A US household light bulb on AC power actually flickers at 120 Hz, yet people never notice this flickering in everyday use. Why not?
AThe current never actually reaches zero
BTransformers eliminate the flicker before it reaches the bulb
C120 Hz is slower than the refresh rate of the human eye
D120 Hz is faster than the refresh rate of the human eye
Answer & Solution
Correct answer: D. 120 Hz is faster than the refresh rate of the human eye
1. The eye cannot resolve extremely rapid changes in brightness; it perceives fast enough flicker as continuous light.
2. The text explains: "This is faster than the refresh rate of the human eye, so you don't notice the flicker of the lights."
3. The claim that current never reaches zero is false -- AC crosses zero by definition twice per cycle, which is exactly why the flicker exists in the first place.
4. Transformers change voltage levels; they play no role in smoothing out this particular flicker, which is purely a perceptual (not electrical) effect.
5. So the flicker goes unnoticed because 120 Hz outpaces what the human eye can perceive as separate flashes.
_Source: OpenStax Physics Ch 20 "Magnetism", p.688 §20.2 Motors, Generators, and Transformers_
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