# Changes

## Holography Transmission Equations Part I

, 21:05, 14 May 2013
m
Spatial Frequency
==Spatial Frequency==
[[Image:InterferenceEQ.gif]]<center>$\displaystyle f = \frac{sin{\theta_2} - sin{\theta_1}}{\lambda}$</center>
Frequency, how often something happens, the frequencies most often coming to mind are the 20 to 20,000 cycles per second (abbreviated as Hertz) of good human hearing (not mine, for sure!) or the radio frequencies of 550 kiloHertz to 1800 kHZ of the AM band, or the GigaHertz processing speed of computer chips.
Related to this equation is finding the distance between fringes; instead of fringes per mm, how big is a fringe cycle? (= the distance from the center of one bright fringe to the next.) It is simply the inverse of the spatial frequency equation:
[[Image:SpatialEQ.gif]]<center>$d = \frac{\lambda}{sin{\theta_2} - sin{\theta_1}}$</center>
(And don’t forget theta 2 is a negative angle, otherwise you will have an error!) (Or to make life easier, forget about the sign convention, and just add the sines of the two angles as positives together! But watch your step when getting too far off the track!)
Alternatively, to find the fringe spacing distance, you could simply use the 1/x button on the calculator once you have your spatial frequency. In either case the distance will be in millimeters, and the digits will usually all pop up behind the decimal point, so if you want to convert to microns, move the decimal point over three places to the right.
Onward and upward, as my Abstract Algebra teacher used to say. (while I nodded off!).