Photoresistors detection
It might work. Let's think of the LDR (light dependent resistor) as a long thin resistor with, for example, a resistance when dark of 1 MΩ/mm. Let's say the resistor path is 10 mm long. Then its resistance will be 10 MΩ.



Image from Wikipedia Commons.

Note from the image that the light areas are conductive and the dark area is the photo-sensitive area. If we could get a tiny portion of the brown track to go close to zero ohms then, theoretically, the device should present a low resistance between the terminals.

Now if you shine a spot of light on 0.5 mm of the resistor and it's resistance dropped by a factor of 1000. In our model above that section would provide a 1 kΩ bridge between the terminals. 

This sounds promising but we might find that concentrating the current in a small bridge causes local heating and may damage the device at that spot.

You should try it out and report your results. Note that you may have to focus the light to hit the dark trace. If all the light hits only one conductor or the other there will be no reduction in resistance.