LDR Photoresistive Track
Try it! Fix the LDR to your desk with tape, connect your multimeter on resistance mode to the LEDs, fix a desk lamp above the LDR and gradually shade it with a piece of card sliding it across the surface of the LDR. You should see the resistance gradually increase.

Why?







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Imagine the LDR as 15 x 1 mm long sections of LDR.

Your photo clearly shows the two silver electrical contacts. The LDR is the brown portion between (and around) the contacts. Judging from your photo and some that I have used the gap is about 0.5 mm and the length of the zig-zag is about 15 mm. You could imagine this being split into 15 x 1 mm LDRs in parallel. Clearly the more that are illuminated the more the resistance will drop.