Tuning forks are great for tuning musical instruments. So good that it would be easy to believe that they would be good to tune of check the frequency of many things, such as a radar.
Sound is transmitted by pressure waves. These move outward from a tuning fork almost evenly in all directions. Get a tuning fork and turn it about its stem and there is little difference in the sound that you can hear.
In this video there is a section that shows how tuning forks reflect a laser.
Lasers, like radar don’t have pressure waves, so if you reflect light off the tuning fork from different angles you can get interesting shapes. The math behind the shapes depends on the angle that the laser reflects off the tuning fork. The cosine of the angle relates to the percentage of the movement that as viewed from that angle.
Looking at the Y of the tuning fork, there is a zero or 180 degree angle and not much interesting happening, just the tines moving left and right.
Turning the tuning fork 90 degrees looking at one tine with the other behind they are moving forward and back. Just the way a radar or laser reflects to get the images above.
At those 90 degree and 270 degree angles, the frequency of the tuning fork is added or subtracted from the laser. A few angles and you get these interesting patterns.
In calibrating a speed radar, having a precise alignment is required. Just like the reflection off a moving object the cosine rule needs to be applied. An angle of 15 degrees gives 3% error, 30 degrees gives 13% error and 45 degrees gives about 30% error. The cosine rule applies to the tuning calibration, if there is any angle at all, the radar will be incorrectly calibrated and result only in one that overstates the speed of a vehicle. I.E. a 3, 13, or 30% error that’s like having a true speed of 45 MPH read between 47 and 58 MPH.
Alignment is one of those practices that need traceable and verifiable calibration.