Persistence of vision, flickering LED

Power an LED with using SQR1 and change its frequency gradually from 1Hz to 50Hz to observe at what frequency the flickering stops. It may depend on the observer and the intensity of light. I felt the flickering stopping at around 40 Hz, and my son says it still flickers at that frequency. Connection: SQR1 --LED-- GND. No need of series resistor since SQR1 comes out via a 100 Ohm resistor.

The program listed below gives a slider to change the SQR1 frequency.

from Tkinter import *
import expeyes.eyesj

def set_freq(w):
    state = int(Sqr1.get())
    fr = float(Sqr1.get())
    res = p.set_sqrs(fr)

w = Tk()
Sqr1 = Scale(w,command = set_freq, orient=HORIZONTAL, length=200, showvalue=True, from_ = 1, to=50, resolution=.1)