L.E.D. headlamps are a wonder of the modern age.
When I think back to what flashlights were when I was Boy Scout, it was 2 D cell units with an incandescent bulb. A couple of hours of use and then it went dim.
Now it's a couple of AAA batteries, it fits on with a headband, and it's bright enough to hike with, and the batteries last for a week of regular use.
There's a side effect of these head lamps I discovered, though, that gave me pause. It was 4 or 5 years ago. I had just gotten a new L.E.D. headlamp and was camped in a site that had a large grassy field nearby. I had gotten up to go to the latrine and as the light from my headlamp swept the field, I saw thousands of tiny, very bright reflections. They looked like prisms reflecting rainbows, sharp points of multicolored light.
I had to see, so I walked in, following one of them to the source. It was a spider. A small spider with eyes that reflected the light from that headlamp. A moment later came the realization that there where tens of thousands of spiders in that field. Every one defending a territory of a circle about eight inches around.
This only works with headlamps and only of a certain type of lamp I think, but I have shown this to others to my amusement. That dawning moment when they grok the idea of just how many spiders are in every patch of grass and leaf litter is priceless.
I was reminded of this today by an article in the news. A mom took a cellphone picture of her toddler and the reflection of the child's eye was white instead of red. It's indicative of a particular form of cancer that starts in the eye and then can grow out into the brain. That white reflection is a common symptom. It was the light from the flash that revealed what was hidden in plain sight.