (That is why you can generate them with an electrical circuit). An electric field is something that causes an electrical charge to move in the direction the field is pointing, and similarly a magnetic field causes a magnet to orient itself along the direction it is pointing. So, at any instant, the radio waves have a certain collection of these electric and magnetic fields spread out all over the place. Imagine freezing this collection of fields, and testing the directions while moving towards and away from the station transmitter. What you will find is that first of all the fields are always pointing perpendicular to the line-of-sight from the transmitter (they never point along the direction they are going) and that the electric field is always perpendicular to the magnetic field. You will also find that every 1.5 meters (1/2 of a wavelength) both the electric and magnetic fields flip direction, so that after a full 3 meters, they are back where they started again. That is why they are called waves! Now unfreeze it. 1 of over the frequency is 10^-8 seconds, which is called the period. You will find that every 1/2 period (5 x 10^-9 seconds) in time, the electric and magnetic fields everywhere flip directions, so that after a full 10^-8 seconds, they are again back where the started. So the frequency (100 million inverse seconds) is the number of times a second the fields do one full cycle. The same (much faster and smaller) is true of light.

Arthur Smith

Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory