At the beginning of the thermal lens experiment you will need to adjust the chop

Question

At the beginning of the thermal lens experiment you will need to adjust the chopper frequency to approximately 60 Hz. A chopper is a rotating disk that has windows in it. The laser light will reach the detector when a window is in front of the beam; no light will reach the detector when the solid part of the chopper blade intersects the beam. The signal on the oscilloscope screen will be a square wave and look like the one in Figure 2a of the handout. The width of the square wave is the time that the window is open. The faster the disk is rotating, the shorter the time the window is open. The oscilloscope screen has a grid on it and the scale can be adjusted by changing the settings on the knob on the front of the oscilloscope. The "duty cycle" of the chopper can be determined by displaying at least two square waves on the screen and then measuring the time from the start of one square wave to the start of the next. The chopping frequency is the number of duty cycles per second.

If the knob is set at 5 msec/div and the distance between the start of one square wave to the start of the next is 1.6 divisions, what is the chopping frequency?

Explanation / Answer

to complete 1 square wave = 1.6 div are taken

so Time period = 1.6 div = 1.6 x 5m sec = 8 mSec = 8 x 10^-3 sec

frequency = 1/ T = 125 Hz

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