The intensity of an ordiary incandescent lightbulb flickers with time in the sam

Question

The intensity of an ordiary incandescent lightbulb flickers with time in the same way that the instantaneous power in a resistor varies with time. In the United States, the AC voltage across the lightbulb has the frequency of f = 60 Hz. Is the frequency of the flickering (a) 30 Hz, (b) 60 Hz, (c) 120 Hz, (d) 240 Hz? How does the frequency of the flickering compare with the response time of your eyes?

The answer is C, but I do not know why is it 120 Hz? Also don't know the second answer to the second question.

Explanation / Answer

Here ,

for the sinsoidal wave ,

the fucntion goes from 0 to maximum amplitude 2 times in a cycle ,

as the brightness of bulb is independent on the direction of current ,

hence , the frequency of flickering = 2 * 60 Hz

the frequency of flickering = 120 Hz

the the frequency of flickering is 120 Hz

as the human eye refesh rate is about 24 Hz ,

we will not be able to see the flickering

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