Vibrating screen

[Alex VA]

I have noticed that when I put a neck massager on my head while I am watching TV or using the computer, the screen vibrates. First real fast, then it slows down, speeds up, slows down, ect. Why is this?
Also, if I put one of those little handheld moterized fan dealies in front of the moniter, the propeller motions seem to change in the manner above. Again, why is this?

[Q.E.D.]

Both effects are caused by differences in the rates of oscillation of the light source (in this case, the TV screen or the monitor screen) and in the object being illuminated (your eyes in the first case, and the fan blades in the second). It's called the stoboscopic effect, and is the same reason the wheels of cars seem to change between turning forward and then backwards in the movies.

[netscape 6]

i think i can explain the fan. your monitor as a light source, a fast blinking light (it brings a new frame thirty times a second, so it blinks thirty times a second). this pretty much makes your monitor a strobe light.

experiment: look at a wall. now move your hand back and fourth so fast it is a blur. now try this in front of your monitor. of course a real strobe light is more fun.

[Alex VA]

I meant why does it change speed

[Q.E.D.]

It changes speed because while the TV and computer monitor refresh rates are constant, the variations in line voltage mean the fan and the vibrator are not. When their rates of oscillation change, the perceived stobe effect speeds up or slows down too.