The human eye cannot discern flicker with a frequency greater than roughly 30 Hz. A light turning on and off faster than this does not appear to be turning on and off. Instead it appears to have a constant intensity to the eye.
Human ears however are much faster, that is, better able to perceive a signal changing quickly in time. The human ear can detect frequencies from roughly 20 Hz to 20 000 Hz, provided those frequencies arrive as sound.
Now suppose there were an easy way to change light of time-varying intensity, into sound of time varying intensity. Then you could "hear" light that was flickering too fast for you to see it.
The humble silicon solar-cell can convert a time-varying light signal into a time-varying electrical signal. This small electrical signal can then be fed to an audio signal amplifier and converted into sound.
Step 1: Parts:
1 audio amplifier (Radio Shack Catalog #:277-1008)
1 1/8-inch (3mm) headphone plug, can be mono or stereo
1 small Silicon solar cell with approximately 1 cm2 in area
1 1/2-inch PVC pipe cap
some wire, solder, epoxy, black paint, etc.